Adding upstream version 3.0.36.upstream/3.0.36upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

13 files changed, 7578 insertions, 0 deletions

diff --git a/src/prompt_toolkit/layout/__init__.py b/src/prompt_toolkit/layout/__init__.py
new file mode 100644
index 0000000..6669da5
--- /dev/null
+++ b/src/prompt_toolkit/layout/__init__.py
@@ -0,0 +1,144 @@
+"""
+Command line layout definitions
+-------------------------------
+
+The layout of a command line interface is defined by a Container instance.
+There are two main groups of classes here. Containers and controls:
+
+- A container can contain other containers or controls, it can have multiple
+  children and it decides about the dimensions.
+- A control is responsible for rendering the actual content to a screen.
+  A control can propose some dimensions, but it's the container who decides
+  about the dimensions -- or when the control consumes more space -- which part
+  of the control will be visible.
+
+
+Container classes::
+
+    - Container (Abstract base class)
+       |- HSplit (Horizontal split)
+       |- VSplit (Vertical split)
+       |- FloatContainer (Container which can also contain menus and other floats)
+       `- Window (Container which contains one actual control
+
+Control classes::
+
+    - UIControl (Abstract base class)
+       |- FormattedTextControl (Renders formatted text, or a simple list of text fragments)
+       `- BufferControl (Renders an input buffer.)
+
+
+Usually, you end up wrapping every control inside a `Window` object, because
+that's the only way to render it in a layout.
+
+There are some prepared toolbars which are ready to use::
+
+- SystemToolbar (Shows the 'system' input buffer, for entering system commands.)
+- ArgToolbar (Shows the input 'arg', for repetition of input commands.)
+- SearchToolbar (Shows the 'search' input buffer, for incremental search.)
+- CompletionsToolbar (Shows the completions of the current buffer.)
+- ValidationToolbar (Shows validation errors of the current buffer.)
+
+And one prepared menu:
+
+- CompletionsMenu
+
+"""
+from .containers import (
+    AnyContainer,
+    ColorColumn,
+    ConditionalContainer,
+    Container,
+    DynamicContainer,
+    Float,
+    FloatContainer,
+    HorizontalAlign,
+    HSplit,
+    ScrollOffsets,
+    VerticalAlign,
+    VSplit,
+    Window,
+    WindowAlign,
+    WindowRenderInfo,
+    is_container,
+    to_container,
+    to_window,
+)
+from .controls import (
+    BufferControl,
+    DummyControl,
+    FormattedTextControl,
+    SearchBufferControl,
+    UIContent,
+    UIControl,
+)
+from .dimension import (
+    AnyDimension,
+    D,
+    Dimension,
+    is_dimension,
+    max_layout_dimensions,
+    sum_layout_dimensions,
+    to_dimension,
+)
+from .layout import InvalidLayoutError, Layout, walk
+from .margins import (
+    ConditionalMargin,
+    Margin,
+    NumberedMargin,
+    PromptMargin,
+    ScrollbarMargin,
+)
+from .menus import CompletionsMenu, MultiColumnCompletionsMenu
+from .scrollable_pane import ScrollablePane
+
+__all__ = [
+    # Layout.
+    "Layout",
+    "InvalidLayoutError",
+    "walk",
+    # Dimensions.
+    "AnyDimension",
+    "Dimension",
+    "D",
+    "sum_layout_dimensions",
+    "max_layout_dimensions",
+    "to_dimension",
+    "is_dimension",
+    # Containers.
+    "AnyContainer",
+    "Container",
+    "HorizontalAlign",
+    "VerticalAlign",
+    "HSplit",
+    "VSplit",
+    "FloatContainer",
+    "Float",
+    "WindowAlign",
+    "Window",
+    "WindowRenderInfo",
+    "ConditionalContainer",
+    "ScrollOffsets",
+    "ColorColumn",
+    "to_container",
+    "to_window",
+    "is_container",
+    "DynamicContainer",
+    "ScrollablePane",
+    # Controls.
+    "BufferControl",
+    "SearchBufferControl",
+    "DummyControl",
+    "FormattedTextControl",
+    "UIControl",
+    "UIContent",
+    # Margins.
+    "Margin",
+    "NumberedMargin",
+    "ScrollbarMargin",
+    "ConditionalMargin",
+    "PromptMargin",
+    # Menus.
+    "CompletionsMenu",
+    "MultiColumnCompletionsMenu",
+]
diff --git a/src/prompt_toolkit/layout/containers.py b/src/prompt_toolkit/layout/containers.py
new file mode 100644
index 0000000..03f9e7d
--- /dev/null
+++ b/src/prompt_toolkit/layout/containers.py
@@ -0,0 +1,2757 @@
+"""
+Container for the layout.
+(Containers can contain other containers or user interface controls.)
+"""
+from abc import ABCMeta, abstractmethod
+from enum import Enum
+from functools import partial
+from typing import (
+    TYPE_CHECKING,
+    Callable,
+    Dict,
+    List,
+    Optional,
+    Sequence,
+    Tuple,
+    Union,
+    cast,
+)
+
+from prompt_toolkit.application.current import get_app
+from prompt_toolkit.cache import SimpleCache
+from prompt_toolkit.data_structures import Point
+from prompt_toolkit.filters import (
+    FilterOrBool,
+    emacs_insert_mode,
+    to_filter,
+    vi_insert_mode,
+)
+from prompt_toolkit.formatted_text import (
+    AnyFormattedText,
+    StyleAndTextTuples,
+    to_formatted_text,
+)
+from prompt_toolkit.formatted_text.utils import (
+    fragment_list_to_text,
+    fragment_list_width,
+)
+from prompt_toolkit.key_binding import KeyBindingsBase
+from prompt_toolkit.mouse_events import MouseEvent, MouseEventType
+from prompt_toolkit.utils import get_cwidth, take_using_weights, to_int, to_str
+
+from .controls import (
+    DummyControl,
+    FormattedTextControl,
+    GetLinePrefixCallable,
+    UIContent,
+    UIControl,
+)
+from .dimension import (
+    AnyDimension,
+    Dimension,
+    max_layout_dimensions,
+    sum_layout_dimensions,
+    to_dimension,
+)
+from .margins import Margin
+from .mouse_handlers import MouseHandlers
+from .screen import _CHAR_CACHE, Screen, WritePosition
+from .utils import explode_text_fragments
+
+if TYPE_CHECKING:
+    from typing_extensions import Protocol, TypeGuard
+
+    from prompt_toolkit.key_binding.key_bindings import NotImplementedOrNone
+
+
+__all__ = [
+    "AnyContainer",
+    "Container",
+    "HorizontalAlign",
+    "VerticalAlign",
+    "HSplit",
+    "VSplit",
+    "FloatContainer",
+    "Float",
+    "WindowAlign",
+    "Window",
+    "WindowRenderInfo",
+    "ConditionalContainer",
+    "ScrollOffsets",
+    "ColorColumn",
+    "to_container",
+    "to_window",
+    "is_container",
+    "DynamicContainer",
+]
+
+
+class Container(metaclass=ABCMeta):
+    """
+    Base class for user interface layout.
+    """
+
+    @abstractmethod
+    def reset(self) -> None:
+        """
+        Reset the state of this container and all the children.
+        (E.g. reset scroll offsets, etc...)
+        """
+
+    @abstractmethod
+    def preferred_width(self, max_available_width: int) -> Dimension:
+        """
+        Return a :class:`~prompt_toolkit.layout.Dimension` that represents the
+        desired width for this container.
+        """
+
+    @abstractmethod
+    def preferred_height(self, width: int, max_available_height: int) -> Dimension:
+        """
+        Return a :class:`~prompt_toolkit.layout.Dimension` that represents the
+        desired height for this container.
+        """
+
+    @abstractmethod
+    def write_to_screen(
+        self,
+        screen: Screen,
+        mouse_handlers: MouseHandlers,
+        write_position: WritePosition,
+        parent_style: str,
+        erase_bg: bool,
+        z_index: Optional[int],
+    ) -> None:
+        """
+        Write the actual content to the screen.
+
+        :param screen: :class:`~prompt_toolkit.layout.screen.Screen`
+        :param mouse_handlers: :class:`~prompt_toolkit.layout.mouse_handlers.MouseHandlers`.
+        :param parent_style: Style string to pass to the :class:`.Window`
+            object. This will be applied to all content of the windows.
+            :class:`.VSplit` and :class:`.HSplit` can use it to pass their
+            style down to the windows that they contain.
+        :param z_index: Used for propagating z_index from parent to child.
+        """
+
+    def is_modal(self) -> bool:
+        """
+        When this container is modal, key bindings from parent containers are
+        not taken into account if a user control in this container is focused.
+        """
+        return False
+
+    def get_key_bindings(self) -> Optional[KeyBindingsBase]:
+        """
+        Returns a :class:`.KeyBindings` object. These bindings become active when any
+        user control in this container has the focus, except if any containers
+        between this container and the focused user control is modal.
+        """
+        return None
+
+    @abstractmethod
+    def get_children(self) -> List["Container"]:
+        """
+        Return the list of child :class:`.Container` objects.
+        """
+        return []
+
+
+if TYPE_CHECKING:
+
+    class MagicContainer(Protocol):
+        """
+        Any object that implements ``__pt_container__`` represents a container.
+        """
+
+        def __pt_container__(self) -> "AnyContainer":
+            ...
+
+
+AnyContainer = Union[Container, "MagicContainer"]
+
+
+def _window_too_small() -> "Window":
+    "Create a `Window` that displays the 'Window too small' text."
+    return Window(
+        FormattedTextControl(text=[("class:window-too-small", " Window too small... ")])
+    )
+
+
+class VerticalAlign(Enum):
+    "Alignment for `HSplit`."
+    TOP = "TOP"
+    CENTER = "CENTER"
+    BOTTOM = "BOTTOM"
+    JUSTIFY = "JUSTIFY"
+
+
+class HorizontalAlign(Enum):
+    "Alignment for `VSplit`."
+    LEFT = "LEFT"
+    CENTER = "CENTER"
+    RIGHT = "RIGHT"
+    JUSTIFY = "JUSTIFY"
+
+
+class _Split(Container):
+    """
+    The common parts of `VSplit` and `HSplit`.
+    """
+
+    def __init__(
+        self,
+        children: Sequence[AnyContainer],
+        window_too_small: Optional[Container] = None,
+        padding: AnyDimension = Dimension.exact(0),
+        padding_char: Optional[str] = None,
+        padding_style: str = "",
+        width: AnyDimension = None,
+        height: AnyDimension = None,
+        z_index: Optional[int] = None,
+        modal: bool = False,
+        key_bindings: Optional[KeyBindingsBase] = None,
+        style: Union[str, Callable[[], str]] = "",
+    ) -> None:
+
+        self.children = [to_container(c) for c in children]
+        self.window_too_small = window_too_small or _window_too_small()
+        self.padding = padding
+        self.padding_char = padding_char
+        self.padding_style = padding_style
+
+        self.width = width
+        self.height = height
+        self.z_index = z_index
+
+        self.modal = modal
+        self.key_bindings = key_bindings
+        self.style = style
+
+    def is_modal(self) -> bool:
+        return self.modal
+
+    def get_key_bindings(self) -> Optional[KeyBindingsBase]:
+        return self.key_bindings
+
+    def get_children(self) -> List[Container]:
+        return self.children
+
+
+class HSplit(_Split):
+    """
+    Several layouts, one stacked above/under the other. ::
+
+        +--------------------+
+        |                    |
+        +--------------------+
+        |                    |
+        +--------------------+
+
+    By default, this doesn't display a horizontal line between the children,
+    but if this is something you need, then create a HSplit as follows::
+
+        HSplit(children=[ ... ], padding_char='-',
+               padding=1, padding_style='#ffff00')
+
+    :param children: List of child :class:`.Container` objects.
+    :param window_too_small: A :class:`.Container` object that is displayed if
+        there is not enough space for all the children. By default, this is a
+        "Window too small" message.
+    :param align: `VerticalAlign` value.
+    :param width: When given, use this width instead of looking at the children.
+    :param height: When given, use this height instead of looking at the children.
+    :param z_index: (int or None) When specified, this can be used to bring
+        element in front of floating elements.  `None` means: inherit from parent.
+    :param style: A style string.
+    :param modal: ``True`` or ``False``.
+    :param key_bindings: ``None`` or a :class:`.KeyBindings` object.
+
+    :param padding: (`Dimension` or int), size to be used for the padding.
+    :param padding_char: Character to be used for filling in the padding.
+    :param padding_style: Style to applied to the padding.
+    """
+
+    def __init__(
+        self,
+        children: Sequence[AnyContainer],
+        window_too_small: Optional[Container] = None,
+        align: VerticalAlign = VerticalAlign.JUSTIFY,
+        padding: AnyDimension = 0,
+        padding_char: Optional[str] = None,
+        padding_style: str = "",
+        width: AnyDimension = None,
+        height: AnyDimension = None,
+        z_index: Optional[int] = None,
+        modal: bool = False,
+        key_bindings: Optional[KeyBindingsBase] = None,
+        style: Union[str, Callable[[], str]] = "",
+    ) -> None:
+
+        super().__init__(
+            children=children,
+            window_too_small=window_too_small,
+            padding=padding,
+            padding_char=padding_char,
+            padding_style=padding_style,
+            width=width,
+            height=height,
+            z_index=z_index,
+            modal=modal,
+            key_bindings=key_bindings,
+            style=style,
+        )
+
+        self.align = align
+
+        self._children_cache: SimpleCache[
+            Tuple[Container, ...], List[Container]
+        ] = SimpleCache(maxsize=1)
+        self._remaining_space_window = Window()  # Dummy window.
+
+    def preferred_width(self, max_available_width: int) -> Dimension:
+        if self.width is not None:
+            return to_dimension(self.width)
+
+        if self.children:
+            dimensions = [c.preferred_width(max_available_width) for c in self.children]
+            return max_layout_dimensions(dimensions)
+        else:
+            return Dimension()
+
+    def preferred_height(self, width: int, max_available_height: int) -> Dimension:
+        if self.height is not None:
+            return to_dimension(self.height)
+
+        dimensions = [
+            c.preferred_height(width, max_available_height) for c in self._all_children
+        ]
+        return sum_layout_dimensions(dimensions)
+
+    def reset(self) -> None:
+        for c in self.children:
+            c.reset()
+
+    @property
+    def _all_children(self) -> List[Container]:
+        """
+        List of child objects, including padding.
+        """
+
+        def get() -> List[Container]:
+            result: List[Container] = []
+
+            # Padding Top.
+            if self.align in (VerticalAlign.CENTER, VerticalAlign.BOTTOM):
+                result.append(Window(width=Dimension(preferred=0)))
+
+            # The children with padding.
+            for child in self.children:
+                result.append(child)
+                result.append(
+                    Window(
+                        height=self.padding,
+                        char=self.padding_char,
+                        style=self.padding_style,
+                    )
+                )
+            if result:
+                result.pop()
+
+            # Padding right.
+            if self.align in (VerticalAlign.CENTER, VerticalAlign.TOP):
+                result.append(Window(width=Dimension(preferred=0)))
+
+            return result
+
+        return self._children_cache.get(tuple(self.children), get)
+
+    def write_to_screen(
+        self,
+        screen: Screen,
+        mouse_handlers: MouseHandlers,
+        write_position: WritePosition,
+        parent_style: str,
+        erase_bg: bool,
+        z_index: Optional[int],
+    ) -> None:
+        """
+        Render the prompt to a `Screen` instance.
+
+        :param screen: The :class:`~prompt_toolkit.layout.screen.Screen` class
+            to which the output has to be written.
+        """
+        sizes = self._divide_heights(write_position)
+        style = parent_style + " " + to_str(self.style)
+        z_index = z_index if self.z_index is None else self.z_index
+
+        if sizes is None:
+            self.window_too_small.write_to_screen(
+                screen, mouse_handlers, write_position, style, erase_bg, z_index
+            )
+        else:
+            #
+            ypos = write_position.ypos
+            xpos = write_position.xpos
+            width = write_position.width
+
+            # Draw child panes.
+            for s, c in zip(sizes, self._all_children):
+                c.write_to_screen(
+                    screen,
+                    mouse_handlers,
+                    WritePosition(xpos, ypos, width, s),
+                    style,
+                    erase_bg,
+                    z_index,
+                )
+                ypos += s
+
+            # Fill in the remaining space. This happens when a child control
+            # refuses to take more space and we don't have any padding. Adding a
+            # dummy child control for this (in `self._all_children`) is not
+            # desired, because in some situations, it would take more space, even
+            # when it's not required. This is required to apply the styling.
+            remaining_height = write_position.ypos + write_position.height - ypos
+            if remaining_height > 0:
+                self._remaining_space_window.write_to_screen(
+                    screen,
+                    mouse_handlers,
+                    WritePosition(xpos, ypos, width, remaining_height),
+                    style,
+                    erase_bg,
+                    z_index,
+                )
+
+    def _divide_heights(self, write_position: WritePosition) -> Optional[List[int]]:
+        """
+        Return the heights for all rows.
+        Or None when there is not enough space.
+        """
+        if not self.children:
+            return []
+
+        width = write_position.width
+        height = write_position.height
+
+        # Calculate heights.
+        dimensions = [c.preferred_height(width, height) for c in self._all_children]
+
+        # Sum dimensions
+        sum_dimensions = sum_layout_dimensions(dimensions)
+
+        # If there is not enough space for both.
+        # Don't do anything.
+        if sum_dimensions.min > height:
+            return None
+
+        # Find optimal sizes. (Start with minimal size, increase until we cover
+        # the whole height.)
+        sizes = [d.min for d in dimensions]
+
+        child_generator = take_using_weights(
+            items=list(range(len(dimensions))), weights=[d.weight for d in dimensions]
+        )
+
+        i = next(child_generator)
+
+        # Increase until we meet at least the 'preferred' size.
+        preferred_stop = min(height, sum_dimensions.preferred)
+        preferred_dimensions = [d.preferred for d in dimensions]
+
+        while sum(sizes) < preferred_stop:
+            if sizes[i] < preferred_dimensions[i]:
+                sizes[i] += 1
+            i = next(child_generator)
+
+        # Increase until we use all the available space. (or until "max")
+        if not get_app().is_done:
+            max_stop = min(height, sum_dimensions.max)
+            max_dimensions = [d.max for d in dimensions]
+
+            while sum(sizes) < max_stop:
+                if sizes[i] < max_dimensions[i]:
+                    sizes[i] += 1
+                i = next(child_generator)
+
+        return sizes
+
+
+class VSplit(_Split):
+    """
+    Several layouts, one stacked left/right of the other. ::
+
+        +---------+----------+
+        |         |          |
+        |         |          |
+        +---------+----------+
+
+    By default, this doesn't display a vertical line between the children, but
+    if this is something you need, then create a HSplit as follows::
+
+        VSplit(children=[ ... ], padding_char='|',
+               padding=1, padding_style='#ffff00')
+
+    :param children: List of child :class:`.Container` objects.
+    :param window_too_small: A :class:`.Container` object that is displayed if
+        there is not enough space for all the children. By default, this is a
+        "Window too small" message.
+    :param align: `HorizontalAlign` value.
+    :param width: When given, use this width instead of looking at the children.
+    :param height: When given, use this height instead of looking at the children.
+    :param z_index: (int or None) When specified, this can be used to bring
+        element in front of floating elements.  `None` means: inherit from parent.
+    :param style: A style string.
+    :param modal: ``True`` or ``False``.
+    :param key_bindings: ``None`` or a :class:`.KeyBindings` object.
+
+    :param padding: (`Dimension` or int), size to be used for the padding.
+    :param padding_char: Character to be used for filling in the padding.
+    :param padding_style: Style to applied to the padding.
+    """
+
+    def __init__(
+        self,
+        children: Sequence[AnyContainer],
+        window_too_small: Optional[Container] = None,
+        align: HorizontalAlign = HorizontalAlign.JUSTIFY,
+        padding: AnyDimension = 0,
+        padding_char: Optional[str] = None,
+        padding_style: str = "",
+        width: AnyDimension = None,
+        height: AnyDimension = None,
+        z_index: Optional[int] = None,
+        modal: bool = False,
+        key_bindings: Optional[KeyBindingsBase] = None,
+        style: Union[str, Callable[[], str]] = "",
+    ) -> None:
+
+        super().__init__(
+            children=children,
+            window_too_small=window_too_small,
+            padding=padding,
+            padding_char=padding_char,
+            padding_style=padding_style,
+            width=width,
+            height=height,
+            z_index=z_index,
+            modal=modal,
+            key_bindings=key_bindings,
+            style=style,
+        )
+
+        self.align = align
+
+        self._children_cache: SimpleCache[
+            Tuple[Container, ...], List[Container]
+        ] = SimpleCache(maxsize=1)
+        self._remaining_space_window = Window()  # Dummy window.
+
+    def preferred_width(self, max_available_width: int) -> Dimension:
+        if self.width is not None:
+            return to_dimension(self.width)
+
+        dimensions = [
+            c.preferred_width(max_available_width) for c in self._all_children
+        ]
+
+        return sum_layout_dimensions(dimensions)
+
+    def preferred_height(self, width: int, max_available_height: int) -> Dimension:
+        if self.height is not None:
+            return to_dimension(self.height)
+
+        # At the point where we want to calculate the heights, the widths have
+        # already been decided. So we can trust `width` to be the actual
+        # `width` that's going to be used for the rendering. So,
+        # `divide_widths` is supposed to use all of the available width.
+        # Using only the `preferred` width caused a bug where the reported
+        # height was more than required. (we had a `BufferControl` which did
+        # wrap lines because of the smaller width returned by `_divide_widths`.
+
+        sizes = self._divide_widths(width)
+        children = self._all_children
+
+        if sizes is None:
+            return Dimension()
+        else:
+            dimensions = [
+                c.preferred_height(s, max_available_height)
+                for s, c in zip(sizes, children)
+            ]
+            return max_layout_dimensions(dimensions)
+
+    def reset(self) -> None:
+        for c in self.children:
+            c.reset()
+
+    @property
+    def _all_children(self) -> List[Container]:
+        """
+        List of child objects, including padding.
+        """
+
+        def get() -> List[Container]:
+            result: List[Container] = []
+
+            # Padding left.
+            if self.align in (HorizontalAlign.CENTER, HorizontalAlign.RIGHT):
+                result.append(Window(width=Dimension(preferred=0)))
+
+            # The children with padding.
+            for child in self.children:
+                result.append(child)
+                result.append(
+                    Window(
+                        width=self.padding,
+                        char=self.padding_char,
+                        style=self.padding_style,
+                    )
+                )
+            if result:
+                result.pop()
+
+            # Padding right.
+            if self.align in (HorizontalAlign.CENTER, HorizontalAlign.LEFT):
+                result.append(Window(width=Dimension(preferred=0)))
+
+            return result
+
+        return self._children_cache.get(tuple(self.children), get)
+
+    def _divide_widths(self, width: int) -> Optional[List[int]]:
+        """
+        Return the widths for all columns.
+        Or None when there is not enough space.
+        """
+        children = self._all_children
+
+        if not children:
+            return []
+
+        # Calculate widths.
+        dimensions = [c.preferred_width(width) for c in children]
+        preferred_dimensions = [d.preferred for d in dimensions]
+
+        # Sum dimensions
+        sum_dimensions = sum_layout_dimensions(dimensions)
+
+        # If there is not enough space for both.
+        # Don't do anything.
+        if sum_dimensions.min > width:
+            return None
+
+        # Find optimal sizes. (Start with minimal size, increase until we cover
+        # the whole width.)
+        sizes = [d.min for d in dimensions]
+
+        child_generator = take_using_weights(
+            items=list(range(len(dimensions))), weights=[d.weight for d in dimensions]
+        )
+
+        i = next(child_generator)
+
+        # Increase until we meet at least the 'preferred' size.
+        preferred_stop = min(width, sum_dimensions.preferred)
+
+        while sum(sizes) < preferred_stop:
+            if sizes[i] < preferred_dimensions[i]:
+                sizes[i] += 1
+            i = next(child_generator)
+
+        # Increase until we use all the available space.
+        max_dimensions = [d.max for d in dimensions]
+        max_stop = min(width, sum_dimensions.max)
+
+        while sum(sizes) < max_stop:
+            if sizes[i] < max_dimensions[i]:
+                sizes[i] += 1
+            i = next(child_generator)
+
+        return sizes
+
+    def write_to_screen(
+        self,
+        screen: Screen,
+        mouse_handlers: MouseHandlers,
+        write_position: WritePosition,
+        parent_style: str,
+        erase_bg: bool,
+        z_index: Optional[int],
+    ) -> None:
+        """
+        Render the prompt to a `Screen` instance.
+
+        :param screen: The :class:`~prompt_toolkit.layout.screen.Screen` class
+            to which the output has to be written.
+        """
+        if not self.children:
+            return
+
+        children = self._all_children
+        sizes = self._divide_widths(write_position.width)
+        style = parent_style + " " + to_str(self.style)
+        z_index = z_index if self.z_index is None else self.z_index
+
+        # If there is not enough space.
+        if sizes is None:
+            self.window_too_small.write_to_screen(
+                screen, mouse_handlers, write_position, style, erase_bg, z_index
+            )
+            return
+
+        # Calculate heights, take the largest possible, but not larger than
+        # write_position.height.
+        heights = [
+            child.preferred_height(width, write_position.height).preferred
+            for width, child in zip(sizes, children)
+        ]
+        height = max(write_position.height, min(write_position.height, max(heights)))
+
+        #
+        ypos = write_position.ypos
+        xpos = write_position.xpos
+
+        # Draw all child panes.
+        for s, c in zip(sizes, children):
+            c.write_to_screen(
+                screen,
+                mouse_handlers,
+                WritePosition(xpos, ypos, s, height),
+                style,
+                erase_bg,
+                z_index,
+            )
+            xpos += s
+
+        # Fill in the remaining space. This happens when a child control
+        # refuses to take more space and we don't have any padding. Adding a
+        # dummy child control for this (in `self._all_children`) is not
+        # desired, because in some situations, it would take more space, even
+        # when it's not required. This is required to apply the styling.
+        remaining_width = write_position.xpos + write_position.width - xpos
+        if remaining_width > 0:
+            self._remaining_space_window.write_to_screen(
+                screen,
+                mouse_handlers,
+                WritePosition(xpos, ypos, remaining_width, height),
+                style,
+                erase_bg,
+                z_index,
+            )
+
+
+class FloatContainer(Container):
+    """
+    Container which can contain another container for the background, as well
+    as a list of floating containers on top of it.
+
+    Example Usage::
+
+        FloatContainer(content=Window(...),
+                       floats=[
+                           Float(xcursor=True,
+                                ycursor=True,
+                                content=CompletionsMenu(...))
+                       ])
+
+    :param z_index: (int or None) When specified, this can be used to bring
+        element in front of floating elements.  `None` means: inherit from parent.
+        This is the z_index for the whole `Float` container as a whole.
+    """
+
+    def __init__(
+        self,
+        content: AnyContainer,
+        floats: List["Float"],
+        modal: bool = False,
+        key_bindings: Optional[KeyBindingsBase] = None,
+        style: Union[str, Callable[[], str]] = "",
+        z_index: Optional[int] = None,
+    ) -> None:
+
+        self.content = to_container(content)
+        self.floats = floats
+
+        self.modal = modal
+        self.key_bindings = key_bindings
+        self.style = style
+        self.z_index = z_index
+
+    def reset(self) -> None:
+        self.content.reset()
+
+        for f in self.floats:
+            f.content.reset()
+
+    def preferred_width(self, max_available_width: int) -> Dimension:
+        return self.content.preferred_width(max_available_width)
+
+    def preferred_height(self, width: int, max_available_height: int) -> Dimension:
+        """
+        Return the preferred height of the float container.
+        (We don't care about the height of the floats, they should always fit
+        into the dimensions provided by the container.)
+        """
+        return self.content.preferred_height(width, max_available_height)
+
+    def write_to_screen(
+        self,
+        screen: Screen,
+        mouse_handlers: MouseHandlers,
+        write_position: WritePosition,
+        parent_style: str,
+        erase_bg: bool,
+        z_index: Optional[int],
+    ) -> None:
+        style = parent_style + " " + to_str(self.style)
+        z_index = z_index if self.z_index is None else self.z_index
+
+        self.content.write_to_screen(
+            screen, mouse_handlers, write_position, style, erase_bg, z_index
+        )
+
+        for number, fl in enumerate(self.floats):
+            # z_index of a Float is computed by summing the z_index of the
+            # container and the `Float`.
+            new_z_index = (z_index or 0) + fl.z_index
+            style = parent_style + " " + to_str(self.style)
+
+            # If the float that we have here, is positioned relative to the
+            # cursor position, but the Window that specifies the cursor
+            # position is not drawn yet, because it's a Float itself, we have
+            # to postpone this calculation. (This is a work-around, but good
+            # enough for now.)
+            postpone = fl.xcursor is not None or fl.ycursor is not None
+
+            if postpone:
+                new_z_index = (
+                    number + 10**8
+                )  # Draw as late as possible, but keep the order.
+                screen.draw_with_z_index(
+                    z_index=new_z_index,
+                    draw_func=partial(
+                        self._draw_float,
+                        fl,
+                        screen,
+                        mouse_handlers,
+                        write_position,
+                        style,
+                        erase_bg,
+                        new_z_index,
+                    ),
+                )
+            else:
+                self._draw_float(
+                    fl,
+                    screen,
+                    mouse_handlers,
+                    write_position,
+                    style,
+                    erase_bg,
+                    new_z_index,
+                )
+
+    def _draw_float(
+        self,
+        fl: "Float",
+        screen: Screen,
+        mouse_handlers: MouseHandlers,
+        write_position: WritePosition,
+        style: str,
+        erase_bg: bool,
+        z_index: Optional[int],
+    ) -> None:
+        "Draw a single Float."
+        # When a menu_position was given, use this instead of the cursor
+        # position. (These cursor positions are absolute, translate again
+        # relative to the write_position.)
+        # Note: This should be inside the for-loop, because one float could
+        #       set the cursor position to be used for the next one.
+        cpos = screen.get_menu_position(
+            fl.attach_to_window or get_app().layout.current_window
+        )
+        cursor_position = Point(
+            x=cpos.x - write_position.xpos, y=cpos.y - write_position.ypos
+        )
+
+        fl_width = fl.get_width()
+        fl_height = fl.get_height()
+        width: int
+        height: int
+        xpos: int
+        ypos: int
+
+        # Left & width given.
+        if fl.left is not None and fl_width is not None:
+            xpos = fl.left
+            width = fl_width
+        # Left & right given -> calculate width.
+        elif fl.left is not None and fl.right is not None:
+            xpos = fl.left
+            width = write_position.width - fl.left - fl.right
+        # Width & right given -> calculate left.
+        elif fl_width is not None and fl.right is not None:
+            xpos = write_position.width - fl.right - fl_width
+            width = fl_width
+        # Near x position of cursor.
+        elif fl.xcursor:
+            if fl_width is None:
+                width = fl.content.preferred_width(write_position.width).preferred
+                width = min(write_position.width, width)
+            else:
+                width = fl_width
+
+            xpos = cursor_position.x
+            if xpos + width > write_position.width:
+                xpos = max(0, write_position.width - width)
+        # Only width given -> center horizontally.
+        elif fl_width:
+            xpos = int((write_position.width - fl_width) / 2)
+            width = fl_width
+        # Otherwise, take preferred width from float content.
+        else:
+            width = fl.content.preferred_width(write_position.width).preferred
+
+            if fl.left is not None:
+                xpos = fl.left
+            elif fl.right is not None:
+                xpos = max(0, write_position.width - width - fl.right)
+            else:  # Center horizontally.
+                xpos = max(0, int((write_position.width - width) / 2))
+
+            # Trim.
+            width = min(width, write_position.width - xpos)
+
+        # Top & height given.
+        if fl.top is not None and fl_height is not None:
+            ypos = fl.top
+            height = fl_height
+        # Top & bottom given -> calculate height.
+        elif fl.top is not None and fl.bottom is not None:
+            ypos = fl.top
+            height = write_position.height - fl.top - fl.bottom
+        # Height & bottom given -> calculate top.
+        elif fl_height is not None and fl.bottom is not None:
+            ypos = write_position.height - fl_height - fl.bottom
+            height = fl_height
+        # Near cursor.
+        elif fl.ycursor:
+            ypos = cursor_position.y + (0 if fl.allow_cover_cursor else 1)
+
+            if fl_height is None:
+                height = fl.content.preferred_height(
+                    width, write_position.height
+                ).preferred
+            else:
+                height = fl_height
+
+            # Reduce height if not enough space. (We can use the height
+            # when the content requires it.)
+            if height > write_position.height - ypos:
+                if write_position.height - ypos + 1 >= ypos:
+                    # When the space below the cursor is more than
+                    # the space above, just reduce the height.
+                    height = write_position.height - ypos
+                else:
+                    # Otherwise, fit the float above the cursor.
+                    height = min(height, cursor_position.y)
+                    ypos = cursor_position.y - height
+
+        # Only height given -> center vertically.
+        elif fl_height:
+            ypos = int((write_position.height - fl_height) / 2)
+            height = fl_height
+        # Otherwise, take preferred height from content.
+        else:
+            height = fl.content.preferred_height(width, write_position.height).preferred
+
+            if fl.top is not None:
+                ypos = fl.top
+            elif fl.bottom is not None:
+                ypos = max(0, write_position.height - height - fl.bottom)
+            else:  # Center vertically.
+                ypos = max(0, int((write_position.height - height) / 2))
+
+            # Trim.
+            height = min(height, write_position.height - ypos)
+
+        # Write float.
+        # (xpos and ypos can be negative: a float can be partially visible.)
+        if height > 0 and width > 0:
+            wp = WritePosition(
+                xpos=xpos + write_position.xpos,
+                ypos=ypos + write_position.ypos,
+                width=width,
+                height=height,
+            )
+
+            if not fl.hide_when_covering_content or self._area_is_empty(screen, wp):
+                fl.content.write_to_screen(
+                    screen,
+                    mouse_handlers,
+                    wp,
+                    style,
+                    erase_bg=not fl.transparent(),
+                    z_index=z_index,
+                )
+
+    def _area_is_empty(self, screen: Screen, write_position: WritePosition) -> bool:
+        """
+        Return True when the area below the write position is still empty.
+        (For floats that should not hide content underneath.)
+        """
+        wp = write_position
+
+        for y in range(wp.ypos, wp.ypos + wp.height):
+            if y in screen.data_buffer:
+                row = screen.data_buffer[y]
+
+                for x in range(wp.xpos, wp.xpos + wp.width):
+                    c = row[x]
+                    if c.char != " ":
+                        return False
+
+        return True
+
+    def is_modal(self) -> bool:
+        return self.modal
+
+    def get_key_bindings(self) -> Optional[KeyBindingsBase]:
+        return self.key_bindings
+
+    def get_children(self) -> List[Container]:
+        children = [self.content]
+        children.extend(f.content for f in self.floats)
+        return children
+
+
+class Float:
+    """
+    Float for use in a :class:`.FloatContainer`.
+    Except for the `content` parameter, all other options are optional.
+
+    :param content: :class:`.Container` instance.
+
+    :param width: :class:`.Dimension` or callable which returns a :class:`.Dimension`.
+    :param height: :class:`.Dimension` or callable which returns a :class:`.Dimension`.
+
+    :param left: Distance to the left edge of the :class:`.FloatContainer`.
+    :param right: Distance to the right edge of the :class:`.FloatContainer`.
+    :param top: Distance to the top of the :class:`.FloatContainer`.
+    :param bottom: Distance to the bottom of the :class:`.FloatContainer`.
+
+    :param attach_to_window: Attach to the cursor from this window, instead of
+        the current window.
+    :param hide_when_covering_content: Hide the float when it covers content underneath.
+    :param allow_cover_cursor: When `False`, make sure to display the float
+        below the cursor. Not on top of the indicated position.
+    :param z_index: Z-index position. For a Float, this needs to be at least
+        one. It is relative to the z_index of the parent container.
+    :param transparent: :class:`.Filter` indicating whether this float needs to be
+        drawn transparently.
+    """
+
+    def __init__(
+        self,
+        content: AnyContainer,
+        top: Optional[int] = None,
+        right: Optional[int] = None,
+        bottom: Optional[int] = None,
+        left: Optional[int] = None,
+        width: Optional[Union[int, Callable[[], int]]] = None,
+        height: Optional[Union[int, Callable[[], int]]] = None,
+        xcursor: bool = False,
+        ycursor: bool = False,
+        attach_to_window: Optional[AnyContainer] = None,
+        hide_when_covering_content: bool = False,
+        allow_cover_cursor: bool = False,
+        z_index: int = 1,
+        transparent: bool = False,
+    ) -> None:
+
+        assert z_index >= 1
+
+        self.left = left
+        self.right = right
+        self.top = top
+        self.bottom = bottom
+
+        self.width = width
+        self.height = height
+
+        self.xcursor = xcursor
+        self.ycursor = ycursor
+
+        self.attach_to_window = (
+            to_window(attach_to_window) if attach_to_window else None
+        )
+
+        self.content = to_container(content)
+        self.hide_when_covering_content = hide_when_covering_content
+        self.allow_cover_cursor = allow_cover_cursor
+        self.z_index = z_index
+        self.transparent = to_filter(transparent)
+
+    def get_width(self) -> Optional[int]:
+        if callable(self.width):
+            return self.width()
+        return self.width
+
+    def get_height(self) -> Optional[int]:
+        if callable(self.height):
+            return self.height()
+        return self.height
+
+    def __repr__(self) -> str:
+        return "Float(content=%r)" % self.content
+
+
+class WindowRenderInfo:
+    """
+    Render information for the last render time of this control.
+    It stores mapping information between the input buffers (in case of a
+    :class:`~prompt_toolkit.layout.controls.BufferControl`) and the actual
+    render position on the output screen.
+
+    (Could be used for implementation of the Vi 'H' and 'L' key bindings as
+    well as implementing mouse support.)
+
+    :param ui_content: The original :class:`.UIContent` instance that contains
+        the whole input, without clipping. (ui_content)
+    :param horizontal_scroll: The horizontal scroll of the :class:`.Window` instance.
+    :param vertical_scroll: The vertical scroll of the :class:`.Window` instance.
+    :param window_width: The width of the window that displays the content,
+        without the margins.
+    :param window_height: The height of the window that displays the content.
+    :param configured_scroll_offsets: The scroll offsets as configured for the
+        :class:`Window` instance.
+    :param visible_line_to_row_col: Mapping that maps the row numbers on the
+        displayed screen (starting from zero for the first visible line) to
+        (row, col) tuples pointing to the row and column of the :class:`.UIContent`.
+    :param rowcol_to_yx: Mapping that maps (row, column) tuples representing
+        coordinates of the :class:`UIContent` to (y, x) absolute coordinates at
+        the rendered screen.
+    """
+
+    def __init__(
+        self,
+        window: "Window",
+        ui_content: UIContent,
+        horizontal_scroll: int,
+        vertical_scroll: int,
+        window_width: int,
+        window_height: int,
+        configured_scroll_offsets: "ScrollOffsets",
+        visible_line_to_row_col: Dict[int, Tuple[int, int]],
+        rowcol_to_yx: Dict[Tuple[int, int], Tuple[int, int]],
+        x_offset: int,
+        y_offset: int,
+        wrap_lines: bool,
+    ) -> None:
+
+        self.window = window
+        self.ui_content = ui_content
+        self.vertical_scroll = vertical_scroll
+        self.window_width = window_width  # Width without margins.
+        self.window_height = window_height
+
+        self.configured_scroll_offsets = configured_scroll_offsets
+        self.visible_line_to_row_col = visible_line_to_row_col
+        self.wrap_lines = wrap_lines
+
+        self._rowcol_to_yx = rowcol_to_yx  # row/col from input to absolute y/x
+        # screen coordinates.
+        self._x_offset = x_offset
+        self._y_offset = y_offset
+
+    @property
+    def visible_line_to_input_line(self) -> Dict[int, int]:
+        return {
+            visible_line: rowcol[0]
+            for visible_line, rowcol in self.visible_line_to_row_col.items()
+        }
+
+    @property
+    def cursor_position(self) -> Point:
+        """
+        Return the cursor position coordinates, relative to the left/top corner
+        of the rendered screen.
+        """
+        cpos = self.ui_content.cursor_position
+        try:
+            y, x = self._rowcol_to_yx[cpos.y, cpos.x]
+        except KeyError:
+            # For `DummyControl` for instance, the content can be empty, and so
+            # will `_rowcol_to_yx` be. Return 0/0 by default.
+            return Point(x=0, y=0)
+        else:
+            return Point(x=x - self._x_offset, y=y - self._y_offset)
+
+    @property
+    def applied_scroll_offsets(self) -> "ScrollOffsets":
+        """
+        Return a :class:`.ScrollOffsets` instance that indicates the actual
+        offset. This can be less than or equal to what's configured. E.g, when
+        the cursor is completely at the top, the top offset will be zero rather
+        than what's configured.
+        """
+        if self.displayed_lines[0] == 0:
+            top = 0
+        else:
+            # Get row where the cursor is displayed.
+            y = self.input_line_to_visible_line[self.ui_content.cursor_position.y]
+            top = min(y, self.configured_scroll_offsets.top)
+
+        return ScrollOffsets(
+            top=top,
+            bottom=min(
+                self.ui_content.line_count - self.displayed_lines[-1] - 1,
+                self.configured_scroll_offsets.bottom,
+            ),
+            # For left/right, it probably doesn't make sense to return something.
+            # (We would have to calculate the widths of all the lines and keep
+            # double width characters in mind.)
+            left=0,
+            right=0,
+        )
+
+    @property
+    def displayed_lines(self) -> List[int]:
+        """
+        List of all the visible rows. (Line numbers of the input buffer.)
+        The last line may not be entirely visible.
+        """
+        return sorted(row for row, col in self.visible_line_to_row_col.values())
+
+    @property
+    def input_line_to_visible_line(self) -> Dict[int, int]:
+        """
+        Return the dictionary mapping the line numbers of the input buffer to
+        the lines of the screen. When a line spans several rows at the screen,
+        the first row appears in the dictionary.
+        """
+        result: Dict[int, int] = {}
+        for k, v in self.visible_line_to_input_line.items():
+            if v in result:
+                result[v] = min(result[v], k)
+            else:
+                result[v] = k
+        return result
+
+    def first_visible_line(self, after_scroll_offset: bool = False) -> int:
+        """
+        Return the line number (0 based) of the input document that corresponds
+        with the first visible line.
+        """
+        if after_scroll_offset:
+            return self.displayed_lines[self.applied_scroll_offsets.top]
+        else:
+            return self.displayed_lines[0]
+
+    def last_visible_line(self, before_scroll_offset: bool = False) -> int:
+        """
+        Like `first_visible_line`, but for the last visible line.
+        """
+        if before_scroll_offset:
+            return self.displayed_lines[-1 - self.applied_scroll_offsets.bottom]
+        else:
+            return self.displayed_lines[-1]
+
+    def center_visible_line(
+        self, before_scroll_offset: bool = False, after_scroll_offset: bool = False
+    ) -> int:
+        """
+        Like `first_visible_line`, but for the center visible line.
+        """
+        return (
+            self.first_visible_line(after_scroll_offset)
+            + (
+                self.last_visible_line(before_scroll_offset)
+                - self.first_visible_line(after_scroll_offset)
+            )
+            // 2
+        )
+
+    @property
+    def content_height(self) -> int:
+        """
+        The full height of the user control.
+        """
+        return self.ui_content.line_count
+
+    @property
+    def full_height_visible(self) -> bool:
+        """
+        True when the full height is visible (There is no vertical scroll.)
+        """
+        return (
+            self.vertical_scroll == 0
+            and self.last_visible_line() == self.content_height
+        )
+
+    @property
+    def top_visible(self) -> bool:
+        """
+        True when the top of the buffer is visible.
+        """
+        return self.vertical_scroll == 0
+
+    @property
+    def bottom_visible(self) -> bool:
+        """
+        True when the bottom of the buffer is visible.
+        """
+        return self.last_visible_line() == self.content_height - 1
+
+    @property
+    def vertical_scroll_percentage(self) -> int:
+        """
+        Vertical scroll as a percentage. (0 means: the top is visible,
+        100 means: the bottom is visible.)
+        """
+        if self.bottom_visible:
+            return 100
+        else:
+            return 100 * self.vertical_scroll // self.content_height
+
+    def get_height_for_line(self, lineno: int) -> int:
+        """
+        Return the height of the given line.
+        (The height that it would take, if this line became visible.)
+        """
+        if self.wrap_lines:
+            return self.ui_content.get_height_for_line(
+                lineno, self.window_width, self.window.get_line_prefix
+            )
+        else:
+            return 1
+
+
+class ScrollOffsets:
+    """
+    Scroll offsets for the :class:`.Window` class.
+
+    Note that left/right offsets only make sense if line wrapping is disabled.
+    """
+
+    def __init__(
+        self,
+        top: Union[int, Callable[[], int]] = 0,
+        bottom: Union[int, Callable[[], int]] = 0,
+        left: Union[int, Callable[[], int]] = 0,
+        right: Union[int, Callable[[], int]] = 0,
+    ) -> None:
+
+        self._top = top
+        self._bottom = bottom
+        self._left = left
+        self._right = right
+
+    @property
+    def top(self) -> int:
+        return to_int(self._top)
+
+    @property
+    def bottom(self) -> int:
+        return to_int(self._bottom)
+
+    @property
+    def left(self) -> int:
+        return to_int(self._left)
+
+    @property
+    def right(self) -> int:
+        return to_int(self._right)
+
+    def __repr__(self) -> str:
+        return "ScrollOffsets(top={!r}, bottom={!r}, left={!r}, right={!r})".format(
+            self._top,
+            self._bottom,
+            self._left,
+            self._right,
+        )
+
+
+class ColorColumn:
+    """
+    Column for a :class:`.Window` to be colored.
+    """
+
+    def __init__(self, position: int, style: str = "class:color-column") -> None:
+        self.position = position
+        self.style = style
+
+
+_in_insert_mode = vi_insert_mode | emacs_insert_mode
+
+
+class WindowAlign(Enum):
+    """
+    Alignment of the Window content.
+
+    Note that this is different from `HorizontalAlign` and `VerticalAlign`,
+    which are used for the alignment of the child containers in respectively
+    `VSplit` and `HSplit`.
+    """
+
+    LEFT = "LEFT"
+    RIGHT = "RIGHT"
+    CENTER = "CENTER"
+
+
+class Window(Container):
+    """
+    Container that holds a control.
+
+    :param content: :class:`.UIControl` instance.
+    :param width: :class:`.Dimension` instance or callable.
+    :param height: :class:`.Dimension` instance or callable.
+    :param z_index: When specified, this can be used to bring element in front
+        of floating elements.
+    :param dont_extend_width: When `True`, don't take up more width then the
+                              preferred width reported by the control.
+    :param dont_extend_height: When `True`, don't take up more width then the
+                               preferred height reported by the control.
+    :param ignore_content_width: A `bool` or :class:`.Filter` instance. Ignore
+        the :class:`.UIContent` width when calculating the dimensions.
+    :param ignore_content_height: A `bool` or :class:`.Filter` instance. Ignore
+        the :class:`.UIContent` height when calculating the dimensions.
+    :param left_margins: A list of :class:`.Margin` instance to be displayed on
+        the left. For instance: :class:`~prompt_toolkit.layout.NumberedMargin`
+        can be one of them in order to show line numbers.
+    :param right_margins: Like `left_margins`, but on the other side.
+    :param scroll_offsets: :class:`.ScrollOffsets` instance, representing the
+        preferred amount of lines/columns to be always visible before/after the
+        cursor. When both top and bottom are a very high number, the cursor
+        will be centered vertically most of the time.
+    :param allow_scroll_beyond_bottom: A `bool` or
+        :class:`.Filter` instance. When True, allow scrolling so far, that the
+        top part of the content is not visible anymore, while there is still
+        empty space available at the bottom of the window. In the Vi editor for
+        instance, this is possible. You will see tildes while the top part of
+        the body is hidden.
+    :param wrap_lines: A `bool` or :class:`.Filter` instance. When True, don't
+        scroll horizontally, but wrap lines instead.
+    :param get_vertical_scroll: Callable that takes this window
+        instance as input and returns a preferred vertical scroll.
+        (When this is `None`, the scroll is only determined by the last and
+        current cursor position.)
+    :param get_horizontal_scroll: Callable that takes this window
+        instance as input and returns a preferred vertical scroll.
+    :param always_hide_cursor: A `bool` or
+        :class:`.Filter` instance. When True, never display the cursor, even
+        when the user control specifies a cursor position.
+    :param cursorline: A `bool` or :class:`.Filter` instance. When True,
+        display a cursorline.
+    :param cursorcolumn: A `bool` or :class:`.Filter` instance. When True,
+        display a cursorcolumn.
+    :param colorcolumns: A list of :class:`.ColorColumn` instances that
+        describe the columns to be highlighted, or a callable that returns such
+        a list.
+    :param align: :class:`.WindowAlign` value or callable that returns an
+        :class:`.WindowAlign` value. alignment of content.
+    :param style: A style string. Style to be applied to all the cells in this
+        window. (This can be a callable that returns a string.)
+    :param char: (string) Character to be used for filling the background. This
+        can also be a callable that returns a character.
+    :param get_line_prefix: None or a callable that returns formatted text to
+        be inserted before a line. It takes a line number (int) and a
+        wrap_count and returns formatted text. This can be used for
+        implementation of line continuations, things like Vim "breakindent" and
+        so on.
+    """
+
+    def __init__(
+        self,
+        content: Optional[UIControl] = None,
+        width: AnyDimension = None,
+        height: AnyDimension = None,
+        z_index: Optional[int] = None,
+        dont_extend_width: FilterOrBool = False,
+        dont_extend_height: FilterOrBool = False,
+        ignore_content_width: FilterOrBool = False,
+        ignore_content_height: FilterOrBool = False,
+        left_margins: Optional[Sequence[Margin]] = None,
+        right_margins: Optional[Sequence[Margin]] = None,
+        scroll_offsets: Optional[ScrollOffsets] = None,
+        allow_scroll_beyond_bottom: FilterOrBool = False,
+        wrap_lines: FilterOrBool = False,
+        get_vertical_scroll: Optional[Callable[["Window"], int]] = None,
+        get_horizontal_scroll: Optional[Callable[["Window"], int]] = None,
+        always_hide_cursor: FilterOrBool = False,
+        cursorline: FilterOrBool = False,
+        cursorcolumn: FilterOrBool = False,
+        colorcolumns: Union[
+            None, List[ColorColumn], Callable[[], List[ColorColumn]]
+        ] = None,
+        align: Union[WindowAlign, Callable[[], WindowAlign]] = WindowAlign.LEFT,
+        style: Union[str, Callable[[], str]] = "",
+        char: Union[None, str, Callable[[], str]] = None,
+        get_line_prefix: Optional[GetLinePrefixCallable] = None,
+    ) -> None:
+
+        self.allow_scroll_beyond_bottom = to_filter(allow_scroll_beyond_bottom)
+        self.always_hide_cursor = to_filter(always_hide_cursor)
+        self.wrap_lines = to_filter(wrap_lines)
+        self.cursorline = to_filter(cursorline)
+        self.cursorcolumn = to_filter(cursorcolumn)
+
+        self.content = content or DummyControl()
+        self.dont_extend_width = to_filter(dont_extend_width)
+        self.dont_extend_height = to_filter(dont_extend_height)
+        self.ignore_content_width = to_filter(ignore_content_width)
+        self.ignore_content_height = to_filter(ignore_content_height)
+        self.left_margins = left_margins or []
+        self.right_margins = right_margins or []
+        self.scroll_offsets = scroll_offsets or ScrollOffsets()
+        self.get_vertical_scroll = get_vertical_scroll
+        self.get_horizontal_scroll = get_horizontal_scroll
+        self.colorcolumns = colorcolumns or []
+        self.align = align
+        self.style = style
+        self.char = char
+        self.get_line_prefix = get_line_prefix
+
+        self.width = width
+        self.height = height
+        self.z_index = z_index
+
+        # Cache for the screens generated by the margin.
+        self._ui_content_cache: SimpleCache[
+            Tuple[int, int, int], UIContent
+        ] = SimpleCache(maxsize=8)
+        self._margin_width_cache: SimpleCache[Tuple[Margin, int], int] = SimpleCache(
+            maxsize=1
+        )
+
+        self.reset()
+
+    def __repr__(self) -> str:
+        return "Window(content=%r)" % self.content
+
+    def reset(self) -> None:
+        self.content.reset()
+
+        #: Scrolling position of the main content.
+        self.vertical_scroll = 0
+        self.horizontal_scroll = 0
+
+        # Vertical scroll 2: this is the vertical offset that a line is
+        # scrolled if a single line (the one that contains the cursor) consumes
+        # all of the vertical space.
+        self.vertical_scroll_2 = 0
+
+        #: Keep render information (mappings between buffer input and render
+        #: output.)
+        self.render_info: Optional[WindowRenderInfo] = None
+
+    def _get_margin_width(self, margin: Margin) -> int:
+        """
+        Return the width for this margin.
+        (Calculate only once per render time.)
+        """
+        # Margin.get_width, needs to have a UIContent instance.
+        def get_ui_content() -> UIContent:
+            return self._get_ui_content(width=0, height=0)
+
+        def get_width() -> int:
+            return margin.get_width(get_ui_content)
+
+        key = (margin, get_app().render_counter)
+        return self._margin_width_cache.get(key, get_width)
+
+    def _get_total_margin_width(self) -> int:
+        """
+        Calculate and return the width of the margin (left + right).
+        """
+        return sum(self._get_margin_width(m) for m in self.left_margins) + sum(
+            self._get_margin_width(m) for m in self.right_margins
+        )
+
+    def preferred_width(self, max_available_width: int) -> Dimension:
+        """
+        Calculate the preferred width for this window.
+        """
+
+        def preferred_content_width() -> Optional[int]:
+            """Content width: is only calculated if no exact width for the
+            window was given."""
+            if self.ignore_content_width():
+                return None
+
+            # Calculate the width of the margin.
+            total_margin_width = self._get_total_margin_width()
+
+            # Window of the content. (Can be `None`.)
+            preferred_width = self.content.preferred_width(
+                max_available_width - total_margin_width
+            )
+
+            if preferred_width is not None:
+                # Include width of the margins.
+                preferred_width += total_margin_width
+            return preferred_width
+
+        # Merge.
+        return self._merge_dimensions(
+            dimension=to_dimension(self.width),
+            get_preferred=preferred_content_width,
+            dont_extend=self.dont_extend_width(),
+        )
+
+    def preferred_height(self, width: int, max_available_height: int) -> Dimension:
+        """
+        Calculate the preferred height for this window.
+        """
+
+        def preferred_content_height() -> Optional[int]:
+            """Content height: is only calculated if no exact height for the
+            window was given."""
+            if self.ignore_content_height():
+                return None
+
+            total_margin_width = self._get_total_margin_width()
+            wrap_lines = self.wrap_lines()
+
+            return self.content.preferred_height(
+                width - total_margin_width,
+                max_available_height,
+                wrap_lines,
+                self.get_line_prefix,
+            )
+
+        return self._merge_dimensions(
+            dimension=to_dimension(self.height),
+            get_preferred=preferred_content_height,
+            dont_extend=self.dont_extend_height(),
+        )
+
+    @staticmethod
+    def _merge_dimensions(
+        dimension: Optional[Dimension],
+        get_preferred: Callable[[], Optional[int]],
+        dont_extend: bool = False,
+    ) -> Dimension:
+        """
+        Take the Dimension from this `Window` class and the received preferred
+        size from the `UIControl` and return a `Dimension` to report to the
+        parent container.
+        """
+        dimension = dimension or Dimension()
+
+        # When a preferred dimension was explicitly given to the Window,
+        # ignore the UIControl.
+        preferred: Optional[int]
+
+        if dimension.preferred_specified:
+            preferred = dimension.preferred
+        else:
+            # Otherwise, calculate the preferred dimension from the UI control
+            # content.
+            preferred = get_preferred()
+
+        # When a 'preferred' dimension is given by the UIControl, make sure
+        # that it stays within the bounds of the Window.
+        if preferred is not None:
+            if dimension.max_specified:
+                preferred = min(preferred, dimension.max)
+
+            if dimension.min_specified:
+                preferred = max(preferred, dimension.min)
+
+        # When a `dont_extend` flag has been given, use the preferred dimension
+        # also as the max dimension.
+        max_: Optional[int]
+        min_: Optional[int]
+
+        if dont_extend and preferred is not None:
+            max_ = min(dimension.max, preferred)
+        else:
+            max_ = dimension.max if dimension.max_specified else None
+
+        min_ = dimension.min if dimension.min_specified else None
+
+        return Dimension(
+            min=min_, max=max_, preferred=preferred, weight=dimension.weight
+        )
+
+    def _get_ui_content(self, width: int, height: int) -> UIContent:
+        """
+        Create a `UIContent` instance.
+        """
+
+        def get_content() -> UIContent:
+            return self.content.create_content(width=width, height=height)
+
+        key = (get_app().render_counter, width, height)
+        return self._ui_content_cache.get(key, get_content)
+
+    def _get_digraph_char(self) -> Optional[str]:
+        "Return `False`, or the Digraph symbol to be used."
+        app = get_app()
+        if app.quoted_insert:
+            return "^"
+        if app.vi_state.waiting_for_digraph:
+            if app.vi_state.digraph_symbol1:
+                return app.vi_state.digraph_symbol1
+            return "?"
+        return None
+
+    def write_to_screen(
+        self,
+        screen: Screen,
+        mouse_handlers: MouseHandlers,
+        write_position: WritePosition,
+        parent_style: str,
+        erase_bg: bool,
+        z_index: Optional[int],
+    ) -> None:
+        """
+        Write window to screen. This renders the user control, the margins and
+        copies everything over to the absolute position at the given screen.
+        """
+        # If dont_extend_width/height was given. Then reduce width/height in
+        # WritePosition if the parent wanted us to paint in a bigger area.
+        # (This happens if this window is bundled with another window in a
+        # HSplit/VSplit, but with different size requirements.)
+        write_position = WritePosition(
+            xpos=write_position.xpos,
+            ypos=write_position.ypos,
+            width=write_position.width,
+            height=write_position.height,
+        )
+
+        if self.dont_extend_width():
+            write_position.width = min(
+                write_position.width,
+                self.preferred_width(write_position.width).preferred,
+            )
+
+        if self.dont_extend_height():
+            write_position.height = min(
+                write_position.height,
+                self.preferred_height(
+                    write_position.width, write_position.height
+                ).preferred,
+            )
+
+        # Draw
+        z_index = z_index if self.z_index is None else self.z_index
+
+        draw_func = partial(
+            self._write_to_screen_at_index,
+            screen,
+            mouse_handlers,
+            write_position,
+            parent_style,
+            erase_bg,
+        )
+
+        if z_index is None or z_index <= 0:
+            # When no z_index is given, draw right away.
+            draw_func()
+        else:
+            # Otherwise, postpone.
+            screen.draw_with_z_index(z_index=z_index, draw_func=draw_func)
+
+    def _write_to_screen_at_index(
+        self,
+        screen: Screen,
+        mouse_handlers: MouseHandlers,
+        write_position: WritePosition,
+        parent_style: str,
+        erase_bg: bool,
+    ) -> None:
+        # Don't bother writing invisible windows.
+        # (We save some time, but also avoid applying last-line styling.)
+        if write_position.height <= 0 or write_position.width <= 0:
+            return
+
+        # Calculate margin sizes.
+        left_margin_widths = [self._get_margin_width(m) for m in self.left_margins]
+        right_margin_widths = [self._get_margin_width(m) for m in self.right_margins]
+        total_margin_width = sum(left_margin_widths + right_margin_widths)
+
+        # Render UserControl.
+        ui_content = self.content.create_content(
+            write_position.width - total_margin_width, write_position.height
+        )
+        assert isinstance(ui_content, UIContent)
+
+        # Scroll content.
+        wrap_lines = self.wrap_lines()
+        self._scroll(
+            ui_content, write_position.width - total_margin_width, write_position.height
+        )
+
+        # Erase background and fill with `char`.
+        self._fill_bg(screen, write_position, erase_bg)
+
+        # Resolve `align` attribute.
+        align = self.align() if callable(self.align) else self.align
+
+        # Write body
+        visible_line_to_row_col, rowcol_to_yx = self._copy_body(
+            ui_content,
+            screen,
+            write_position,
+            sum(left_margin_widths),
+            write_position.width - total_margin_width,
+            self.vertical_scroll,
+            self.horizontal_scroll,
+            wrap_lines=wrap_lines,
+            highlight_lines=True,
+            vertical_scroll_2=self.vertical_scroll_2,
+            always_hide_cursor=self.always_hide_cursor(),
+            has_focus=get_app().layout.current_control == self.content,
+            align=align,
+            get_line_prefix=self.get_line_prefix,
+        )
+
+        # Remember render info. (Set before generating the margins. They need this.)
+        x_offset = write_position.xpos + sum(left_margin_widths)
+        y_offset = write_position.ypos
+
+        render_info = WindowRenderInfo(
+            window=self,
+            ui_content=ui_content,
+            horizontal_scroll=self.horizontal_scroll,
+            vertical_scroll=self.vertical_scroll,
+            window_width=write_position.width - total_margin_width,
+            window_height=write_position.height,
+            configured_scroll_offsets=self.scroll_offsets,
+            visible_line_to_row_col=visible_line_to_row_col,
+            rowcol_to_yx=rowcol_to_yx,
+            x_offset=x_offset,
+            y_offset=y_offset,
+            wrap_lines=wrap_lines,
+        )
+        self.render_info = render_info
+
+        # Set mouse handlers.
+        def mouse_handler(mouse_event: MouseEvent) -> "NotImplementedOrNone":
+            """
+            Wrapper around the mouse_handler of the `UIControl` that turns
+            screen coordinates into line coordinates.
+            Returns `NotImplemented` if no UI invalidation should be done.
+            """
+            # Don't handle mouse events outside of the current modal part of
+            # the UI.
+            if self not in get_app().layout.walk_through_modal_area():
+                return NotImplemented
+
+            # Find row/col position first.
+            yx_to_rowcol = {v: k for k, v in rowcol_to_yx.items()}
+            y = mouse_event.position.y
+            x = mouse_event.position.x
+
+            # If clicked below the content area, look for a position in the
+            # last line instead.
+            max_y = write_position.ypos + len(visible_line_to_row_col) - 1
+            y = min(max_y, y)
+            result: NotImplementedOrNone
+
+            while x >= 0:
+                try:
+                    row, col = yx_to_rowcol[y, x]
+                except KeyError:
+                    # Try again. (When clicking on the right side of double
+                    # width characters, or on the right side of the input.)
+                    x -= 1
+                else:
+                    # Found position, call handler of UIControl.
+                    result = self.content.mouse_handler(
+                        MouseEvent(
+                            position=Point(x=col, y=row),
+                            event_type=mouse_event.event_type,
+                            button=mouse_event.button,
+                            modifiers=mouse_event.modifiers,
+                        )
+                    )
+                    break
+            else:
+                # nobreak.
+                # (No x/y coordinate found for the content. This happens in
+                # case of a DummyControl, that does not have any content.
+                # Report (0,0) instead.)
+                result = self.content.mouse_handler(
+                    MouseEvent(
+                        position=Point(x=0, y=0),
+                        event_type=mouse_event.event_type,
+                        button=mouse_event.button,
+                        modifiers=mouse_event.modifiers,
+                    )
+                )
+
+            # If it returns NotImplemented, handle it here.
+            if result == NotImplemented:
+                result = self._mouse_handler(mouse_event)
+
+            return result
+
+        mouse_handlers.set_mouse_handler_for_range(
+            x_min=write_position.xpos + sum(left_margin_widths),
+            x_max=write_position.xpos + write_position.width - total_margin_width,
+            y_min=write_position.ypos,
+            y_max=write_position.ypos + write_position.height,
+            handler=mouse_handler,
+        )
+
+        # Render and copy margins.
+        move_x = 0
+
+        def render_margin(m: Margin, width: int) -> UIContent:
+            "Render margin. Return `Screen`."
+            # Retrieve margin fragments.
+            fragments = m.create_margin(render_info, width, write_position.height)
+
+            # Turn it into a UIContent object.
+            # already rendered those fragments using this size.)
+            return FormattedTextControl(fragments).create_content(
+                width + 1, write_position.height
+            )
+
+        for m, width in zip(self.left_margins, left_margin_widths):
+            if width > 0:  # (ConditionalMargin returns a zero width. -- Don't render.)
+                # Create screen for margin.
+                margin_content = render_margin(m, width)
+
+                # Copy and shift X.
+                self._copy_margin(margin_content, screen, write_position, move_x, width)
+                move_x += width
+
+        move_x = write_position.width - sum(right_margin_widths)
+
+        for m, width in zip(self.right_margins, right_margin_widths):
+            # Create screen for margin.
+            margin_content = render_margin(m, width)
+
+            # Copy and shift X.
+            self._copy_margin(margin_content, screen, write_position, move_x, width)
+            move_x += width
+
+        # Apply 'self.style'
+        self._apply_style(screen, write_position, parent_style)
+
+        # Tell the screen that this user control has been painted at this
+        # position.
+        screen.visible_windows_to_write_positions[self] = write_position
+
+    def _copy_body(
+        self,
+        ui_content: UIContent,
+        new_screen: Screen,
+        write_position: WritePosition,
+        move_x: int,
+        width: int,
+        vertical_scroll: int = 0,
+        horizontal_scroll: int = 0,
+        wrap_lines: bool = False,
+        highlight_lines: bool = False,
+        vertical_scroll_2: int = 0,
+        always_hide_cursor: bool = False,
+        has_focus: bool = False,
+        align: WindowAlign = WindowAlign.LEFT,
+        get_line_prefix: Optional[Callable[[int, int], AnyFormattedText]] = None,
+    ) -> Tuple[Dict[int, Tuple[int, int]], Dict[Tuple[int, int], Tuple[int, int]]]:
+        """
+        Copy the UIContent into the output screen.
+        Return (visible_line_to_row_col, rowcol_to_yx) tuple.
+
+        :param get_line_prefix: None or a callable that takes a line number
+            (int) and a wrap_count (int) and returns formatted text.
+        """
+        xpos = write_position.xpos + move_x
+        ypos = write_position.ypos
+        line_count = ui_content.line_count
+        new_buffer = new_screen.data_buffer
+        empty_char = _CHAR_CACHE["", ""]
+
+        # Map visible line number to (row, col) of input.
+        # 'col' will always be zero if line wrapping is off.
+        visible_line_to_row_col: Dict[int, Tuple[int, int]] = {}
+
+        # Maps (row, col) from the input to (y, x) screen coordinates.
+        rowcol_to_yx: Dict[Tuple[int, int], Tuple[int, int]] = {}
+
+        def copy_line(
+            line: StyleAndTextTuples,
+            lineno: int,
+            x: int,
+            y: int,
+            is_input: bool = False,
+        ) -> Tuple[int, int]:
+            """
+            Copy over a single line to the output screen. This can wrap over
+            multiple lines in the output. It will call the prefix (prompt)
+            function before every line.
+            """
+            if is_input:
+                current_rowcol_to_yx = rowcol_to_yx
+            else:
+                current_rowcol_to_yx = {}  # Throwaway dictionary.
+
+            # Draw line prefix.
+            if is_input and get_line_prefix:
+                prompt = to_formatted_text(get_line_prefix(lineno, 0))
+                x, y = copy_line(prompt, lineno, x, y, is_input=False)
+
+            # Scroll horizontally.
+            skipped = 0  # Characters skipped because of horizontal scrolling.
+            if horizontal_scroll and is_input:
+                h_scroll = horizontal_scroll
+                line = explode_text_fragments(line)
+                while h_scroll > 0 and line:
+                    h_scroll -= get_cwidth(line[0][1])
+                    skipped += 1
+                    del line[:1]  # Remove first character.
+
+                x -= h_scroll  # When scrolling over double width character,
+                # this can end up being negative.
+
+            # Align this line. (Note that this doesn't work well when we use
+            # get_line_prefix and that function returns variable width prefixes.)
+            if align == WindowAlign.CENTER:
+                line_width = fragment_list_width(line)
+                if line_width < width:
+                    x += (width - line_width) // 2
+            elif align == WindowAlign.RIGHT:
+                line_width = fragment_list_width(line)
+                if line_width < width:
+                    x += width - line_width
+
+            col = 0
+            wrap_count = 0
+            for style, text, *_ in line:
+                new_buffer_row = new_buffer[y + ypos]
+
+                # Remember raw VT escape sequences. (E.g. FinalTerm's
+                # escape sequences.)
+                if "[ZeroWidthEscape]" in style:
+                    new_screen.zero_width_escapes[y + ypos][x + xpos] += text
+                    continue
+
+                for c in text:
+                    char = _CHAR_CACHE[c, style]
+                    char_width = char.width
+
+                    # Wrap when the line width is exceeded.
+                    if wrap_lines and x + char_width > width:
+                        visible_line_to_row_col[y + 1] = (
+                            lineno,
+                            visible_line_to_row_col[y][1] + x,
+                        )
+                        y += 1
+                        wrap_count += 1
+                        x = 0
+
+                        # Insert line prefix (continuation prompt).
+                        if is_input and get_line_prefix:
+                            prompt = to_formatted_text(
+                                get_line_prefix(lineno, wrap_count)
+                            )
+                            x, y = copy_line(prompt, lineno, x, y, is_input=False)
+
+                        new_buffer_row = new_buffer[y + ypos]
+
+                        if y >= write_position.height:
+                            return x, y  # Break out of all for loops.
+
+                    # Set character in screen and shift 'x'.
+                    if x >= 0 and y >= 0 and x < width:
+                        new_buffer_row[x + xpos] = char
+
+                        # When we print a multi width character, make sure
+                        # to erase the neighbours positions in the screen.
+                        # (The empty string if different from everything,
+                        # so next redraw this cell will repaint anyway.)
+                        if char_width > 1:
+                            for i in range(1, char_width):
+                                new_buffer_row[x + xpos + i] = empty_char
+
+                        # If this is a zero width characters, then it's
+                        # probably part of a decomposed unicode character.
+                        # See: https://en.wikipedia.org/wiki/Unicode_equivalence
+                        # Merge it in the previous cell.
+                        elif char_width == 0:
+                            # Handle all character widths. If the previous
+                            # character is a multiwidth character, then
+                            # merge it two positions back.
+                            for pw in [2, 1]:  # Previous character width.
+                                if (
+                                    x - pw >= 0
+                                    and new_buffer_row[x + xpos - pw].width == pw
+                                ):
+                                    prev_char = new_buffer_row[x + xpos - pw]
+                                    char2 = _CHAR_CACHE[
+                                        prev_char.char + c, prev_char.style
+                                    ]
+                                    new_buffer_row[x + xpos - pw] = char2
+
+                        # Keep track of write position for each character.
+                        current_rowcol_to_yx[lineno, col + skipped] = (
+                            y + ypos,
+                            x + xpos,
+                        )
+
+                    col += 1
+                    x += char_width
+            return x, y
+
+        # Copy content.
+        def copy() -> int:
+            y = -vertical_scroll_2
+            lineno = vertical_scroll
+
+            while y < write_position.height and lineno < line_count:
+                # Take the next line and copy it in the real screen.
+                line = ui_content.get_line(lineno)
+
+                visible_line_to_row_col[y] = (lineno, horizontal_scroll)
+
+                # Copy margin and actual line.
+                x = 0
+                x, y = copy_line(line, lineno, x, y, is_input=True)
+
+                lineno += 1
+                y += 1
+            return y
+
+        copy()
+
+        def cursor_pos_to_screen_pos(row: int, col: int) -> Point:
+            "Translate row/col from UIContent to real Screen coordinates."
+            try:
+                y, x = rowcol_to_yx[row, col]
+            except KeyError:
+                # Normally this should never happen. (It is a bug, if it happens.)
+                # But to be sure, return (0, 0)
+                return Point(x=0, y=0)
+
+                # raise ValueError(
+                #     'Invalid position. row=%r col=%r, vertical_scroll=%r, '
+                #     'horizontal_scroll=%r, height=%r' %
+                #     (row, col, vertical_scroll, horizontal_scroll, write_position.height))
+            else:
+                return Point(x=x, y=y)
+
+        # Set cursor and menu positions.
+        if ui_content.cursor_position:
+            screen_cursor_position = cursor_pos_to_screen_pos(
+                ui_content.cursor_position.y, ui_content.cursor_position.x
+            )
+
+            if has_focus:
+                new_screen.set_cursor_position(self, screen_cursor_position)
+
+                if always_hide_cursor:
+                    new_screen.show_cursor = False
+                else:
+                    new_screen.show_cursor = ui_content.show_cursor
+
+                self._highlight_digraph(new_screen)
+
+            if highlight_lines:
+                self._highlight_cursorlines(
+                    new_screen,
+                    screen_cursor_position,
+                    xpos,
+                    ypos,
+                    width,
+                    write_position.height,
+                )
+
+        # Draw input characters from the input processor queue.
+        if has_focus and ui_content.cursor_position:
+            self._show_key_processor_key_buffer(new_screen)
+
+        # Set menu position.
+        if ui_content.menu_position:
+            new_screen.set_menu_position(
+                self,
+                cursor_pos_to_screen_pos(
+                    ui_content.menu_position.y, ui_content.menu_position.x
+                ),
+            )
+
+        # Update output screen height.
+        new_screen.height = max(new_screen.height, ypos + write_position.height)
+
+        return visible_line_to_row_col, rowcol_to_yx
+
+    def _fill_bg(
+        self, screen: Screen, write_position: WritePosition, erase_bg: bool
+    ) -> None:
+        """
+        Erase/fill the background.
+        (Useful for floats and when a `char` has been given.)
+        """
+        char: Optional[str]
+        if callable(self.char):
+            char = self.char()
+        else:
+            char = self.char
+
+        if erase_bg or char:
+            wp = write_position
+            char_obj = _CHAR_CACHE[char or " ", ""]
+
+            for y in range(wp.ypos, wp.ypos + wp.height):
+                row = screen.data_buffer[y]
+                for x in range(wp.xpos, wp.xpos + wp.width):
+                    row[x] = char_obj
+
+    def _apply_style(
+        self, new_screen: Screen, write_position: WritePosition, parent_style: str
+    ) -> None:
+
+        # Apply `self.style`.
+        style = parent_style + " " + to_str(self.style)
+
+        new_screen.fill_area(write_position, style=style, after=False)
+
+        # Apply the 'last-line' class to the last line of each Window. This can
+        # be used to apply an 'underline' to the user control.
+        wp = WritePosition(
+            write_position.xpos,
+            write_position.ypos + write_position.height - 1,
+            write_position.width,
+            1,
+        )
+        new_screen.fill_area(wp, "class:last-line", after=True)
+
+    def _highlight_digraph(self, new_screen: Screen) -> None:
+        """
+        When we are in Vi digraph mode, put a question mark underneath the
+        cursor.
+        """
+        digraph_char = self._get_digraph_char()
+        if digraph_char:
+            cpos = new_screen.get_cursor_position(self)
+            new_screen.data_buffer[cpos.y][cpos.x] = _CHAR_CACHE[
+                digraph_char, "class:digraph"
+            ]
+
+    def _show_key_processor_key_buffer(self, new_screen: Screen) -> None:
+        """
+        When the user is typing a key binding that consists of several keys,
+        display the last pressed key if the user is in insert mode and the key
+        is meaningful to be displayed.
+        E.g. Some people want to bind 'jj' to escape in Vi insert mode. But the
+             first 'j' needs to be displayed in order to get some feedback.
+        """
+        app = get_app()
+        key_buffer = app.key_processor.key_buffer
+
+        if key_buffer and _in_insert_mode() and not app.is_done:
+            # The textual data for the given key. (Can be a VT100 escape
+            # sequence.)
+            data = key_buffer[-1].data
+
+            # Display only if this is a 1 cell width character.
+            if get_cwidth(data) == 1:
+                cpos = new_screen.get_cursor_position(self)
+                new_screen.data_buffer[cpos.y][cpos.x] = _CHAR_CACHE[
+                    data, "class:partial-key-binding"
+                ]
+
+    def _highlight_cursorlines(
+        self, new_screen: Screen, cpos: Point, x: int, y: int, width: int, height: int
+    ) -> None:
+        """
+        Highlight cursor row/column.
+        """
+        cursor_line_style = " class:cursor-line "
+        cursor_column_style = " class:cursor-column "
+
+        data_buffer = new_screen.data_buffer
+
+        # Highlight cursor line.
+        if self.cursorline():
+            row = data_buffer[cpos.y]
+            for x in range(x, x + width):
+                original_char = row[x]
+                row[x] = _CHAR_CACHE[
+                    original_char.char, original_char.style + cursor_line_style
+                ]
+
+        # Highlight cursor column.
+        if self.cursorcolumn():
+            for y2 in range(y, y + height):
+                row = data_buffer[y2]
+                original_char = row[cpos.x]
+                row[cpos.x] = _CHAR_CACHE[
+                    original_char.char, original_char.style + cursor_column_style
+                ]
+
+        # Highlight color columns
+        colorcolumns = self.colorcolumns
+        if callable(colorcolumns):
+            colorcolumns = colorcolumns()
+
+        for cc in colorcolumns:
+            assert isinstance(cc, ColorColumn)
+            column = cc.position
+
+            if column < x + width:  # Only draw when visible.
+                color_column_style = " " + cc.style
+
+                for y2 in range(y, y + height):
+                    row = data_buffer[y2]
+                    original_char = row[column + x]
+                    row[column + x] = _CHAR_CACHE[
+                        original_char.char, original_char.style + color_column_style
+                    ]
+
+    def _copy_margin(
+        self,
+        margin_content: UIContent,
+        new_screen: Screen,
+        write_position: WritePosition,
+        move_x: int,
+        width: int,
+    ) -> None:
+        """
+        Copy characters from the margin screen to the real screen.
+        """
+        xpos = write_position.xpos + move_x
+        ypos = write_position.ypos
+
+        margin_write_position = WritePosition(xpos, ypos, width, write_position.height)
+        self._copy_body(margin_content, new_screen, margin_write_position, 0, width)
+
+    def _scroll(self, ui_content: UIContent, width: int, height: int) -> None:
+        """
+        Scroll body. Ensure that the cursor is visible.
+        """
+        if self.wrap_lines():
+            func = self._scroll_when_linewrapping
+        else:
+            func = self._scroll_without_linewrapping
+
+        func(ui_content, width, height)
+
+    def _scroll_when_linewrapping(
+        self, ui_content: UIContent, width: int, height: int
+    ) -> None:
+        """
+        Scroll to make sure the cursor position is visible and that we maintain
+        the requested scroll offset.
+
+        Set `self.horizontal_scroll/vertical_scroll`.
+        """
+        scroll_offsets_bottom = self.scroll_offsets.bottom
+        scroll_offsets_top = self.scroll_offsets.top
+
+        # We don't have horizontal scrolling.
+        self.horizontal_scroll = 0
+
+        def get_line_height(lineno: int) -> int:
+            return ui_content.get_height_for_line(lineno, width, self.get_line_prefix)
+
+        # When there is no space, reset `vertical_scroll_2` to zero and abort.
+        # This can happen if the margin is bigger than the window width.
+        # Otherwise the text height will become "infinite" (a big number) and
+        # the copy_line will spend a huge amount of iterations trying to render
+        # nothing.
+        if width <= 0:
+            self.vertical_scroll = ui_content.cursor_position.y
+            self.vertical_scroll_2 = 0
+            return
+
+        # If the current line consumes more than the whole window height,
+        # then we have to scroll vertically inside this line. (We don't take
+        # the scroll offsets into account for this.)
+        # Also, ignore the scroll offsets in this case. Just set the vertical
+        # scroll to this line.
+        line_height = get_line_height(ui_content.cursor_position.y)
+        if line_height > height - scroll_offsets_top:
+            # Calculate the height of the text before the cursor (including
+            # line prefixes).
+            text_before_height = ui_content.get_height_for_line(
+                ui_content.cursor_position.y,
+                width,
+                self.get_line_prefix,
+                slice_stop=ui_content.cursor_position.x,
+            )
+
+            # Adjust scroll offset.
+            self.vertical_scroll = ui_content.cursor_position.y
+            self.vertical_scroll_2 = min(
+                text_before_height - 1,  # Keep the cursor visible.
+                line_height
+                - height,  # Avoid blank lines at the bottom when scolling up again.
+                self.vertical_scroll_2,
+            )
+            self.vertical_scroll_2 = max(
+                0, text_before_height - height, self.vertical_scroll_2
+            )
+            return
+        else:
+            self.vertical_scroll_2 = 0
+
+        # Current line doesn't consume the whole height. Take scroll offsets into account.
+        def get_min_vertical_scroll() -> int:
+            # Make sure that the cursor line is not below the bottom.
+            # (Calculate how many lines can be shown between the cursor and the .)
+            used_height = 0
+            prev_lineno = ui_content.cursor_position.y
+
+            for lineno in range(ui_content.cursor_position.y, -1, -1):
+                used_height += get_line_height(lineno)
+
+                if used_height > height - scroll_offsets_bottom:
+                    return prev_lineno
+                else:
+                    prev_lineno = lineno
+            return 0
+
+        def get_max_vertical_scroll() -> int:
+            # Make sure that the cursor line is not above the top.
+            prev_lineno = ui_content.cursor_position.y
+            used_height = 0
+
+            for lineno in range(ui_content.cursor_position.y - 1, -1, -1):
+                used_height += get_line_height(lineno)
+
+                if used_height > scroll_offsets_top:
+                    return prev_lineno
+                else:
+                    prev_lineno = lineno
+            return prev_lineno
+
+        def get_topmost_visible() -> int:
+            """
+            Calculate the upper most line that can be visible, while the bottom
+            is still visible. We should not allow scroll more than this if
+            `allow_scroll_beyond_bottom` is false.
+            """
+            prev_lineno = ui_content.line_count - 1
+            used_height = 0
+            for lineno in range(ui_content.line_count - 1, -1, -1):
+                used_height += get_line_height(lineno)
+                if used_height > height:
+                    return prev_lineno
+                else:
+                    prev_lineno = lineno
+            return prev_lineno
+
+        # Scroll vertically. (Make sure that the whole line which contains the
+        # cursor is visible.
+        topmost_visible = get_topmost_visible()
+
+        # Note: the `min(topmost_visible, ...)` is to make sure that we
+        # don't require scrolling up because of the bottom scroll offset,
+        # when we are at the end of the document.
+        self.vertical_scroll = max(
+            self.vertical_scroll, min(topmost_visible, get_min_vertical_scroll())
+        )
+        self.vertical_scroll = min(self.vertical_scroll, get_max_vertical_scroll())
+
+        # Disallow scrolling beyond bottom?
+        if not self.allow_scroll_beyond_bottom():
+            self.vertical_scroll = min(self.vertical_scroll, topmost_visible)
+
+    def _scroll_without_linewrapping(
+        self, ui_content: UIContent, width: int, height: int
+    ) -> None:
+        """
+        Scroll to make sure the cursor position is visible and that we maintain
+        the requested scroll offset.
+
+        Set `self.horizontal_scroll/vertical_scroll`.
+        """
+        cursor_position = ui_content.cursor_position or Point(x=0, y=0)
+
+        # Without line wrapping, we will never have to scroll vertically inside
+        # a single line.
+        self.vertical_scroll_2 = 0
+
+        if ui_content.line_count == 0:
+            self.vertical_scroll = 0
+            self.horizontal_scroll = 0
+            return
+        else:
+            current_line_text = fragment_list_to_text(
+                ui_content.get_line(cursor_position.y)
+            )
+
+        def do_scroll(
+            current_scroll: int,
+            scroll_offset_start: int,
+            scroll_offset_end: int,
+            cursor_pos: int,
+            window_size: int,
+            content_size: int,
+        ) -> int:
+            "Scrolling algorithm. Used for both horizontal and vertical scrolling."
+            # Calculate the scroll offset to apply.
+            # This can obviously never be more than have the screen size. Also, when the
+            # cursor appears at the top or bottom, we don't apply the offset.
+            scroll_offset_start = int(
+                min(scroll_offset_start, window_size / 2, cursor_pos)
+            )
+            scroll_offset_end = int(
+                min(scroll_offset_end, window_size / 2, content_size - 1 - cursor_pos)
+            )
+
+            # Prevent negative scroll offsets.
+            if current_scroll < 0:
+                current_scroll = 0
+
+            # Scroll back if we scrolled to much and there's still space to show more of the document.
+            if (
+                not self.allow_scroll_beyond_bottom()
+                and current_scroll > content_size - window_size
+            ):
+                current_scroll = max(0, content_size - window_size)
+
+            # Scroll up if cursor is before visible part.
+            if current_scroll > cursor_pos - scroll_offset_start:
+                current_scroll = max(0, cursor_pos - scroll_offset_start)
+
+            # Scroll down if cursor is after visible part.
+            if current_scroll < (cursor_pos + 1) - window_size + scroll_offset_end:
+                current_scroll = (cursor_pos + 1) - window_size + scroll_offset_end
+
+            return current_scroll
+
+        # When a preferred scroll is given, take that first into account.
+        if self.get_vertical_scroll:
+            self.vertical_scroll = self.get_vertical_scroll(self)
+            assert isinstance(self.vertical_scroll, int)
+        if self.get_horizontal_scroll:
+            self.horizontal_scroll = self.get_horizontal_scroll(self)
+            assert isinstance(self.horizontal_scroll, int)
+
+        # Update horizontal/vertical scroll to make sure that the cursor
+        # remains visible.
+        offsets = self.scroll_offsets
+
+        self.vertical_scroll = do_scroll(
+            current_scroll=self.vertical_scroll,
+            scroll_offset_start=offsets.top,
+            scroll_offset_end=offsets.bottom,
+            cursor_pos=ui_content.cursor_position.y,
+            window_size=height,
+            content_size=ui_content.line_count,
+        )
+
+        if self.get_line_prefix:
+            current_line_prefix_width = fragment_list_width(
+                to_formatted_text(self.get_line_prefix(ui_content.cursor_position.y, 0))
+            )
+        else:
+            current_line_prefix_width = 0
+
+        self.horizontal_scroll = do_scroll(
+            current_scroll=self.horizontal_scroll,
+            scroll_offset_start=offsets.left,
+            scroll_offset_end=offsets.right,
+            cursor_pos=get_cwidth(current_line_text[: ui_content.cursor_position.x]),
+            window_size=width - current_line_prefix_width,
+            # We can only analyse the current line. Calculating the width off
+            # all the lines is too expensive.
+            content_size=max(
+                get_cwidth(current_line_text), self.horizontal_scroll + width
+            ),
+        )
+
+    def _mouse_handler(self, mouse_event: MouseEvent) -> "NotImplementedOrNone":
+        """
+        Mouse handler. Called when the UI control doesn't handle this
+        particular event.
+
+        Return `NotImplemented` if nothing was done as a consequence of this
+        key binding (no UI invalidate required in that case).
+        """
+        if mouse_event.event_type == MouseEventType.SCROLL_DOWN:
+            self._scroll_down()
+            return None
+        elif mouse_event.event_type == MouseEventType.SCROLL_UP:
+            self._scroll_up()
+            return None
+
+        return NotImplemented
+
+    def _scroll_down(self) -> None:
+        "Scroll window down."
+        info = self.render_info
+
+        if info is None:
+            return
+
+        if self.vertical_scroll < info.content_height - info.window_height:
+            if info.cursor_position.y <= info.configured_scroll_offsets.top:
+                self.content.move_cursor_down()
+
+            self.vertical_scroll += 1
+
+    def _scroll_up(self) -> None:
+        "Scroll window up."
+        info = self.render_info
+
+        if info is None:
+            return
+
+        if info.vertical_scroll > 0:
+            # TODO: not entirely correct yet in case of line wrapping and long lines.
+            if (
+                info.cursor_position.y
+                >= info.window_height - 1 - info.configured_scroll_offsets.bottom
+            ):
+                self.content.move_cursor_up()
+
+            self.vertical_scroll -= 1
+
+    def get_key_bindings(self) -> Optional[KeyBindingsBase]:
+        return self.content.get_key_bindings()
+
+    def get_children(self) -> List[Container]:
+        return []
+
+
+class ConditionalContainer(Container):
+    """
+    Wrapper around any other container that can change the visibility. The
+    received `filter` determines whether the given container should be
+    displayed or not.
+
+    :param content: :class:`.Container` instance.
+    :param filter: :class:`.Filter` instance.
+    """
+
+    def __init__(self, content: AnyContainer, filter: FilterOrBool) -> None:
+        self.content = to_container(content)
+        self.filter = to_filter(filter)
+
+    def __repr__(self) -> str:
+        return f"ConditionalContainer({self.content!r}, filter={self.filter!r})"
+
+    def reset(self) -> None:
+        self.content.reset()
+
+    def preferred_width(self, max_available_width: int) -> Dimension:
+        if self.filter():
+            return self.content.preferred_width(max_available_width)
+        else:
+            return Dimension.zero()
+
+    def preferred_height(self, width: int, max_available_height: int) -> Dimension:
+        if self.filter():
+            return self.content.preferred_height(width, max_available_height)
+        else:
+            return Dimension.zero()
+
+    def write_to_screen(
+        self,
+        screen: Screen,
+        mouse_handlers: MouseHandlers,
+        write_position: WritePosition,
+        parent_style: str,
+        erase_bg: bool,
+        z_index: Optional[int],
+    ) -> None:
+        if self.filter():
+            return self.content.write_to_screen(
+                screen, mouse_handlers, write_position, parent_style, erase_bg, z_index
+            )
+
+    def get_children(self) -> List[Container]:
+        return [self.content]
+
+
+class DynamicContainer(Container):
+    """
+    Container class that dynamically returns any Container.
+
+    :param get_container: Callable that returns a :class:`.Container` instance
+        or any widget with a ``__pt_container__`` method.
+    """
+
+    def __init__(self, get_container: Callable[[], AnyContainer]) -> None:
+        self.get_container = get_container
+
+    def _get_container(self) -> Container:
+        """
+        Return the current container object.
+
+        We call `to_container`, because `get_container` can also return a
+        widget with a ``__pt_container__`` method.
+        """
+        obj = self.get_container()
+        return to_container(obj)
+
+    def reset(self) -> None:
+        self._get_container().reset()
+
+    def preferred_width(self, max_available_width: int) -> Dimension:
+        return self._get_container().preferred_width(max_available_width)
+
+    def preferred_height(self, width: int, max_available_height: int) -> Dimension:
+        return self._get_container().preferred_height(width, max_available_height)
+
+    def write_to_screen(
+        self,
+        screen: Screen,
+        mouse_handlers: MouseHandlers,
+        write_position: WritePosition,
+        parent_style: str,
+        erase_bg: bool,
+        z_index: Optional[int],
+    ) -> None:
+        self._get_container().write_to_screen(
+            screen, mouse_handlers, write_position, parent_style, erase_bg, z_index
+        )
+
+    def is_modal(self) -> bool:
+        return False
+
+    def get_key_bindings(self) -> Optional[KeyBindingsBase]:
+        # Key bindings will be collected when `layout.walk()` finds the child
+        # container.
+        return None
+
+    def get_children(self) -> List[Container]:
+        # Here we have to return the current active container itself, not its
+        # children. Otherwise, we run into issues where `layout.walk()` will
+        # never see an object of type `Window` if this contains a window. We
+        # can't/shouldn't proxy the "isinstance" check.
+        return [self._get_container()]
+
+
+def to_container(container: AnyContainer) -> Container:
+    """
+    Make sure that the given object is a :class:`.Container`.
+    """
+    if isinstance(container, Container):
+        return container
+    elif hasattr(container, "__pt_container__"):
+        return to_container(container.__pt_container__())
+    else:
+        raise ValueError(f"Not a container object: {container!r}")
+
+
+def to_window(container: AnyContainer) -> Window:
+    """
+    Make sure that the given argument is a :class:`.Window`.
+    """
+    if isinstance(container, Window):
+        return container
+    elif hasattr(container, "__pt_container__"):
+        return to_window(cast("MagicContainer", container).__pt_container__())
+    else:
+        raise ValueError(f"Not a Window object: {container!r}.")
+
+
+def is_container(value: object) -> "TypeGuard[AnyContainer]":
+    """
+    Checks whether the given value is a container object
+    (for use in assert statements).
+    """
+    if isinstance(value, Container):
+        return True
+    if hasattr(value, "__pt_container__"):
+        return is_container(cast("MagicContainer", value).__pt_container__())
+    return False
diff --git a/src/prompt_toolkit/layout/controls.py b/src/prompt_toolkit/layout/controls.py
new file mode 100644
index 0000000..016d289
--- /dev/null
+++ b/src/prompt_toolkit/layout/controls.py
@@ -0,0 +1,957 @@
+"""
+User interface Controls for the layout.
+"""
+import time
+from abc import ABCMeta, abstractmethod
+from typing import (
+    TYPE_CHECKING,
+    Callable,
+    Dict,
+    Hashable,
+    Iterable,
+    List,
+    NamedTuple,
+    Optional,
+    Union,
+)
+
+from prompt_toolkit.application.current import get_app
+from prompt_toolkit.buffer import Buffer
+from prompt_toolkit.cache import SimpleCache
+from prompt_toolkit.data_structures import Point
+from prompt_toolkit.document import Document
+from prompt_toolkit.filters import FilterOrBool, to_filter
+from prompt_toolkit.formatted_text import (
+    AnyFormattedText,
+    StyleAndTextTuples,
+    to_formatted_text,
+)
+from prompt_toolkit.formatted_text.utils import (
+    fragment_list_to_text,
+    fragment_list_width,
+    split_lines,
+)
+from prompt_toolkit.lexers import Lexer, SimpleLexer
+from prompt_toolkit.mouse_events import MouseButton, MouseEvent, MouseEventType
+from prompt_toolkit.search import SearchState
+from prompt_toolkit.selection import SelectionType
+from prompt_toolkit.utils import get_cwidth
+
+from .processors import (
+    DisplayMultipleCursors,
+    HighlightIncrementalSearchProcessor,
+    HighlightSearchProcessor,
+    HighlightSelectionProcessor,
+    Processor,
+    TransformationInput,
+    merge_processors,
+)
+
+if TYPE_CHECKING:
+    from prompt_toolkit.key_binding.key_bindings import (
+        KeyBindingsBase,
+        NotImplementedOrNone,
+    )
+    from prompt_toolkit.utils import Event
+
+
+__all__ = [
+    "BufferControl",
+    "SearchBufferControl",
+    "DummyControl",
+    "FormattedTextControl",
+    "UIControl",
+    "UIContent",
+]
+
+GetLinePrefixCallable = Callable[[int, int], AnyFormattedText]
+
+
+class UIControl(metaclass=ABCMeta):
+    """
+    Base class for all user interface controls.
+    """
+
+    def reset(self) -> None:
+        # Default reset. (Doesn't have to be implemented.)
+        pass
+
+    def preferred_width(self, max_available_width: int) -> Optional[int]:
+        return None
+
+    def preferred_height(
+        self,
+        width: int,
+        max_available_height: int,
+        wrap_lines: bool,
+        get_line_prefix: Optional[GetLinePrefixCallable],
+    ) -> Optional[int]:
+        return None
+
+    def is_focusable(self) -> bool:
+        """
+        Tell whether this user control is focusable.
+        """
+        return False
+
+    @abstractmethod
+    def create_content(self, width: int, height: int) -> "UIContent":
+        """
+        Generate the content for this user control.
+
+        Returns a :class:`.UIContent` instance.
+        """
+
+    def mouse_handler(self, mouse_event: MouseEvent) -> "NotImplementedOrNone":
+        """
+        Handle mouse events.
+
+        When `NotImplemented` is returned, it means that the given event is not
+        handled by the `UIControl` itself. The `Window` or key bindings can
+        decide to handle this event as scrolling or changing focus.
+
+        :param mouse_event: `MouseEvent` instance.
+        """
+        return NotImplemented
+
+    def move_cursor_down(self) -> None:
+        """
+        Request to move the cursor down.
+        This happens when scrolling down and the cursor is completely at the
+        top.
+        """
+
+    def move_cursor_up(self) -> None:
+        """
+        Request to move the cursor up.
+        """
+
+    def get_key_bindings(self) -> Optional["KeyBindingsBase"]:
+        """
+        The key bindings that are specific for this user control.
+
+        Return a :class:`.KeyBindings` object if some key bindings are
+        specified, or `None` otherwise.
+        """
+
+    def get_invalidate_events(self) -> Iterable["Event[object]"]:
+        """
+        Return a list of `Event` objects. This can be a generator.
+        (The application collects all these events, in order to bind redraw
+        handlers to these events.)
+        """
+        return []
+
+
+class UIContent:
+    """
+    Content generated by a user control. This content consists of a list of
+    lines.
+
+    :param get_line: Callable that takes a line number and returns the current
+        line. This is a list of (style_str, text) tuples.
+    :param line_count: The number of lines.
+    :param cursor_position: a :class:`.Point` for the cursor position.
+    :param menu_position: a :class:`.Point` for the menu position.
+    :param show_cursor: Make the cursor visible.
+    """
+
+    def __init__(
+        self,
+        get_line: Callable[[int], StyleAndTextTuples] = (lambda i: []),
+        line_count: int = 0,
+        cursor_position: Optional[Point] = None,
+        menu_position: Optional[Point] = None,
+        show_cursor: bool = True,
+    ):
+
+        self.get_line = get_line
+        self.line_count = line_count
+        self.cursor_position = cursor_position or Point(x=0, y=0)
+        self.menu_position = menu_position
+        self.show_cursor = show_cursor
+
+        # Cache for line heights. Maps cache key -> height
+        self._line_heights_cache: Dict[Hashable, int] = {}
+
+    def __getitem__(self, lineno: int) -> StyleAndTextTuples:
+        "Make it iterable (iterate line by line)."
+        if lineno < self.line_count:
+            return self.get_line(lineno)
+        else:
+            raise IndexError
+
+    def get_height_for_line(
+        self,
+        lineno: int,
+        width: int,
+        get_line_prefix: Optional[GetLinePrefixCallable],
+        slice_stop: Optional[int] = None,
+    ) -> int:
+        """
+        Return the height that a given line would need if it is rendered in a
+        space with the given width (using line wrapping).
+
+        :param get_line_prefix: None or a `Window.get_line_prefix` callable
+            that returns the prefix to be inserted before this line.
+        :param slice_stop: Wrap only "line[:slice_stop]" and return that
+            partial result. This is needed for scrolling the window correctly
+            when line wrapping.
+        :returns: The computed height.
+        """
+        # Instead of using `get_line_prefix` as key, we use render_counter
+        # instead. This is more reliable, because this function could still be
+        # the same, while the content would change over time.
+        key = get_app().render_counter, lineno, width, slice_stop
+
+        try:
+            return self._line_heights_cache[key]
+        except KeyError:
+            if width == 0:
+                height = 10**8
+            else:
+                # Calculate line width first.
+                line = fragment_list_to_text(self.get_line(lineno))[:slice_stop]
+                text_width = get_cwidth(line)
+
+                if get_line_prefix:
+                    # Add prefix width.
+                    text_width += fragment_list_width(
+                        to_formatted_text(get_line_prefix(lineno, 0))
+                    )
+
+                    # Slower path: compute path when there's a line prefix.
+                    height = 1
+
+                    # Keep wrapping as long as the line doesn't fit.
+                    # Keep adding new prefixes for every wrapped line.
+                    while text_width > width:
+                        height += 1
+                        text_width -= width
+
+                        fragments2 = to_formatted_text(
+                            get_line_prefix(lineno, height - 1)
+                        )
+                        prefix_width = get_cwidth(fragment_list_to_text(fragments2))
+
+                        if prefix_width >= width:  # Prefix doesn't fit.
+                            height = 10**8
+                            break
+
+                        text_width += prefix_width
+                else:
+                    # Fast path: compute height when there's no line prefix.
+                    try:
+                        quotient, remainder = divmod(text_width, width)
+                    except ZeroDivisionError:
+                        height = 10**8
+                    else:
+                        if remainder:
+                            quotient += 1  # Like math.ceil.
+                        height = max(1, quotient)
+
+            # Cache and return
+            self._line_heights_cache[key] = height
+            return height
+
+
+class FormattedTextControl(UIControl):
+    """
+    Control that displays formatted text. This can be either plain text, an
+    :class:`~prompt_toolkit.formatted_text.HTML` object an
+    :class:`~prompt_toolkit.formatted_text.ANSI` object, a list of ``(style_str,
+    text)`` tuples or a callable that takes no argument and returns one of
+    those, depending on how you prefer to do the formatting. See
+    ``prompt_toolkit.layout.formatted_text`` for more information.
+
+    (It's mostly optimized for rather small widgets, like toolbars, menus, etc...)
+
+    When this UI control has the focus, the cursor will be shown in the upper
+    left corner of this control by default. There are two ways for specifying
+    the cursor position:
+
+    - Pass a `get_cursor_position` function which returns a `Point` instance
+      with the current cursor position.
+
+    - If the (formatted) text is passed as a list of ``(style, text)`` tuples
+      and there is one that looks like ``('[SetCursorPosition]', '')``, then
+      this will specify the cursor position.
+
+    Mouse support:
+
+        The list of fragments can also contain tuples of three items, looking like:
+        (style_str, text, handler). When mouse support is enabled and the user
+        clicks on this fragment, then the given handler is called. That handler
+        should accept two inputs: (Application, MouseEvent) and it should
+        either handle the event or return `NotImplemented` in case we want the
+        containing Window to handle this event.
+
+    :param focusable: `bool` or :class:`.Filter`: Tell whether this control is
+        focusable.
+
+    :param text: Text or formatted text to be displayed.
+    :param style: Style string applied to the content. (If you want to style
+        the whole :class:`~prompt_toolkit.layout.Window`, pass the style to the
+        :class:`~prompt_toolkit.layout.Window` instead.)
+    :param key_bindings: a :class:`.KeyBindings` object.
+    :param get_cursor_position: A callable that returns the cursor position as
+        a `Point` instance.
+    """
+
+    def __init__(
+        self,
+        text: AnyFormattedText = "",
+        style: str = "",
+        focusable: FilterOrBool = False,
+        key_bindings: Optional["KeyBindingsBase"] = None,
+        show_cursor: bool = True,
+        modal: bool = False,
+        get_cursor_position: Optional[Callable[[], Optional[Point]]] = None,
+    ) -> None:
+
+        self.text = text  # No type check on 'text'. This is done dynamically.
+        self.style = style
+        self.focusable = to_filter(focusable)
+
+        # Key bindings.
+        self.key_bindings = key_bindings
+        self.show_cursor = show_cursor
+        self.modal = modal
+        self.get_cursor_position = get_cursor_position
+
+        #: Cache for the content.
+        self._content_cache: SimpleCache[Hashable, UIContent] = SimpleCache(maxsize=18)
+        self._fragment_cache: SimpleCache[int, StyleAndTextTuples] = SimpleCache(
+            maxsize=1
+        )
+        # Only cache one fragment list. We don't need the previous item.
+
+        # Render info for the mouse support.
+        self._fragments: Optional[StyleAndTextTuples] = None
+
+    def reset(self) -> None:
+        self._fragments = None
+
+    def is_focusable(self) -> bool:
+        return self.focusable()
+
+    def __repr__(self) -> str:
+        return f"{self.__class__.__name__}({self.text!r})"
+
+    def _get_formatted_text_cached(self) -> StyleAndTextTuples:
+        """
+        Get fragments, but only retrieve fragments once during one render run.
+        (This function is called several times during one rendering, because
+        we also need those for calculating the dimensions.)
+        """
+        return self._fragment_cache.get(
+            get_app().render_counter, lambda: to_formatted_text(self.text, self.style)
+        )
+
+    def preferred_width(self, max_available_width: int) -> int:
+        """
+        Return the preferred width for this control.
+        That is the width of the longest line.
+        """
+        text = fragment_list_to_text(self._get_formatted_text_cached())
+        line_lengths = [get_cwidth(l) for l in text.split("\n")]
+        return max(line_lengths)
+
+    def preferred_height(
+        self,
+        width: int,
+        max_available_height: int,
+        wrap_lines: bool,
+        get_line_prefix: Optional[GetLinePrefixCallable],
+    ) -> Optional[int]:
+        """
+        Return the preferred height for this control.
+        """
+        content = self.create_content(width, None)
+        if wrap_lines:
+            height = 0
+            for i in range(content.line_count):
+                height += content.get_height_for_line(i, width, get_line_prefix)
+                if height >= max_available_height:
+                    return max_available_height
+            return height
+        else:
+            return content.line_count
+
+    def create_content(self, width: int, height: Optional[int]) -> UIContent:
+        # Get fragments
+        fragments_with_mouse_handlers = self._get_formatted_text_cached()
+        fragment_lines_with_mouse_handlers = list(
+            split_lines(fragments_with_mouse_handlers)
+        )
+
+        # Strip mouse handlers from fragments.
+        fragment_lines: List[StyleAndTextTuples] = [
+            [(item[0], item[1]) for item in line]
+            for line in fragment_lines_with_mouse_handlers
+        ]
+
+        # Keep track of the fragments with mouse handler, for later use in
+        # `mouse_handler`.
+        self._fragments = fragments_with_mouse_handlers
+
+        # If there is a `[SetCursorPosition]` in the fragment list, set the
+        # cursor position here.
+        def get_cursor_position(
+            fragment: str = "[SetCursorPosition]",
+        ) -> Optional[Point]:
+            for y, line in enumerate(fragment_lines):
+                x = 0
+                for style_str, text, *_ in line:
+                    if fragment in style_str:
+                        return Point(x=x, y=y)
+                    x += len(text)
+            return None
+
+        # If there is a `[SetMenuPosition]`, set the menu over here.
+        def get_menu_position() -> Optional[Point]:
+            return get_cursor_position("[SetMenuPosition]")
+
+        cursor_position = (self.get_cursor_position or get_cursor_position)()
+
+        # Create content, or take it from the cache.
+        key = (tuple(fragments_with_mouse_handlers), width, cursor_position)
+
+        def get_content() -> UIContent:
+            return UIContent(
+                get_line=lambda i: fragment_lines[i],
+                line_count=len(fragment_lines),
+                show_cursor=self.show_cursor,
+                cursor_position=cursor_position,
+                menu_position=get_menu_position(),
+            )
+
+        return self._content_cache.get(key, get_content)
+
+    def mouse_handler(self, mouse_event: MouseEvent) -> "NotImplementedOrNone":
+        """
+        Handle mouse events.
+
+        (When the fragment list contained mouse handlers and the user clicked on
+        on any of these, the matching handler is called. This handler can still
+        return `NotImplemented` in case we want the
+        :class:`~prompt_toolkit.layout.Window` to handle this particular
+        event.)
+        """
+        if self._fragments:
+            # Read the generator.
+            fragments_for_line = list(split_lines(self._fragments))
+
+            try:
+                fragments = fragments_for_line[mouse_event.position.y]
+            except IndexError:
+                return NotImplemented
+            else:
+                # Find position in the fragment list.
+                xpos = mouse_event.position.x
+
+                # Find mouse handler for this character.
+                count = 0
+                for item in fragments:
+                    count += len(item[1])
+                    if count > xpos:
+                        if len(item) >= 3:
+                            # Handler found. Call it.
+                            # (Handler can return NotImplemented, so return
+                            # that result.)
+                            handler = item[2]  # type: ignore
+                            return handler(mouse_event)
+                        else:
+                            break
+
+        # Otherwise, don't handle here.
+        return NotImplemented
+
+    def is_modal(self) -> bool:
+        return self.modal
+
+    def get_key_bindings(self) -> Optional["KeyBindingsBase"]:
+        return self.key_bindings
+
+
+class DummyControl(UIControl):
+    """
+    A dummy control object that doesn't paint any content.
+
+    Useful for filling a :class:`~prompt_toolkit.layout.Window`. (The
+    `fragment` and `char` attributes of the `Window` class can be used to
+    define the filling.)
+    """
+
+    def create_content(self, width: int, height: int) -> UIContent:
+        def get_line(i: int) -> StyleAndTextTuples:
+            return []
+
+        return UIContent(
+            get_line=get_line, line_count=100**100
+        )  # Something very big.
+
+    def is_focusable(self) -> bool:
+        return False
+
+
+class _ProcessedLine(NamedTuple):
+    fragments: StyleAndTextTuples
+    source_to_display: Callable[[int], int]
+    display_to_source: Callable[[int], int]
+
+
+class BufferControl(UIControl):
+    """
+    Control for visualising the content of a :class:`.Buffer`.
+
+    :param buffer: The :class:`.Buffer` object to be displayed.
+    :param input_processors: A list of
+        :class:`~prompt_toolkit.layout.processors.Processor` objects.
+    :param include_default_input_processors: When True, include the default
+        processors for highlighting of selection, search and displaying of
+        multiple cursors.
+    :param lexer: :class:`.Lexer` instance for syntax highlighting.
+    :param preview_search: `bool` or :class:`.Filter`: Show search while
+        typing. When this is `True`, probably you want to add a
+        ``HighlightIncrementalSearchProcessor`` as well. Otherwise only the
+        cursor position will move, but the text won't be highlighted.
+    :param focusable: `bool` or :class:`.Filter`: Tell whether this control is focusable.
+    :param focus_on_click: Focus this buffer when it's click, but not yet focused.
+    :param key_bindings: a :class:`.KeyBindings` object.
+    """
+
+    def __init__(
+        self,
+        buffer: Optional[Buffer] = None,
+        input_processors: Optional[List[Processor]] = None,
+        include_default_input_processors: bool = True,
+        lexer: Optional[Lexer] = None,
+        preview_search: FilterOrBool = False,
+        focusable: FilterOrBool = True,
+        search_buffer_control: Union[
+            None, "SearchBufferControl", Callable[[], "SearchBufferControl"]
+        ] = None,
+        menu_position: Optional[Callable[[], Optional[int]]] = None,
+        focus_on_click: FilterOrBool = False,
+        key_bindings: Optional["KeyBindingsBase"] = None,
+    ):
+
+        self.input_processors = input_processors
+        self.include_default_input_processors = include_default_input_processors
+
+        self.default_input_processors = [
+            HighlightSearchProcessor(),
+            HighlightIncrementalSearchProcessor(),
+            HighlightSelectionProcessor(),
+            DisplayMultipleCursors(),
+        ]
+
+        self.preview_search = to_filter(preview_search)
+        self.focusable = to_filter(focusable)
+        self.focus_on_click = to_filter(focus_on_click)
+
+        self.buffer = buffer or Buffer()
+        self.menu_position = menu_position
+        self.lexer = lexer or SimpleLexer()
+        self.key_bindings = key_bindings
+        self._search_buffer_control = search_buffer_control
+
+        #: Cache for the lexer.
+        #: Often, due to cursor movement, undo/redo and window resizing
+        #: operations, it happens that a short time, the same document has to be
+        #: lexed. This is a fairly easy way to cache such an expensive operation.
+        self._fragment_cache: SimpleCache[
+            Hashable, Callable[[int], StyleAndTextTuples]
+        ] = SimpleCache(maxsize=8)
+
+        self._last_click_timestamp: Optional[float] = None
+        self._last_get_processed_line: Optional[Callable[[int], _ProcessedLine]] = None
+
+    def __repr__(self) -> str:
+        return f"<{self.__class__.__name__} buffer={self.buffer!r} at {id(self)!r}>"
+
+    @property
+    def search_buffer_control(self) -> Optional["SearchBufferControl"]:
+        result: Optional[SearchBufferControl]
+
+        if callable(self._search_buffer_control):
+            result = self._search_buffer_control()
+        else:
+            result = self._search_buffer_control
+
+        assert result is None or isinstance(result, SearchBufferControl)
+        return result
+
+    @property
+    def search_buffer(self) -> Optional[Buffer]:
+        control = self.search_buffer_control
+        if control is not None:
+            return control.buffer
+        return None
+
+    @property
+    def search_state(self) -> SearchState:
+        """
+        Return the `SearchState` for searching this `BufferControl`. This is
+        always associated with the search control. If one search bar is used
+        for searching multiple `BufferControls`, then they share the same
+        `SearchState`.
+        """
+        search_buffer_control = self.search_buffer_control
+        if search_buffer_control:
+            return search_buffer_control.searcher_search_state
+        else:
+            return SearchState()
+
+    def is_focusable(self) -> bool:
+        return self.focusable()
+
+    def preferred_width(self, max_available_width: int) -> Optional[int]:
+        """
+        This should return the preferred width.
+
+        Note: We don't specify a preferred width according to the content,
+              because it would be too expensive. Calculating the preferred
+              width can be done by calculating the longest line, but this would
+              require applying all the processors to each line. This is
+              unfeasible for a larger document, and doing it for small
+              documents only would result in inconsistent behaviour.
+        """
+        return None
+
+    def preferred_height(
+        self,
+        width: int,
+        max_available_height: int,
+        wrap_lines: bool,
+        get_line_prefix: Optional[GetLinePrefixCallable],
+    ) -> Optional[int]:
+
+        # Calculate the content height, if it was drawn on a screen with the
+        # given width.
+        height = 0
+        content = self.create_content(width, height=1)  # Pass a dummy '1' as height.
+
+        # When line wrapping is off, the height should be equal to the amount
+        # of lines.
+        if not wrap_lines:
+            return content.line_count
+
+        # When the number of lines exceeds the max_available_height, just
+        # return max_available_height. No need to calculate anything.
+        if content.line_count >= max_available_height:
+            return max_available_height
+
+        for i in range(content.line_count):
+            height += content.get_height_for_line(i, width, get_line_prefix)
+
+            if height >= max_available_height:
+                return max_available_height
+
+        return height
+
+    def _get_formatted_text_for_line_func(
+        self, document: Document
+    ) -> Callable[[int], StyleAndTextTuples]:
+        """
+        Create a function that returns the fragments for a given line.
+        """
+        # Cache using `document.text`.
+        def get_formatted_text_for_line() -> Callable[[int], StyleAndTextTuples]:
+            return self.lexer.lex_document(document)
+
+        key = (document.text, self.lexer.invalidation_hash())
+        return self._fragment_cache.get(key, get_formatted_text_for_line)
+
+    def _create_get_processed_line_func(
+        self, document: Document, width: int, height: int
+    ) -> Callable[[int], _ProcessedLine]:
+        """
+        Create a function that takes a line number of the current document and
+        returns a _ProcessedLine(processed_fragments, source_to_display, display_to_source)
+        tuple.
+        """
+        # Merge all input processors together.
+        input_processors = self.input_processors or []
+        if self.include_default_input_processors:
+            input_processors = self.default_input_processors + input_processors
+
+        merged_processor = merge_processors(input_processors)
+
+        def transform(lineno: int, fragments: StyleAndTextTuples) -> _ProcessedLine:
+            "Transform the fragments for a given line number."
+            # Get cursor position at this line.
+            def source_to_display(i: int) -> int:
+                """X position from the buffer to the x position in the
+                processed fragment list. By default, we start from the 'identity'
+                operation."""
+                return i
+
+            transformation = merged_processor.apply_transformation(
+                TransformationInput(
+                    self, document, lineno, source_to_display, fragments, width, height
+                )
+            )
+
+            return _ProcessedLine(
+                transformation.fragments,
+                transformation.source_to_display,
+                transformation.display_to_source,
+            )
+
+        def create_func() -> Callable[[int], _ProcessedLine]:
+            get_line = self._get_formatted_text_for_line_func(document)
+            cache: Dict[int, _ProcessedLine] = {}
+
+            def get_processed_line(i: int) -> _ProcessedLine:
+                try:
+                    return cache[i]
+                except KeyError:
+                    processed_line = transform(i, get_line(i))
+                    cache[i] = processed_line
+                    return processed_line
+
+            return get_processed_line
+
+        return create_func()
+
+    def create_content(
+        self, width: int, height: int, preview_search: bool = False
+    ) -> UIContent:
+        """
+        Create a UIContent.
+        """
+        buffer = self.buffer
+
+        # Trigger history loading of the buffer. We do this during the
+        # rendering of the UI here, because it needs to happen when an
+        # `Application` with its event loop is running. During the rendering of
+        # the buffer control is the earliest place we can achieve this, where
+        # we're sure the right event loop is active, and don't require user
+        # interaction (like in a key binding).
+        buffer.load_history_if_not_yet_loaded()
+
+        # Get the document to be shown. If we are currently searching (the
+        # search buffer has focus, and the preview_search filter is enabled),
+        # then use the search document, which has possibly a different
+        # text/cursor position.)
+        search_control = self.search_buffer_control
+        preview_now = preview_search or bool(
+            # Only if this feature is enabled.
+            self.preview_search()
+            and
+            # And something was typed in the associated search field.
+            search_control
+            and search_control.buffer.text
+            and
+            # And we are searching in this control. (Many controls can point to
+            # the same search field, like in Pyvim.)
+            get_app().layout.search_target_buffer_control == self
+        )
+
+        if preview_now and search_control is not None:
+            ss = self.search_state
+
+            document = buffer.document_for_search(
+                SearchState(
+                    text=search_control.buffer.text,
+                    direction=ss.direction,
+                    ignore_case=ss.ignore_case,
+                )
+            )
+        else:
+            document = buffer.document
+
+        get_processed_line = self._create_get_processed_line_func(
+            document, width, height
+        )
+        self._last_get_processed_line = get_processed_line
+
+        def translate_rowcol(row: int, col: int) -> Point:
+            "Return the content column for this coordinate."
+            return Point(x=get_processed_line(row).source_to_display(col), y=row)
+
+        def get_line(i: int) -> StyleAndTextTuples:
+            "Return the fragments for a given line number."
+            fragments = get_processed_line(i).fragments
+
+            # Add a space at the end, because that is a possible cursor
+            # position. (When inserting after the input.) We should do this on
+            # all the lines, not just the line containing the cursor. (Because
+            # otherwise, line wrapping/scrolling could change when moving the
+            # cursor around.)
+            fragments = fragments + [("", " ")]
+            return fragments
+
+        content = UIContent(
+            get_line=get_line,
+            line_count=document.line_count,
+            cursor_position=translate_rowcol(
+                document.cursor_position_row, document.cursor_position_col
+            ),
+        )
+
+        # If there is an auto completion going on, use that start point for a
+        # pop-up menu position. (But only when this buffer has the focus --
+        # there is only one place for a menu, determined by the focused buffer.)
+        if get_app().layout.current_control == self:
+            menu_position = self.menu_position() if self.menu_position else None
+            if menu_position is not None:
+                assert isinstance(menu_position, int)
+                menu_row, menu_col = buffer.document.translate_index_to_position(
+                    menu_position
+                )
+                content.menu_position = translate_rowcol(menu_row, menu_col)
+            elif buffer.complete_state:
+                # Position for completion menu.
+                # Note: We use 'min', because the original cursor position could be
+                #       behind the input string when the actual completion is for
+                #       some reason shorter than the text we had before. (A completion
+                #       can change and shorten the input.)
+                menu_row, menu_col = buffer.document.translate_index_to_position(
+                    min(
+                        buffer.cursor_position,
+                        buffer.complete_state.original_document.cursor_position,
+                    )
+                )
+                content.menu_position = translate_rowcol(menu_row, menu_col)
+            else:
+                content.menu_position = None
+
+        return content
+
+    def mouse_handler(self, mouse_event: MouseEvent) -> "NotImplementedOrNone":
+        """
+        Mouse handler for this control.
+        """
+        buffer = self.buffer
+        position = mouse_event.position
+
+        # Focus buffer when clicked.
+        if get_app().layout.current_control == self:
+            if self._last_get_processed_line:
+                processed_line = self._last_get_processed_line(position.y)
+
+                # Translate coordinates back to the cursor position of the
+                # original input.
+                xpos = processed_line.display_to_source(position.x)
+                index = buffer.document.translate_row_col_to_index(position.y, xpos)
+
+                # Set the cursor position.
+                if mouse_event.event_type == MouseEventType.MOUSE_DOWN:
+                    buffer.exit_selection()
+                    buffer.cursor_position = index
+
+                elif (
+                    mouse_event.event_type == MouseEventType.MOUSE_MOVE
+                    and mouse_event.button != MouseButton.NONE
+                ):
+                    # Click and drag to highlight a selection
+                    if (
+                        buffer.selection_state is None
+                        and abs(buffer.cursor_position - index) > 0
+                    ):
+                        buffer.start_selection(selection_type=SelectionType.CHARACTERS)
+                    buffer.cursor_position = index
+
+                elif mouse_event.event_type == MouseEventType.MOUSE_UP:
+                    # When the cursor was moved to another place, select the text.
+                    # (The >1 is actually a small but acceptable workaround for
+                    # selecting text in Vi navigation mode. In navigation mode,
+                    # the cursor can never be after the text, so the cursor
+                    # will be repositioned automatically.)
+                    if abs(buffer.cursor_position - index) > 1:
+                        if buffer.selection_state is None:
+                            buffer.start_selection(
+                                selection_type=SelectionType.CHARACTERS
+                            )
+                        buffer.cursor_position = index
+
+                    # Select word around cursor on double click.
+                    # Two MOUSE_UP events in a short timespan are considered a double click.
+                    double_click = (
+                        self._last_click_timestamp
+                        and time.time() - self._last_click_timestamp < 0.3
+                    )
+                    self._last_click_timestamp = time.time()
+
+                    if double_click:
+                        start, end = buffer.document.find_boundaries_of_current_word()
+                        buffer.cursor_position += start
+                        buffer.start_selection(selection_type=SelectionType.CHARACTERS)
+                        buffer.cursor_position += end - start
+                else:
+                    # Don't handle scroll events here.
+                    return NotImplemented
+
+        # Not focused, but focusing on click events.
+        else:
+            if (
+                self.focus_on_click()
+                and mouse_event.event_type == MouseEventType.MOUSE_UP
+            ):
+                # Focus happens on mouseup. (If we did this on mousedown, the
+                # up event will be received at the point where this widget is
+                # focused and be handled anyway.)
+                get_app().layout.current_control = self
+            else:
+                return NotImplemented
+
+        return None
+
+    def move_cursor_down(self) -> None:
+        b = self.buffer
+        b.cursor_position += b.document.get_cursor_down_position()
+
+    def move_cursor_up(self) -> None:
+        b = self.buffer
+        b.cursor_position += b.document.get_cursor_up_position()
+
+    def get_key_bindings(self) -> Optional["KeyBindingsBase"]:
+        """
+        When additional key bindings are given. Return these.
+        """
+        return self.key_bindings
+
+    def get_invalidate_events(self) -> Iterable["Event[object]"]:
+        """
+        Return the Window invalidate events.
+        """
+        # Whenever the buffer changes, the UI has to be updated.
+        yield self.buffer.on_text_changed
+        yield self.buffer.on_cursor_position_changed
+
+        yield self.buffer.on_completions_changed
+        yield self.buffer.on_suggestion_set
+
+
+class SearchBufferControl(BufferControl):
+    """
+    :class:`.BufferControl` which is used for searching another
+    :class:`.BufferControl`.
+
+    :param ignore_case: Search case insensitive.
+    """
+
+    def __init__(
+        self,
+        buffer: Optional[Buffer] = None,
+        input_processors: Optional[List[Processor]] = None,
+        lexer: Optional[Lexer] = None,
+        focus_on_click: FilterOrBool = False,
+        key_bindings: Optional["KeyBindingsBase"] = None,
+        ignore_case: FilterOrBool = False,
+    ):
+
+        super().__init__(
+            buffer=buffer,
+            input_processors=input_processors,
+            lexer=lexer,
+            focus_on_click=focus_on_click,
+            key_bindings=key_bindings,
+        )
+
+        # If this BufferControl is used as a search field for one or more other
+        # BufferControls, then represents the search state.
+        self.searcher_search_state = SearchState(ignore_case=ignore_case)
diff --git a/src/prompt_toolkit/layout/dimension.py b/src/prompt_toolkit/layout/dimension.py
new file mode 100644
index 0000000..04c2163
--- /dev/null
+++ b/src/prompt_toolkit/layout/dimension.py
@@ -0,0 +1,217 @@
+"""
+Layout dimensions are used to give the minimum, maximum and preferred
+dimensions for containers and controls.
+"""
+from typing import TYPE_CHECKING, Any, Callable, List, Optional, Union
+
+__all__ = [
+    "Dimension",
+    "D",
+    "sum_layout_dimensions",
+    "max_layout_dimensions",
+    "AnyDimension",
+    "to_dimension",
+    "is_dimension",
+]
+
+if TYPE_CHECKING:
+    from typing_extensions import TypeGuard
+
+
+class Dimension:
+    """
+    Specified dimension (width/height) of a user control or window.
+
+    The layout engine tries to honor the preferred size. If that is not
+    possible, because the terminal is larger or smaller, it tries to keep in
+    between min and max.
+
+    :param min: Minimum size.
+    :param max: Maximum size.
+    :param weight: For a VSplit/HSplit, the actual size will be determined
+                   by taking the proportion of weights from all the children.
+                   E.g. When there are two children, one with a weight of 1,
+                   and the other with a weight of 2, the second will always be
+                   twice as big as the first, if the min/max values allow it.
+    :param preferred: Preferred size.
+    """
+
+    def __init__(
+        self,
+        min: Optional[int] = None,
+        max: Optional[int] = None,
+        weight: Optional[int] = None,
+        preferred: Optional[int] = None,
+    ) -> None:
+        if weight is not None:
+            assert weight >= 0  # Also cannot be a float.
+
+        assert min is None or min >= 0
+        assert max is None or max >= 0
+        assert preferred is None or preferred >= 0
+
+        self.min_specified = min is not None
+        self.max_specified = max is not None
+        self.preferred_specified = preferred is not None
+        self.weight_specified = weight is not None
+
+        if min is None:
+            min = 0  # Smallest possible value.
+        if max is None:  # 0-values are allowed, so use "is None"
+            max = 1000**10  # Something huge.
+        if preferred is None:
+            preferred = min
+        if weight is None:
+            weight = 1
+
+        self.min = min
+        self.max = max
+        self.preferred = preferred
+        self.weight = weight
+
+        # Don't allow situations where max < min. (This would be a bug.)
+        if max < min:
+            raise ValueError("Invalid Dimension: max < min.")
+
+        # Make sure that the 'preferred' size is always in the min..max range.
+        if self.preferred < self.min:
+            self.preferred = self.min
+
+        if self.preferred > self.max:
+            self.preferred = self.max
+
+    @classmethod
+    def exact(cls, amount: int) -> "Dimension":
+        """
+        Return a :class:`.Dimension` with an exact size. (min, max and
+        preferred set to ``amount``).
+        """
+        return cls(min=amount, max=amount, preferred=amount)
+
+    @classmethod
+    def zero(cls) -> "Dimension":
+        """
+        Create a dimension that represents a zero size. (Used for 'invisible'
+        controls.)
+        """
+        return cls.exact(amount=0)
+
+    def is_zero(self) -> bool:
+        "True if this `Dimension` represents a zero size."
+        return self.preferred == 0 or self.max == 0
+
+    def __repr__(self) -> str:
+        fields = []
+        if self.min_specified:
+            fields.append("min=%r" % self.min)
+        if self.max_specified:
+            fields.append("max=%r" % self.max)
+        if self.preferred_specified:
+            fields.append("preferred=%r" % self.preferred)
+        if self.weight_specified:
+            fields.append("weight=%r" % self.weight)
+
+        return "Dimension(%s)" % ", ".join(fields)
+
+
+def sum_layout_dimensions(dimensions: List[Dimension]) -> Dimension:
+    """
+    Sum a list of :class:`.Dimension` instances.
+    """
+    min = sum(d.min for d in dimensions)
+    max = sum(d.max for d in dimensions)
+    preferred = sum(d.preferred for d in dimensions)
+
+    return Dimension(min=min, max=max, preferred=preferred)
+
+
+def max_layout_dimensions(dimensions: List[Dimension]) -> Dimension:
+    """
+    Take the maximum of a list of :class:`.Dimension` instances.
+    Used when we have a HSplit/VSplit, and we want to get the best width/height.)
+    """
+    if not len(dimensions):
+        return Dimension.zero()
+
+    # If all dimensions are size zero. Return zero.
+    # (This is important for HSplit/VSplit, to report the right values to their
+    # parent when all children are invisible.)
+    if all(d.is_zero() for d in dimensions):
+        return dimensions[0]
+
+    # Ignore empty dimensions. (They should not reduce the size of others.)
+    dimensions = [d for d in dimensions if not d.is_zero()]
+
+    if dimensions:
+        # Take the highest minimum dimension.
+        min_ = max(d.min for d in dimensions)
+
+        # For the maximum, we would prefer not to go larger than then smallest
+        # 'max' value, unless other dimensions have a bigger preferred value.
+        # This seems to work best:
+        #  - We don't want that a widget with a small height in a VSplit would
+        #    shrink other widgets in the split.
+        # If it doesn't work well enough, then it's up to the UI designer to
+        # explicitly pass dimensions.
+        max_ = min(d.max for d in dimensions)
+        max_ = max(max_, max(d.preferred for d in dimensions))
+
+        # Make sure that min>=max. In some scenarios, when certain min..max
+        # ranges don't have any overlap, we can end up in such an impossible
+        # situation. In that case, give priority to the max value.
+        # E.g. taking (1..5) and (8..9) would return (8..5). Instead take (8..8).
+        if min_ > max_:
+            max_ = min_
+
+        preferred = max(d.preferred for d in dimensions)
+
+        return Dimension(min=min_, max=max_, preferred=preferred)
+    else:
+        return Dimension()
+
+
+# Anything that can be converted to a dimension.
+AnyDimension = Union[
+    None,  # None is a valid dimension that will fit anything.
+    int,
+    Dimension,
+    # Callable[[], 'AnyDimension']  # Recursive definition not supported by mypy.
+    Callable[[], Any],
+]
+
+
+def to_dimension(value: AnyDimension) -> Dimension:
+    """
+    Turn the given object into a `Dimension` object.
+    """
+    if value is None:
+        return Dimension()
+    if isinstance(value, int):
+        return Dimension.exact(value)
+    if isinstance(value, Dimension):
+        return value
+    if callable(value):
+        return to_dimension(value())
+
+    raise ValueError("Not an integer or Dimension object.")
+
+
+def is_dimension(value: object) -> "TypeGuard[AnyDimension]":
+    """
+    Test whether the given value could be a valid dimension.
+    (For usage in an assertion. It's not guaranteed in case of a callable.)
+    """
+    if value is None:
+        return True
+    if callable(value):
+        return True  # Assume it's a callable that doesn't take arguments.
+    if isinstance(value, (int, Dimension)):
+        return True
+    return False
+
+
+# Common alias.
+D = Dimension
+
+# For backward-compatibility.
+LayoutDimension = Dimension
diff --git a/src/prompt_toolkit/layout/dummy.py b/src/prompt_toolkit/layout/dummy.py
new file mode 100644
index 0000000..dcd54e9
--- /dev/null
+++ b/src/prompt_toolkit/layout/dummy.py
@@ -0,0 +1,37 @@
+"""
+Dummy layout. Used when somebody creates an `Application` without specifying a
+`Layout`.
+"""
+from prompt_toolkit.formatted_text import HTML
+from prompt_toolkit.key_binding import KeyBindings
+from prompt_toolkit.key_binding.key_processor import KeyPressEvent
+
+from .containers import Window
+from .controls import FormattedTextControl
+from .dimension import D
+from .layout import Layout
+
+__all__ = [
+    "create_dummy_layout",
+]
+
+E = KeyPressEvent
+
+
+def create_dummy_layout() -> Layout:
+    """
+    Create a dummy layout for use in an 'Application' that doesn't have a
+    layout specified. When ENTER is pressed, the application quits.
+    """
+    kb = KeyBindings()
+
+    @kb.add("enter")
+    def enter(event: E) -> None:
+        event.app.exit()
+
+    control = FormattedTextControl(
+        HTML("No layout specified. Press <reverse>ENTER</reverse> to quit."),
+        key_bindings=kb,
+    )
+    window = Window(content=control, height=D(min=1))
+    return Layout(container=window, focused_element=window)
diff --git a/src/prompt_toolkit/layout/layout.py b/src/prompt_toolkit/layout/layout.py
new file mode 100644
index 0000000..62a3184
--- /dev/null
+++ b/src/prompt_toolkit/layout/layout.py
@@ -0,0 +1,411 @@
+"""
+Wrapper for the layout.
+"""
+from typing import Dict, Generator, Iterable, List, Optional, Union
+
+from prompt_toolkit.buffer import Buffer
+
+from .containers import (
+    AnyContainer,
+    ConditionalContainer,
+    Container,
+    Window,
+    to_container,
+)
+from .controls import BufferControl, SearchBufferControl, UIControl
+
+__all__ = [
+    "Layout",
+    "InvalidLayoutError",
+    "walk",
+]
+
+FocusableElement = Union[str, Buffer, UIControl, AnyContainer]
+
+
+class Layout:
+    """
+    The layout for a prompt_toolkit
+    :class:`~prompt_toolkit.application.Application`.
+    This also keeps track of which user control is focused.
+
+    :param container: The "root" container for the layout.
+    :param focused_element: element to be focused initially. (Can be anything
+        the `focus` function accepts.)
+    """
+
+    def __init__(
+        self,
+        container: AnyContainer,
+        focused_element: Optional[FocusableElement] = None,
+    ) -> None:
+
+        self.container = to_container(container)
+        self._stack: List[Window] = []
+
+        # Map search BufferControl back to the original BufferControl.
+        # This is used to keep track of when exactly we are searching, and for
+        # applying the search.
+        # When a link exists in this dictionary, that means the search is
+        # currently active.
+        # Map: search_buffer_control -> original buffer control.
+        self.search_links: Dict[SearchBufferControl, BufferControl] = {}
+
+        # Mapping that maps the children in the layout to their parent.
+        # This relationship is calculated dynamically, each time when the UI
+        # is rendered.  (UI elements have only references to their children.)
+        self._child_to_parent: Dict[Container, Container] = {}
+
+        if focused_element is None:
+            try:
+                self._stack.append(next(self.find_all_windows()))
+            except StopIteration as e:
+                raise InvalidLayoutError(
+                    "Invalid layout. The layout does not contain any Window object."
+                ) from e
+        else:
+            self.focus(focused_element)
+
+        # List of visible windows.
+        self.visible_windows: List[Window] = []  # List of `Window` objects.
+
+    def __repr__(self) -> str:
+        return f"Layout({self.container!r}, current_window={self.current_window!r})"
+
+    def find_all_windows(self) -> Generator[Window, None, None]:
+        """
+        Find all the :class:`.UIControl` objects in this layout.
+        """
+        for item in self.walk():
+            if isinstance(item, Window):
+                yield item
+
+    def find_all_controls(self) -> Iterable[UIControl]:
+        for container in self.find_all_windows():
+            yield container.content
+
+    def focus(self, value: FocusableElement) -> None:
+        """
+        Focus the given UI element.
+
+        `value` can be either:
+
+        - a :class:`.UIControl`
+        - a :class:`.Buffer` instance or the name of a :class:`.Buffer`
+        - a :class:`.Window`
+        - Any container object. In this case we will focus the :class:`.Window`
+          from this container that was focused most recent, or the very first
+          focusable :class:`.Window` of the container.
+        """
+        # BufferControl by buffer name.
+        if isinstance(value, str):
+            for control in self.find_all_controls():
+                if isinstance(control, BufferControl) and control.buffer.name == value:
+                    self.focus(control)
+                    return
+            raise ValueError(f"Couldn't find Buffer in the current layout: {value!r}.")
+
+        # BufferControl by buffer object.
+        elif isinstance(value, Buffer):
+            for control in self.find_all_controls():
+                if isinstance(control, BufferControl) and control.buffer == value:
+                    self.focus(control)
+                    return
+            raise ValueError(f"Couldn't find Buffer in the current layout: {value!r}.")
+
+        # Focus UIControl.
+        elif isinstance(value, UIControl):
+            if value not in self.find_all_controls():
+                raise ValueError(
+                    "Invalid value. Container does not appear in the layout."
+                )
+            if not value.is_focusable():
+                raise ValueError("Invalid value. UIControl is not focusable.")
+
+            self.current_control = value
+
+        # Otherwise, expecting any Container object.
+        else:
+            value = to_container(value)
+
+            if isinstance(value, Window):
+                # This is a `Window`: focus that.
+                if value not in self.find_all_windows():
+                    raise ValueError(
+                        "Invalid value. Window does not appear in the layout: %r"
+                        % (value,)
+                    )
+
+                self.current_window = value
+            else:
+                # Focus a window in this container.
+                # If we have many windows as part of this container, and some
+                # of them have been focused before, take the last focused
+                # item. (This is very useful when the UI is composed of more
+                # complex sub components.)
+                windows = []
+                for c in walk(value, skip_hidden=True):
+                    if isinstance(c, Window) and c.content.is_focusable():
+                        windows.append(c)
+
+                # Take the first one that was focused before.
+                for w in reversed(self._stack):
+                    if w in windows:
+                        self.current_window = w
+                        return
+
+                # None was focused before: take the very first focusable window.
+                if windows:
+                    self.current_window = windows[0]
+                    return
+
+                raise ValueError(
+                    f"Invalid value. Container cannot be focused: {value!r}"
+                )
+
+    def has_focus(self, value: FocusableElement) -> bool:
+        """
+        Check whether the given control has the focus.
+        :param value: :class:`.UIControl` or :class:`.Window` instance.
+        """
+        if isinstance(value, str):
+            if self.current_buffer is None:
+                return False
+            return self.current_buffer.name == value
+        if isinstance(value, Buffer):
+            return self.current_buffer == value
+        if isinstance(value, UIControl):
+            return self.current_control == value
+        else:
+            value = to_container(value)
+            if isinstance(value, Window):
+                return self.current_window == value
+            else:
+                # Check whether this "container" is focused. This is true if
+                # one of the elements inside is focused.
+                for element in walk(value):
+                    if element == self.current_window:
+                        return True
+                return False
+
+    @property
+    def current_control(self) -> UIControl:
+        """
+        Get the :class:`.UIControl` to currently has the focus.
+        """
+        return self._stack[-1].content
+
+    @current_control.setter
+    def current_control(self, control: UIControl) -> None:
+        """
+        Set the :class:`.UIControl` to receive the focus.
+        """
+        for window in self.find_all_windows():
+            if window.content == control:
+                self.current_window = window
+                return
+
+        raise ValueError("Control not found in the user interface.")
+
+    @property
+    def current_window(self) -> Window:
+        "Return the :class:`.Window` object that is currently focused."
+        return self._stack[-1]
+
+    @current_window.setter
+    def current_window(self, value: Window) -> None:
+        "Set the :class:`.Window` object to be currently focused."
+        self._stack.append(value)
+
+    @property
+    def is_searching(self) -> bool:
+        "True if we are searching right now."
+        return self.current_control in self.search_links
+
+    @property
+    def search_target_buffer_control(self) -> Optional[BufferControl]:
+        """
+        Return the :class:`.BufferControl` in which we are searching or `None`.
+        """
+        # Not every `UIControl` is a `BufferControl`. This only applies to
+        # `BufferControl`.
+        control = self.current_control
+
+        if isinstance(control, SearchBufferControl):
+            return self.search_links.get(control)
+        else:
+            return None
+
+    def get_focusable_windows(self) -> Iterable[Window]:
+        """
+        Return all the :class:`.Window` objects which are focusable (in the
+        'modal' area).
+        """
+        for w in self.walk_through_modal_area():
+            if isinstance(w, Window) and w.content.is_focusable():
+                yield w
+
+    def get_visible_focusable_windows(self) -> List[Window]:
+        """
+        Return a list of :class:`.Window` objects that are focusable.
+        """
+        # focusable windows are windows that are visible, but also part of the
+        # modal container. Make sure to keep the ordering.
+        visible_windows = self.visible_windows
+        return [w for w in self.get_focusable_windows() if w in visible_windows]
+
+    @property
+    def current_buffer(self) -> Optional[Buffer]:
+        """
+        The currently focused :class:`~.Buffer` or `None`.
+        """
+        ui_control = self.current_control
+        if isinstance(ui_control, BufferControl):
+            return ui_control.buffer
+        return None
+
+    def get_buffer_by_name(self, buffer_name: str) -> Optional[Buffer]:
+        """
+        Look in the layout for a buffer with the given name.
+        Return `None` when nothing was found.
+        """
+        for w in self.walk():
+            if isinstance(w, Window) and isinstance(w.content, BufferControl):
+                if w.content.buffer.name == buffer_name:
+                    return w.content.buffer
+        return None
+
+    @property
+    def buffer_has_focus(self) -> bool:
+        """
+        Return `True` if the currently focused control is a
+        :class:`.BufferControl`. (For instance, used to determine whether the
+        default key bindings should be active or not.)
+        """
+        ui_control = self.current_control
+        return isinstance(ui_control, BufferControl)
+
+    @property
+    def previous_control(self) -> UIControl:
+        """
+        Get the :class:`.UIControl` to previously had the focus.
+        """
+        try:
+            return self._stack[-2].content
+        except IndexError:
+            return self._stack[-1].content
+
+    def focus_last(self) -> None:
+        """
+        Give the focus to the last focused control.
+        """
+        if len(self._stack) > 1:
+            self._stack = self._stack[:-1]
+
+    def focus_next(self) -> None:
+        """
+        Focus the next visible/focusable Window.
+        """
+        windows = self.get_visible_focusable_windows()
+
+        if len(windows) > 0:
+            try:
+                index = windows.index(self.current_window)
+            except ValueError:
+                index = 0
+            else:
+                index = (index + 1) % len(windows)
+
+            self.focus(windows[index])
+
+    def focus_previous(self) -> None:
+        """
+        Focus the previous visible/focusable Window.
+        """
+        windows = self.get_visible_focusable_windows()
+
+        if len(windows) > 0:
+            try:
+                index = windows.index(self.current_window)
+            except ValueError:
+                index = 0
+            else:
+                index = (index - 1) % len(windows)
+
+            self.focus(windows[index])
+
+    def walk(self) -> Iterable[Container]:
+        """
+        Walk through all the layout nodes (and their children) and yield them.
+        """
+        yield from walk(self.container)
+
+    def walk_through_modal_area(self) -> Iterable[Container]:
+        """
+        Walk through all the containers which are in the current 'modal' part
+        of the layout.
+        """
+        # Go up in the tree, and find the root. (it will be a part of the
+        # layout, if the focus is in a modal part.)
+        root: Container = self.current_window
+        while not root.is_modal() and root in self._child_to_parent:
+            root = self._child_to_parent[root]
+
+        yield from walk(root)
+
+    def update_parents_relations(self) -> None:
+        """
+        Update child->parent relationships mapping.
+        """
+        parents = {}
+
+        def walk(e: Container) -> None:
+            for c in e.get_children():
+                parents[c] = e
+                walk(c)
+
+        walk(self.container)
+
+        self._child_to_parent = parents
+
+    def reset(self) -> None:
+        # Remove all search links when the UI starts.
+        # (Important, for instance when control-c is been pressed while
+        #  searching. The prompt cancels, but next `run()` call the search
+        #  links are still there.)
+        self.search_links.clear()
+
+        self.container.reset()
+
+    def get_parent(self, container: Container) -> Optional[Container]:
+        """
+        Return the parent container for the given container, or ``None``, if it
+        wasn't found.
+        """
+        try:
+            return self._child_to_parent[container]
+        except KeyError:
+            return None
+
+
+class InvalidLayoutError(Exception):
+    pass
+
+
+def walk(container: Container, skip_hidden: bool = False) -> Iterable[Container]:
+    """
+    Walk through layout, starting at this container.
+    """
+    # When `skip_hidden` is set, don't go into disabled ConditionalContainer containers.
+    if (
+        skip_hidden
+        and isinstance(container, ConditionalContainer)
+        and not container.filter()
+    ):
+        return
+
+    yield container
+
+    for c in container.get_children():
+        # yield from walk(c)
+        yield from walk(c, skip_hidden=skip_hidden)
diff --git a/src/prompt_toolkit/layout/margins.py b/src/prompt_toolkit/layout/margins.py
new file mode 100644
index 0000000..7c46819
--- /dev/null
+++ b/src/prompt_toolkit/layout/margins.py
@@ -0,0 +1,305 @@
+"""
+Margin implementations for a :class:`~prompt_toolkit.layout.containers.Window`.
+"""
+from abc import ABCMeta, abstractmethod
+from typing import TYPE_CHECKING, Callable, Optional
+
+from prompt_toolkit.filters import FilterOrBool, to_filter
+from prompt_toolkit.formatted_text import (
+    StyleAndTextTuples,
+    fragment_list_to_text,
+    to_formatted_text,
+)
+from prompt_toolkit.utils import get_cwidth
+
+from .controls import UIContent
+
+if TYPE_CHECKING:
+    from .containers import WindowRenderInfo
+
+__all__ = [
+    "Margin",
+    "NumberedMargin",
+    "ScrollbarMargin",
+    "ConditionalMargin",
+    "PromptMargin",
+]
+
+
+class Margin(metaclass=ABCMeta):
+    """
+    Base interface for a margin.
+    """
+
+    @abstractmethod
+    def get_width(self, get_ui_content: Callable[[], UIContent]) -> int:
+        """
+        Return the width that this margin is going to consume.
+
+        :param get_ui_content: Callable that asks the user control to create
+            a :class:`.UIContent` instance. This can be used for instance to
+            obtain the number of lines.
+        """
+        return 0
+
+    @abstractmethod
+    def create_margin(
+        self, window_render_info: "WindowRenderInfo", width: int, height: int
+    ) -> StyleAndTextTuples:
+        """
+        Creates a margin.
+        This should return a list of (style_str, text) tuples.
+
+        :param window_render_info:
+            :class:`~prompt_toolkit.layout.containers.WindowRenderInfo`
+            instance, generated after rendering and copying the visible part of
+            the :class:`~prompt_toolkit.layout.controls.UIControl` into the
+            :class:`~prompt_toolkit.layout.containers.Window`.
+        :param width: The width that's available for this margin. (As reported
+            by :meth:`.get_width`.)
+        :param height: The height that's available for this margin. (The height
+            of the :class:`~prompt_toolkit.layout.containers.Window`.)
+        """
+        return []
+
+
+class NumberedMargin(Margin):
+    """
+    Margin that displays the line numbers.
+
+    :param relative: Number relative to the cursor position. Similar to the Vi
+                     'relativenumber' option.
+    :param display_tildes: Display tildes after the end of the document, just
+        like Vi does.
+    """
+
+    def __init__(
+        self, relative: FilterOrBool = False, display_tildes: FilterOrBool = False
+    ) -> None:
+
+        self.relative = to_filter(relative)
+        self.display_tildes = to_filter(display_tildes)
+
+    def get_width(self, get_ui_content: Callable[[], UIContent]) -> int:
+        line_count = get_ui_content().line_count
+        return max(3, len("%s" % line_count) + 1)
+
+    def create_margin(
+        self, window_render_info: "WindowRenderInfo", width: int, height: int
+    ) -> StyleAndTextTuples:
+        relative = self.relative()
+
+        style = "class:line-number"
+        style_current = "class:line-number.current"
+
+        # Get current line number.
+        current_lineno = window_render_info.ui_content.cursor_position.y
+
+        # Construct margin.
+        result: StyleAndTextTuples = []
+        last_lineno = None
+
+        for y, lineno in enumerate(window_render_info.displayed_lines):
+            # Only display line number if this line is not a continuation of the previous line.
+            if lineno != last_lineno:
+                if lineno is None:
+                    pass
+                elif lineno == current_lineno:
+                    # Current line.
+                    if relative:
+                        # Left align current number in relative mode.
+                        result.append((style_current, "%i" % (lineno + 1)))
+                    else:
+                        result.append(
+                            (style_current, ("%i " % (lineno + 1)).rjust(width))
+                        )
+                else:
+                    # Other lines.
+                    if relative:
+                        lineno = abs(lineno - current_lineno) - 1
+
+                    result.append((style, ("%i " % (lineno + 1)).rjust(width)))
+
+            last_lineno = lineno
+            result.append(("", "\n"))
+
+        # Fill with tildes.
+        if self.display_tildes():
+            while y < window_render_info.window_height:
+                result.append(("class:tilde", "~\n"))
+                y += 1
+
+        return result
+
+
+class ConditionalMargin(Margin):
+    """
+    Wrapper around other :class:`.Margin` classes to show/hide them.
+    """
+
+    def __init__(self, margin: Margin, filter: FilterOrBool) -> None:
+        self.margin = margin
+        self.filter = to_filter(filter)
+
+    def get_width(self, get_ui_content: Callable[[], UIContent]) -> int:
+        if self.filter():
+            return self.margin.get_width(get_ui_content)
+        else:
+            return 0
+
+    def create_margin(
+        self, window_render_info: "WindowRenderInfo", width: int, height: int
+    ) -> StyleAndTextTuples:
+        if width and self.filter():
+            return self.margin.create_margin(window_render_info, width, height)
+        else:
+            return []
+
+
+class ScrollbarMargin(Margin):
+    """
+    Margin displaying a scrollbar.
+
+    :param display_arrows: Display scroll up/down arrows.
+    """
+
+    def __init__(
+        self,
+        display_arrows: FilterOrBool = False,
+        up_arrow_symbol: str = "^",
+        down_arrow_symbol: str = "v",
+    ) -> None:
+
+        self.display_arrows = to_filter(display_arrows)
+        self.up_arrow_symbol = up_arrow_symbol
+        self.down_arrow_symbol = down_arrow_symbol
+
+    def get_width(self, get_ui_content: Callable[[], UIContent]) -> int:
+        return 1
+
+    def create_margin(
+        self, window_render_info: "WindowRenderInfo", width: int, height: int
+    ) -> StyleAndTextTuples:
+        content_height = window_render_info.content_height
+        window_height = window_render_info.window_height
+        display_arrows = self.display_arrows()
+
+        if display_arrows:
+            window_height -= 2
+
+        try:
+            fraction_visible = len(window_render_info.displayed_lines) / float(
+                content_height
+            )
+            fraction_above = window_render_info.vertical_scroll / float(content_height)
+
+            scrollbar_height = int(
+                min(window_height, max(1, window_height * fraction_visible))
+            )
+            scrollbar_top = int(window_height * fraction_above)
+        except ZeroDivisionError:
+            return []
+        else:
+
+            def is_scroll_button(row: int) -> bool:
+                "True if we should display a button on this row."
+                return scrollbar_top <= row <= scrollbar_top + scrollbar_height
+
+            # Up arrow.
+            result: StyleAndTextTuples = []
+            if display_arrows:
+                result.extend(
+                    [
+                        ("class:scrollbar.arrow", self.up_arrow_symbol),
+                        ("class:scrollbar", "\n"),
+                    ]
+                )
+
+            # Scrollbar body.
+            scrollbar_background = "class:scrollbar.background"
+            scrollbar_background_start = "class:scrollbar.background,scrollbar.start"
+            scrollbar_button = "class:scrollbar.button"
+            scrollbar_button_end = "class:scrollbar.button,scrollbar.end"
+
+            for i in range(window_height):
+                if is_scroll_button(i):
+                    if not is_scroll_button(i + 1):
+                        # Give the last cell a different style, because we
+                        # want to underline this.
+                        result.append((scrollbar_button_end, " "))
+                    else:
+                        result.append((scrollbar_button, " "))
+                else:
+                    if is_scroll_button(i + 1):
+                        result.append((scrollbar_background_start, " "))
+                    else:
+                        result.append((scrollbar_background, " "))
+                result.append(("", "\n"))
+
+            # Down arrow
+            if display_arrows:
+                result.append(("class:scrollbar.arrow", self.down_arrow_symbol))
+
+            return result
+
+
+class PromptMargin(Margin):
+    """
+    [Deprecated]
+
+    Create margin that displays a prompt.
+    This can display one prompt at the first line, and a continuation prompt
+    (e.g, just dots) on all the following lines.
+
+    This `PromptMargin` implementation has been largely superseded in favor of
+    the `get_line_prefix` attribute of `Window`. The reason is that a margin is
+    always a fixed width, while `get_line_prefix` can return a variable width
+    prefix in front of every line, making it more powerful, especially for line
+    continuations.
+
+    :param get_prompt: Callable returns formatted text or a list of
+        `(style_str, type)` tuples to be shown as the prompt at the first line.
+    :param get_continuation: Callable that takes three inputs. The width (int),
+        line_number (int), and is_soft_wrap (bool). It should return formatted
+        text or a list of `(style_str, type)` tuples for the next lines of the
+        input.
+    """
+
+    def __init__(
+        self,
+        get_prompt: Callable[[], StyleAndTextTuples],
+        get_continuation: Optional[
+            Callable[[int, int, bool], StyleAndTextTuples]
+        ] = None,
+    ) -> None:
+
+        self.get_prompt = get_prompt
+        self.get_continuation = get_continuation
+
+    def get_width(self, get_ui_content: Callable[[], UIContent]) -> int:
+        "Width to report to the `Window`."
+        # Take the width from the first line.
+        text = fragment_list_to_text(self.get_prompt())
+        return get_cwidth(text)
+
+    def create_margin(
+        self, window_render_info: "WindowRenderInfo", width: int, height: int
+    ) -> StyleAndTextTuples:
+        get_continuation = self.get_continuation
+        result: StyleAndTextTuples = []
+
+        # First line.
+        result.extend(to_formatted_text(self.get_prompt()))
+
+        # Next lines.
+        if get_continuation:
+            last_y = None
+
+            for y in window_render_info.displayed_lines[1:]:
+                result.append(("", "\n"))
+                result.extend(
+                    to_formatted_text(get_continuation(width, y, y == last_y))
+                )
+                last_y = y
+
+        return result
diff --git a/src/prompt_toolkit/layout/menus.py b/src/prompt_toolkit/layout/menus.py
new file mode 100644
index 0000000..db0f9cb
--- /dev/null
+++ b/src/prompt_toolkit/layout/menus.py
@@ -0,0 +1,765 @@
+import math
+from itertools import zip_longest
+from typing import (
+    TYPE_CHECKING,
+    Callable,
+    Dict,
+    Iterable,
+    List,
+    Optional,
+    Sequence,
+    Tuple,
+    TypeVar,
+    Union,
+    cast,
+)
+from weakref import WeakKeyDictionary, WeakValueDictionary
+
+from prompt_toolkit.application.current import get_app
+from prompt_toolkit.buffer import CompletionState
+from prompt_toolkit.completion import Completion
+from prompt_toolkit.data_structures import Point
+from prompt_toolkit.filters import (
+    Condition,
+    FilterOrBool,
+    has_completions,
+    is_done,
+    to_filter,
+)
+from prompt_toolkit.formatted_text import (
+    StyleAndTextTuples,
+    fragment_list_width,
+    to_formatted_text,
+)
+from prompt_toolkit.key_binding.key_processor import KeyPressEvent
+from prompt_toolkit.layout.utils import explode_text_fragments
+from prompt_toolkit.mouse_events import MouseEvent, MouseEventType
+from prompt_toolkit.utils import get_cwidth
+
+from .containers import ConditionalContainer, HSplit, ScrollOffsets, Window
+from .controls import GetLinePrefixCallable, UIContent, UIControl
+from .dimension import Dimension
+from .margins import ScrollbarMargin
+
+if TYPE_CHECKING:
+    from prompt_toolkit.key_binding.key_bindings import (
+        KeyBindings,
+        NotImplementedOrNone,
+    )
+
+
+__all__ = [
+    "CompletionsMenu",
+    "MultiColumnCompletionsMenu",
+]
+
+E = KeyPressEvent
+
+
+class CompletionsMenuControl(UIControl):
+    """
+    Helper for drawing the complete menu to the screen.
+
+    :param scroll_offset: Number (integer) representing the preferred amount of
+        completions to be displayed before and after the current one. When this
+        is a very high number, the current completion will be shown in the
+        middle most of the time.
+    """
+
+    # Preferred minimum size of the menu control.
+    # The CompletionsMenu class defines a width of 8, and there is a scrollbar
+    # of 1.)
+    MIN_WIDTH = 7
+
+    def has_focus(self) -> bool:
+        return False
+
+    def preferred_width(self, max_available_width: int) -> Optional[int]:
+        complete_state = get_app().current_buffer.complete_state
+        if complete_state:
+            menu_width = self._get_menu_width(500, complete_state)
+            menu_meta_width = self._get_menu_meta_width(500, complete_state)
+
+            return menu_width + menu_meta_width
+        else:
+            return 0
+
+    def preferred_height(
+        self,
+        width: int,
+        max_available_height: int,
+        wrap_lines: bool,
+        get_line_prefix: Optional[GetLinePrefixCallable],
+    ) -> Optional[int]:
+
+        complete_state = get_app().current_buffer.complete_state
+        if complete_state:
+            return len(complete_state.completions)
+        else:
+            return 0
+
+    def create_content(self, width: int, height: int) -> UIContent:
+        """
+        Create a UIContent object for this control.
+        """
+        complete_state = get_app().current_buffer.complete_state
+        if complete_state:
+            completions = complete_state.completions
+            index = complete_state.complete_index  # Can be None!
+
+            # Calculate width of completions menu.
+            menu_width = self._get_menu_width(width, complete_state)
+            menu_meta_width = self._get_menu_meta_width(
+                width - menu_width, complete_state
+            )
+            show_meta = self._show_meta(complete_state)
+
+            def get_line(i: int) -> StyleAndTextTuples:
+                c = completions[i]
+                is_current_completion = i == index
+                result = _get_menu_item_fragments(
+                    c, is_current_completion, menu_width, space_after=True
+                )
+
+                if show_meta:
+                    result += self._get_menu_item_meta_fragments(
+                        c, is_current_completion, menu_meta_width
+                    )
+                return result
+
+            return UIContent(
+                get_line=get_line,
+                cursor_position=Point(x=0, y=index or 0),
+                line_count=len(completions),
+            )
+
+        return UIContent()
+
+    def _show_meta(self, complete_state: CompletionState) -> bool:
+        """
+        Return ``True`` if we need to show a column with meta information.
+        """
+        return any(c.display_meta_text for c in complete_state.completions)
+
+    def _get_menu_width(self, max_width: int, complete_state: CompletionState) -> int:
+        """
+        Return the width of the main column.
+        """
+        return min(
+            max_width,
+            max(
+                self.MIN_WIDTH,
+                max(get_cwidth(c.display_text) for c in complete_state.completions) + 2,
+            ),
+        )
+
+    def _get_menu_meta_width(
+        self, max_width: int, complete_state: CompletionState
+    ) -> int:
+        """
+        Return the width of the meta column.
+        """
+
+        def meta_width(completion: Completion) -> int:
+            return get_cwidth(completion.display_meta_text)
+
+        if self._show_meta(complete_state):
+            # If the amount of completions is over 200, compute the width based
+            # on the first 200 completions, otherwise this can be very slow.
+            completions = complete_state.completions
+            if len(completions) > 200:
+                completions = completions[:200]
+
+            return min(max_width, max(meta_width(c) for c in completions) + 2)
+        else:
+            return 0
+
+    def _get_menu_item_meta_fragments(
+        self, completion: Completion, is_current_completion: bool, width: int
+    ) -> StyleAndTextTuples:
+
+        if is_current_completion:
+            style_str = "class:completion-menu.meta.completion.current"
+        else:
+            style_str = "class:completion-menu.meta.completion"
+
+        text, tw = _trim_formatted_text(completion.display_meta, width - 2)
+        padding = " " * (width - 1 - tw)
+
+        return to_formatted_text(
+            cast(StyleAndTextTuples, []) + [("", " ")] + text + [("", padding)],
+            style=style_str,
+        )
+
+    def mouse_handler(self, mouse_event: MouseEvent) -> "NotImplementedOrNone":
+        """
+        Handle mouse events: clicking and scrolling.
+        """
+        b = get_app().current_buffer
+
+        if mouse_event.event_type == MouseEventType.MOUSE_UP:
+            # Select completion.
+            b.go_to_completion(mouse_event.position.y)
+            b.complete_state = None
+
+        elif mouse_event.event_type == MouseEventType.SCROLL_DOWN:
+            # Scroll up.
+            b.complete_next(count=3, disable_wrap_around=True)
+
+        elif mouse_event.event_type == MouseEventType.SCROLL_UP:
+            # Scroll down.
+            b.complete_previous(count=3, disable_wrap_around=True)
+
+        return None
+
+
+def _get_menu_item_fragments(
+    completion: Completion,
+    is_current_completion: bool,
+    width: int,
+    space_after: bool = False,
+) -> StyleAndTextTuples:
+    """
+    Get the style/text tuples for a menu item, styled and trimmed to the given
+    width.
+    """
+    if is_current_completion:
+        style_str = "class:completion-menu.completion.current {} {}".format(
+            completion.style,
+            completion.selected_style,
+        )
+    else:
+        style_str = "class:completion-menu.completion " + completion.style
+
+    text, tw = _trim_formatted_text(
+        completion.display, (width - 2 if space_after else width - 1)
+    )
+
+    padding = " " * (width - 1 - tw)
+
+    return to_formatted_text(
+        cast(StyleAndTextTuples, []) + [("", " ")] + text + [("", padding)],
+        style=style_str,
+    )
+
+
+def _trim_formatted_text(
+    formatted_text: StyleAndTextTuples, max_width: int
+) -> Tuple[StyleAndTextTuples, int]:
+    """
+    Trim the text to `max_width`, append dots when the text is too long.
+    Returns (text, width) tuple.
+    """
+    width = fragment_list_width(formatted_text)
+
+    # When the text is too wide, trim it.
+    if width > max_width:
+        result = []  # Text fragments.
+        remaining_width = max_width - 3
+
+        for style_and_ch in explode_text_fragments(formatted_text):
+            ch_width = get_cwidth(style_and_ch[1])
+
+            if ch_width <= remaining_width:
+                result.append(style_and_ch)
+                remaining_width -= ch_width
+            else:
+                break
+
+        result.append(("", "..."))
+
+        return result, max_width - remaining_width
+    else:
+        return formatted_text, width
+
+
+class CompletionsMenu(ConditionalContainer):
+    # NOTE: We use a pretty big z_index by default. Menus are supposed to be
+    #       above anything else. We also want to make sure that the content is
+    #       visible at the point where we draw this menu.
+    def __init__(
+        self,
+        max_height: Optional[int] = None,
+        scroll_offset: Union[int, Callable[[], int]] = 0,
+        extra_filter: FilterOrBool = True,
+        display_arrows: FilterOrBool = False,
+        z_index: int = 10**8,
+    ) -> None:
+
+        extra_filter = to_filter(extra_filter)
+        display_arrows = to_filter(display_arrows)
+
+        super().__init__(
+            content=Window(
+                content=CompletionsMenuControl(),
+                width=Dimension(min=8),
+                height=Dimension(min=1, max=max_height),
+                scroll_offsets=ScrollOffsets(top=scroll_offset, bottom=scroll_offset),
+                right_margins=[ScrollbarMargin(display_arrows=display_arrows)],
+                dont_extend_width=True,
+                style="class:completion-menu",
+                z_index=z_index,
+            ),
+            # Show when there are completions but not at the point we are
+            # returning the input.
+            filter=has_completions & ~is_done & extra_filter,
+        )
+
+
+class MultiColumnCompletionMenuControl(UIControl):
+    """
+    Completion menu that displays all the completions in several columns.
+    When there are more completions than space for them to be displayed, an
+    arrow is shown on the left or right side.
+
+    `min_rows` indicates how many rows will be available in any possible case.
+    When this is larger than one, it will try to use less columns and more
+    rows until this value is reached.
+    Be careful passing in a too big value, if less than the given amount of
+    rows are available, more columns would have been required, but
+    `preferred_width` doesn't know about that and reports a too small value.
+    This results in less completions displayed and additional scrolling.
+    (It's a limitation of how the layout engine currently works: first the
+    widths are calculated, then the heights.)
+
+    :param suggested_max_column_width: The suggested max width of a column.
+        The column can still be bigger than this, but if there is place for two
+        columns of this width, we will display two columns. This to avoid that
+        if there is one very wide completion, that it doesn't significantly
+        reduce the amount of columns.
+    """
+
+    _required_margin = 3  # One extra padding on the right + space for arrows.
+
+    def __init__(self, min_rows: int = 3, suggested_max_column_width: int = 30) -> None:
+        assert min_rows >= 1
+
+        self.min_rows = min_rows
+        self.suggested_max_column_width = suggested_max_column_width
+        self.scroll = 0
+
+        # Cache for column width computations. This computation is not cheap,
+        # so we don't want to do it over and over again while the user
+        # navigates through the completions.
+        # (map `completion_state` to `(completion_count, width)`. We remember
+        # the count, because a completer can add new completions to the
+        # `CompletionState` while loading.)
+        self._column_width_for_completion_state: "WeakKeyDictionary[CompletionState, Tuple[int, int]]" = (
+            WeakKeyDictionary()
+        )
+
+        # Info of last rendering.
+        self._rendered_rows = 0
+        self._rendered_columns = 0
+        self._total_columns = 0
+        self._render_pos_to_completion: Dict[Tuple[int, int], Completion] = {}
+        self._render_left_arrow = False
+        self._render_right_arrow = False
+        self._render_width = 0
+
+    def reset(self) -> None:
+        self.scroll = 0
+
+    def has_focus(self) -> bool:
+        return False
+
+    def preferred_width(self, max_available_width: int) -> Optional[int]:
+        """
+        Preferred width: prefer to use at least min_rows, but otherwise as much
+        as possible horizontally.
+        """
+        complete_state = get_app().current_buffer.complete_state
+        if complete_state is None:
+            return 0
+
+        column_width = self._get_column_width(complete_state)
+        result = int(
+            column_width
+            * math.ceil(len(complete_state.completions) / float(self.min_rows))
+        )
+
+        # When the desired width is still more than the maximum available,
+        # reduce by removing columns until we are less than the available
+        # width.
+        while (
+            result > column_width
+            and result > max_available_width - self._required_margin
+        ):
+            result -= column_width
+        return result + self._required_margin
+
+    def preferred_height(
+        self,
+        width: int,
+        max_available_height: int,
+        wrap_lines: bool,
+        get_line_prefix: Optional[GetLinePrefixCallable],
+    ) -> Optional[int]:
+        """
+        Preferred height: as much as needed in order to display all the completions.
+        """
+        complete_state = get_app().current_buffer.complete_state
+        if complete_state is None:
+            return 0
+
+        column_width = self._get_column_width(complete_state)
+        column_count = max(1, (width - self._required_margin) // column_width)
+
+        return int(math.ceil(len(complete_state.completions) / float(column_count)))
+
+    def create_content(self, width: int, height: int) -> UIContent:
+        """
+        Create a UIContent object for this menu.
+        """
+        complete_state = get_app().current_buffer.complete_state
+        if complete_state is None:
+            return UIContent()
+
+        column_width = self._get_column_width(complete_state)
+        self._render_pos_to_completion = {}
+
+        _T = TypeVar("_T")
+
+        def grouper(
+            n: int, iterable: Iterable[_T], fillvalue: Optional[_T] = None
+        ) -> Iterable[Sequence[Optional[_T]]]:
+            "grouper(3, 'ABCDEFG', 'x') --> ABC DEF Gxx"
+            args = [iter(iterable)] * n
+            return zip_longest(fillvalue=fillvalue, *args)
+
+        def is_current_completion(completion: Completion) -> bool:
+            "Returns True when this completion is the currently selected one."
+            return (
+                complete_state is not None
+                and complete_state.complete_index is not None
+                and c == complete_state.current_completion
+            )
+
+        # Space required outside of the regular columns, for displaying the
+        # left and right arrow.
+        HORIZONTAL_MARGIN_REQUIRED = 3
+
+        # There should be at least one column, but it cannot be wider than
+        # the available width.
+        column_width = min(width - HORIZONTAL_MARGIN_REQUIRED, column_width)
+
+        # However, when the columns tend to be very wide, because there are
+        # some very wide entries, shrink it anyway.
+        if column_width > self.suggested_max_column_width:
+            # `column_width` can still be bigger that `suggested_max_column_width`,
+            # but if there is place for two columns, we divide by two.
+            column_width //= column_width // self.suggested_max_column_width
+
+        visible_columns = max(1, (width - self._required_margin) // column_width)
+
+        columns_ = list(grouper(height, complete_state.completions))
+        rows_ = list(zip(*columns_))
+
+        # Make sure the current completion is always visible: update scroll offset.
+        selected_column = (complete_state.complete_index or 0) // height
+        self.scroll = min(
+            selected_column, max(self.scroll, selected_column - visible_columns + 1)
+        )
+
+        render_left_arrow = self.scroll > 0
+        render_right_arrow = self.scroll < len(rows_[0]) - visible_columns
+
+        # Write completions to screen.
+        fragments_for_line = []
+
+        for row_index, row in enumerate(rows_):
+            fragments: StyleAndTextTuples = []
+            middle_row = row_index == len(rows_) // 2
+
+            # Draw left arrow if we have hidden completions on the left.
+            if render_left_arrow:
+                fragments.append(("class:scrollbar", "<" if middle_row else " "))
+            elif render_right_arrow:
+                # Reserve one column empty space. (If there is a right
+                # arrow right now, there can be a left arrow as well.)
+                fragments.append(("", " "))
+
+            # Draw row content.
+            for column_index, c in enumerate(row[self.scroll :][:visible_columns]):
+                if c is not None:
+                    fragments += _get_menu_item_fragments(
+                        c, is_current_completion(c), column_width, space_after=False
+                    )
+
+                    # Remember render position for mouse click handler.
+                    for x in range(column_width):
+                        self._render_pos_to_completion[
+                            (column_index * column_width + x, row_index)
+                        ] = c
+                else:
+                    fragments.append(("class:completion", " " * column_width))
+
+            # Draw trailing padding for this row.
+            # (_get_menu_item_fragments only returns padding on the left.)
+            if render_left_arrow or render_right_arrow:
+                fragments.append(("class:completion", " "))
+
+            # Draw right arrow if we have hidden completions on the right.
+            if render_right_arrow:
+                fragments.append(("class:scrollbar", ">" if middle_row else " "))
+            elif render_left_arrow:
+                fragments.append(("class:completion", " "))
+
+            # Add line.
+            fragments_for_line.append(
+                to_formatted_text(fragments, style="class:completion-menu")
+            )
+
+        self._rendered_rows = height
+        self._rendered_columns = visible_columns
+        self._total_columns = len(columns_)
+        self._render_left_arrow = render_left_arrow
+        self._render_right_arrow = render_right_arrow
+        self._render_width = (
+            column_width * visible_columns + render_left_arrow + render_right_arrow + 1
+        )
+
+        def get_line(i: int) -> StyleAndTextTuples:
+            return fragments_for_line[i]
+
+        return UIContent(get_line=get_line, line_count=len(rows_))
+
+    def _get_column_width(self, completion_state: CompletionState) -> int:
+        """
+        Return the width of each column.
+        """
+        try:
+            count, width = self._column_width_for_completion_state[completion_state]
+            if count != len(completion_state.completions):
+                # Number of completions changed, recompute.
+                raise KeyError
+            return width
+        except KeyError:
+            result = (
+                max(get_cwidth(c.display_text) for c in completion_state.completions)
+                + 1
+            )
+            self._column_width_for_completion_state[completion_state] = (
+                len(completion_state.completions),
+                result,
+            )
+            return result
+
+    def mouse_handler(self, mouse_event: MouseEvent) -> "NotImplementedOrNone":
+        """
+        Handle scroll and click events.
+        """
+        b = get_app().current_buffer
+
+        def scroll_left() -> None:
+            b.complete_previous(count=self._rendered_rows, disable_wrap_around=True)
+            self.scroll = max(0, self.scroll - 1)
+
+        def scroll_right() -> None:
+            b.complete_next(count=self._rendered_rows, disable_wrap_around=True)
+            self.scroll = min(
+                self._total_columns - self._rendered_columns, self.scroll + 1
+            )
+
+        if mouse_event.event_type == MouseEventType.SCROLL_DOWN:
+            scroll_right()
+
+        elif mouse_event.event_type == MouseEventType.SCROLL_UP:
+            scroll_left()
+
+        elif mouse_event.event_type == MouseEventType.MOUSE_UP:
+            x = mouse_event.position.x
+            y = mouse_event.position.y
+
+            # Mouse click on left arrow.
+            if x == 0:
+                if self._render_left_arrow:
+                    scroll_left()
+
+            # Mouse click on right arrow.
+            elif x == self._render_width - 1:
+                if self._render_right_arrow:
+                    scroll_right()
+
+            # Mouse click on completion.
+            else:
+                completion = self._render_pos_to_completion.get((x, y))
+                if completion:
+                    b.apply_completion(completion)
+
+        return None
+
+    def get_key_bindings(self) -> "KeyBindings":
+        """
+        Expose key bindings that handle the left/right arrow keys when the menu
+        is displayed.
+        """
+        from prompt_toolkit.key_binding.key_bindings import KeyBindings
+
+        kb = KeyBindings()
+
+        @Condition
+        def filter() -> bool:
+            "Only handle key bindings if this menu is visible."
+            app = get_app()
+            complete_state = app.current_buffer.complete_state
+
+            # There need to be completions, and one needs to be selected.
+            if complete_state is None or complete_state.complete_index is None:
+                return False
+
+            # This menu needs to be visible.
+            return any(window.content == self for window in app.layout.visible_windows)
+
+        def move(right: bool = False) -> None:
+            buff = get_app().current_buffer
+            complete_state = buff.complete_state
+
+            if complete_state is not None and complete_state.complete_index is not None:
+                # Calculate new complete index.
+                new_index = complete_state.complete_index
+                if right:
+                    new_index += self._rendered_rows
+                else:
+                    new_index -= self._rendered_rows
+
+                if 0 <= new_index < len(complete_state.completions):
+                    buff.go_to_completion(new_index)
+
+        # NOTE: the is_global is required because the completion menu will
+        #       never be focussed.
+
+        @kb.add("left", is_global=True, filter=filter)
+        def _left(event: E) -> None:
+            move()
+
+        @kb.add("right", is_global=True, filter=filter)
+        def _right(event: E) -> None:
+            move(True)
+
+        return kb
+
+
+class MultiColumnCompletionsMenu(HSplit):
+    """
+    Container that displays the completions in several columns.
+    When `show_meta` (a :class:`~prompt_toolkit.filters.Filter`) evaluates
+    to True, it shows the meta information at the bottom.
+    """
+
+    def __init__(
+        self,
+        min_rows: int = 3,
+        suggested_max_column_width: int = 30,
+        show_meta: FilterOrBool = True,
+        extra_filter: FilterOrBool = True,
+        z_index: int = 10**8,
+    ) -> None:
+
+        show_meta = to_filter(show_meta)
+        extra_filter = to_filter(extra_filter)
+
+        # Display filter: show when there are completions but not at the point
+        # we are returning the input.
+        full_filter = has_completions & ~is_done & extra_filter
+
+        @Condition
+        def any_completion_has_meta() -> bool:
+            complete_state = get_app().current_buffer.complete_state
+            return complete_state is not None and any(
+                c.display_meta for c in complete_state.completions
+            )
+
+        # Create child windows.
+        # NOTE: We don't set style='class:completion-menu' to the
+        #       `MultiColumnCompletionMenuControl`, because this is used in a
+        #       Float that is made transparent, and the size of the control
+        #       doesn't always correspond exactly with the size of the
+        #       generated content.
+        completions_window = ConditionalContainer(
+            content=Window(
+                content=MultiColumnCompletionMenuControl(
+                    min_rows=min_rows,
+                    suggested_max_column_width=suggested_max_column_width,
+                ),
+                width=Dimension(min=8),
+                height=Dimension(min=1),
+            ),
+            filter=full_filter,
+        )
+
+        meta_window = ConditionalContainer(
+            content=Window(content=_SelectedCompletionMetaControl()),
+            filter=show_meta & full_filter & any_completion_has_meta,
+        )
+
+        # Initialise split.
+        super().__init__([completions_window, meta_window], z_index=z_index)
+
+
+class _SelectedCompletionMetaControl(UIControl):
+    """
+    Control that shows the meta information of the selected completion.
+    """
+
+    def preferred_width(self, max_available_width: int) -> Optional[int]:
+        """
+        Report the width of the longest meta text as the preferred width of this control.
+
+        It could be that we use less width, but this way, we're sure that the
+        layout doesn't change when we select another completion (E.g. that
+        completions are suddenly shown in more or fewer columns.)
+        """
+        app = get_app()
+        if app.current_buffer.complete_state:
+            state = app.current_buffer.complete_state
+
+            if len(state.completions) >= 30:
+                # When there are many completions, calling `get_cwidth` for
+                # every `display_meta_text` is too expensive. In this case,
+                # just return the max available width. There will be enough
+                # columns anyway so that the whole screen is filled with
+                # completions and `create_content` will then take up as much
+                # space as needed.
+                return max_available_width
+
+            return 2 + max(
+                get_cwidth(c.display_meta_text) for c in state.completions[:100]
+            )
+        else:
+            return 0
+
+    def preferred_height(
+        self,
+        width: int,
+        max_available_height: int,
+        wrap_lines: bool,
+        get_line_prefix: Optional[GetLinePrefixCallable],
+    ) -> Optional[int]:
+        return 1
+
+    def create_content(self, width: int, height: int) -> UIContent:
+        fragments = self._get_text_fragments()
+
+        def get_line(i: int) -> StyleAndTextTuples:
+            return fragments
+
+        return UIContent(get_line=get_line, line_count=1 if fragments else 0)
+
+    def _get_text_fragments(self) -> StyleAndTextTuples:
+        style = "class:completion-menu.multi-column-meta"
+        state = get_app().current_buffer.complete_state
+
+        if (
+            state
+            and state.current_completion
+            and state.current_completion.display_meta_text
+        ):
+            return to_formatted_text(
+                cast(StyleAndTextTuples, [("", " ")])
+                + state.current_completion.display_meta
+                + [("", " ")],
+                style=style,
+            )
+
+        return []
diff --git a/src/prompt_toolkit/layout/mouse_handlers.py b/src/prompt_toolkit/layout/mouse_handlers.py
new file mode 100644
index 0000000..2562317
--- /dev/null
+++ b/src/prompt_toolkit/layout/mouse_handlers.py
@@ -0,0 +1,54 @@
+from collections import defaultdict
+from typing import TYPE_CHECKING, Callable, DefaultDict
+
+from prompt_toolkit.mouse_events import MouseEvent
+
+if TYPE_CHECKING:
+    from prompt_toolkit.key_binding.key_bindings import NotImplementedOrNone
+
+__all__ = [
+    "MouseHandler",
+    "MouseHandlers",
+]
+
+
+MouseHandler = Callable[[MouseEvent], "NotImplementedOrNone"]
+
+
+class MouseHandlers:
+    """
+    Two dimensional raster of callbacks for mouse events.
+    """
+
+    def __init__(self) -> None:
+        def dummy_callback(mouse_event: MouseEvent) -> "NotImplementedOrNone":
+            """
+            :param mouse_event: `MouseEvent` instance.
+            """
+            return NotImplemented
+
+        # NOTE: Previously, the data structure was a dictionary mapping (x,y)
+        # to the handlers. This however would be more inefficient when copying
+        # over the mouse handlers of the visible region in the scrollable pane.
+
+        # Map y (row) to x (column) to handlers.
+        self.mouse_handlers: DefaultDict[
+            int, DefaultDict[int, MouseHandler]
+        ] = defaultdict(lambda: defaultdict(lambda: dummy_callback))
+
+    def set_mouse_handler_for_range(
+        self,
+        x_min: int,
+        x_max: int,
+        y_min: int,
+        y_max: int,
+        handler: Callable[[MouseEvent], "NotImplementedOrNone"],
+    ) -> None:
+        """
+        Set mouse handler for a region.
+        """
+        for y in range(y_min, y_max):
+            row = self.mouse_handlers[y]
+
+            for x in range(x_min, x_max):
+                row[x] = handler
diff --git a/src/prompt_toolkit/layout/processors.py b/src/prompt_toolkit/layout/processors.py
new file mode 100644
index 0000000..722658a
--- /dev/null
+++ b/src/prompt_toolkit/layout/processors.py
@@ -0,0 +1,1029 @@
+"""
+Processors are little transformation blocks that transform the fragments list
+from a buffer before the BufferControl will render it to the screen.
+
+They can insert fragments before or after, or highlight fragments by replacing the
+fragment types.
+"""
+import re
+from abc import ABCMeta, abstractmethod
+from typing import (
+    TYPE_CHECKING,
+    Callable,
+    Hashable,
+    List,
+    Optional,
+    Tuple,
+    Type,
+    Union,
+    cast,
+)
+
+from prompt_toolkit.application.current import get_app
+from prompt_toolkit.cache import SimpleCache
+from prompt_toolkit.document import Document
+from prompt_toolkit.filters import FilterOrBool, to_filter, vi_insert_multiple_mode
+from prompt_toolkit.formatted_text import (
+    AnyFormattedText,
+    StyleAndTextTuples,
+    to_formatted_text,
+)
+from prompt_toolkit.formatted_text.utils import fragment_list_len, fragment_list_to_text
+from prompt_toolkit.search import SearchDirection
+from prompt_toolkit.utils import to_int, to_str
+
+from .utils import explode_text_fragments
+
+if TYPE_CHECKING:
+    from .controls import BufferControl, UIContent
+
+__all__ = [
+    "Processor",
+    "TransformationInput",
+    "Transformation",
+    "DummyProcessor",
+    "HighlightSearchProcessor",
+    "HighlightIncrementalSearchProcessor",
+    "HighlightSelectionProcessor",
+    "PasswordProcessor",
+    "HighlightMatchingBracketProcessor",
+    "DisplayMultipleCursors",
+    "BeforeInput",
+    "ShowArg",
+    "AfterInput",
+    "AppendAutoSuggestion",
+    "ConditionalProcessor",
+    "ShowLeadingWhiteSpaceProcessor",
+    "ShowTrailingWhiteSpaceProcessor",
+    "TabsProcessor",
+    "ReverseSearchProcessor",
+    "DynamicProcessor",
+    "merge_processors",
+]
+
+
+class Processor(metaclass=ABCMeta):
+    """
+    Manipulate the fragments for a given line in a
+    :class:`~prompt_toolkit.layout.controls.BufferControl`.
+    """
+
+    @abstractmethod
+    def apply_transformation(
+        self, transformation_input: "TransformationInput"
+    ) -> "Transformation":
+        """
+        Apply transformation. Returns a :class:`.Transformation` instance.
+
+        :param transformation_input: :class:`.TransformationInput` object.
+        """
+        return Transformation(transformation_input.fragments)
+
+
+SourceToDisplay = Callable[[int], int]
+DisplayToSource = Callable[[int], int]
+
+
+class TransformationInput:
+    """
+    :param buffer_control: :class:`.BufferControl` instance.
+    :param lineno: The number of the line to which we apply the processor.
+    :param source_to_display: A function that returns the position in the
+        `fragments` for any position in the source string. (This takes
+        previous processors into account.)
+    :param fragments: List of fragments that we can transform. (Received from the
+        previous processor.)
+    """
+
+    def __init__(
+        self,
+        buffer_control: "BufferControl",
+        document: Document,
+        lineno: int,
+        source_to_display: SourceToDisplay,
+        fragments: StyleAndTextTuples,
+        width: int,
+        height: int,
+    ) -> None:
+
+        self.buffer_control = buffer_control
+        self.document = document
+        self.lineno = lineno
+        self.source_to_display = source_to_display
+        self.fragments = fragments
+        self.width = width
+        self.height = height
+
+    def unpack(
+        self,
+    ) -> Tuple[
+        "BufferControl", Document, int, SourceToDisplay, StyleAndTextTuples, int, int
+    ]:
+        return (
+            self.buffer_control,
+            self.document,
+            self.lineno,
+            self.source_to_display,
+            self.fragments,
+            self.width,
+            self.height,
+        )
+
+
+class Transformation:
+    """
+    Transformation result, as returned by :meth:`.Processor.apply_transformation`.
+
+    Important: Always make sure that the length of `document.text` is equal to
+               the length of all the text in `fragments`!
+
+    :param fragments: The transformed fragments. To be displayed, or to pass to
+        the next processor.
+    :param source_to_display: Cursor position transformation from original
+        string to transformed string.
+    :param display_to_source: Cursor position transformed from source string to
+        original string.
+    """
+
+    def __init__(
+        self,
+        fragments: StyleAndTextTuples,
+        source_to_display: Optional[SourceToDisplay] = None,
+        display_to_source: Optional[DisplayToSource] = None,
+    ) -> None:
+
+        self.fragments = fragments
+        self.source_to_display = source_to_display or (lambda i: i)
+        self.display_to_source = display_to_source or (lambda i: i)
+
+
+class DummyProcessor(Processor):
+    """
+    A `Processor` that doesn't do anything.
+    """
+
+    def apply_transformation(
+        self, transformation_input: TransformationInput
+    ) -> Transformation:
+        return Transformation(transformation_input.fragments)
+
+
+class HighlightSearchProcessor(Processor):
+    """
+    Processor that highlights search matches in the document.
+    Note that this doesn't support multiline search matches yet.
+
+    The style classes 'search' and 'search.current' will be applied to the
+    content.
+    """
+
+    _classname = "search"
+    _classname_current = "search.current"
+
+    def _get_search_text(self, buffer_control: "BufferControl") -> str:
+        """
+        The text we are searching for.
+        """
+        return buffer_control.search_state.text
+
+    def apply_transformation(
+        self, transformation_input: TransformationInput
+    ) -> Transformation:
+
+        (
+            buffer_control,
+            document,
+            lineno,
+            source_to_display,
+            fragments,
+            _,
+            _,
+        ) = transformation_input.unpack()
+
+        search_text = self._get_search_text(buffer_control)
+        searchmatch_fragment = f" class:{self._classname} "
+        searchmatch_current_fragment = f" class:{self._classname_current} "
+
+        if search_text and not get_app().is_done:
+            # For each search match, replace the style string.
+            line_text = fragment_list_to_text(fragments)
+            fragments = explode_text_fragments(fragments)
+
+            if buffer_control.search_state.ignore_case():
+                flags = re.IGNORECASE
+            else:
+                flags = re.RegexFlag(0)
+
+            # Get cursor column.
+            cursor_column: Optional[int]
+            if document.cursor_position_row == lineno:
+                cursor_column = source_to_display(document.cursor_position_col)
+            else:
+                cursor_column = None
+
+            for match in re.finditer(re.escape(search_text), line_text, flags=flags):
+                if cursor_column is not None:
+                    on_cursor = match.start() <= cursor_column < match.end()
+                else:
+                    on_cursor = False
+
+                for i in range(match.start(), match.end()):
+                    old_fragment, text, *_ = fragments[i]
+                    if on_cursor:
+                        fragments[i] = (
+                            old_fragment + searchmatch_current_fragment,
+                            fragments[i][1],
+                        )
+                    else:
+                        fragments[i] = (
+                            old_fragment + searchmatch_fragment,
+                            fragments[i][1],
+                        )
+
+        return Transformation(fragments)
+
+
+class HighlightIncrementalSearchProcessor(HighlightSearchProcessor):
+    """
+    Highlight the search terms that are used for highlighting the incremental
+    search. The style class 'incsearch' will be applied to the content.
+
+    Important: this requires the `preview_search=True` flag to be set for the
+    `BufferControl`. Otherwise, the cursor position won't be set to the search
+    match while searching, and nothing happens.
+    """
+
+    _classname = "incsearch"
+    _classname_current = "incsearch.current"
+
+    def _get_search_text(self, buffer_control: "BufferControl") -> str:
+        """
+        The text we are searching for.
+        """
+        # When the search buffer has focus, take that text.
+        search_buffer = buffer_control.search_buffer
+        if search_buffer is not None and search_buffer.text:
+            return search_buffer.text
+        return ""
+
+
+class HighlightSelectionProcessor(Processor):
+    """
+    Processor that highlights the selection in the document.
+    """
+
+    def apply_transformation(
+        self, transformation_input: TransformationInput
+    ) -> Transformation:
+        (
+            buffer_control,
+            document,
+            lineno,
+            source_to_display,
+            fragments,
+            _,
+            _,
+        ) = transformation_input.unpack()
+
+        selected_fragment = " class:selected "
+
+        # In case of selection, highlight all matches.
+        selection_at_line = document.selection_range_at_line(lineno)
+
+        if selection_at_line:
+            from_, to = selection_at_line
+            from_ = source_to_display(from_)
+            to = source_to_display(to)
+
+            fragments = explode_text_fragments(fragments)
+
+            if from_ == 0 and to == 0 and len(fragments) == 0:
+                # When this is an empty line, insert a space in order to
+                # visualise the selection.
+                return Transformation([(selected_fragment, " ")])
+            else:
+                for i in range(from_, to):
+                    if i < len(fragments):
+                        old_fragment, old_text, *_ = fragments[i]
+                        fragments[i] = (old_fragment + selected_fragment, old_text)
+                    elif i == len(fragments):
+                        fragments.append((selected_fragment, " "))
+
+        return Transformation(fragments)
+
+
+class PasswordProcessor(Processor):
+    """
+    Processor that masks the input. (For passwords.)
+
+    :param char: (string) Character to be used. "*" by default.
+    """
+
+    def __init__(self, char: str = "*") -> None:
+        self.char = char
+
+    def apply_transformation(self, ti: TransformationInput) -> Transformation:
+        fragments: StyleAndTextTuples = cast(
+            StyleAndTextTuples,
+            [
+                (style, self.char * len(text), *handler)
+                for style, text, *handler in ti.fragments
+            ],
+        )
+
+        return Transformation(fragments)
+
+
+class HighlightMatchingBracketProcessor(Processor):
+    """
+    When the cursor is on or right after a bracket, it highlights the matching
+    bracket.
+
+    :param max_cursor_distance: Only highlight matching brackets when the
+        cursor is within this distance. (From inside a `Processor`, we can't
+        know which lines will be visible on the screen. But we also don't want
+        to scan the whole document for matching brackets on each key press, so
+        we limit to this value.)
+    """
+
+    _closing_braces = "])}>"
+
+    def __init__(
+        self, chars: str = "[](){}<>", max_cursor_distance: int = 1000
+    ) -> None:
+        self.chars = chars
+        self.max_cursor_distance = max_cursor_distance
+
+        self._positions_cache: SimpleCache[
+            Hashable, List[Tuple[int, int]]
+        ] = SimpleCache(maxsize=8)
+
+    def _get_positions_to_highlight(self, document: Document) -> List[Tuple[int, int]]:
+        """
+        Return a list of (row, col) tuples that need to be highlighted.
+        """
+        pos: Optional[int]
+
+        # Try for the character under the cursor.
+        if document.current_char and document.current_char in self.chars:
+            pos = document.find_matching_bracket_position(
+                start_pos=document.cursor_position - self.max_cursor_distance,
+                end_pos=document.cursor_position + self.max_cursor_distance,
+            )
+
+        # Try for the character before the cursor.
+        elif (
+            document.char_before_cursor
+            and document.char_before_cursor in self._closing_braces
+            and document.char_before_cursor in self.chars
+        ):
+            document = Document(document.text, document.cursor_position - 1)
+
+            pos = document.find_matching_bracket_position(
+                start_pos=document.cursor_position - self.max_cursor_distance,
+                end_pos=document.cursor_position + self.max_cursor_distance,
+            )
+        else:
+            pos = None
+
+        # Return a list of (row, col) tuples that need to be highlighted.
+        if pos:
+            pos += document.cursor_position  # pos is relative.
+            row, col = document.translate_index_to_position(pos)
+            return [
+                (row, col),
+                (document.cursor_position_row, document.cursor_position_col),
+            ]
+        else:
+            return []
+
+    def apply_transformation(
+        self, transformation_input: TransformationInput
+    ) -> Transformation:
+
+        (
+            buffer_control,
+            document,
+            lineno,
+            source_to_display,
+            fragments,
+            _,
+            _,
+        ) = transformation_input.unpack()
+
+        # When the application is in the 'done' state, don't highlight.
+        if get_app().is_done:
+            return Transformation(fragments)
+
+        # Get the highlight positions.
+        key = (get_app().render_counter, document.text, document.cursor_position)
+        positions = self._positions_cache.get(
+            key, lambda: self._get_positions_to_highlight(document)
+        )
+
+        # Apply if positions were found at this line.
+        if positions:
+            for row, col in positions:
+                if row == lineno:
+                    col = source_to_display(col)
+                    fragments = explode_text_fragments(fragments)
+                    style, text, *_ = fragments[col]
+
+                    if col == document.cursor_position_col:
+                        style += " class:matching-bracket.cursor "
+                    else:
+                        style += " class:matching-bracket.other "
+
+                    fragments[col] = (style, text)
+
+        return Transformation(fragments)
+
+
+class DisplayMultipleCursors(Processor):
+    """
+    When we're in Vi block insert mode, display all the cursors.
+    """
+
+    def apply_transformation(
+        self, transformation_input: TransformationInput
+    ) -> Transformation:
+
+        (
+            buffer_control,
+            document,
+            lineno,
+            source_to_display,
+            fragments,
+            _,
+            _,
+        ) = transformation_input.unpack()
+
+        buff = buffer_control.buffer
+
+        if vi_insert_multiple_mode():
+            cursor_positions = buff.multiple_cursor_positions
+            fragments = explode_text_fragments(fragments)
+
+            # If any cursor appears on the current line, highlight that.
+            start_pos = document.translate_row_col_to_index(lineno, 0)
+            end_pos = start_pos + len(document.lines[lineno])
+
+            fragment_suffix = " class:multiple-cursors"
+
+            for p in cursor_positions:
+                if start_pos <= p <= end_pos:
+                    column = source_to_display(p - start_pos)
+
+                    # Replace fragment.
+                    try:
+                        style, text, *_ = fragments[column]
+                    except IndexError:
+                        # Cursor needs to be displayed after the current text.
+                        fragments.append((fragment_suffix, " "))
+                    else:
+                        style += fragment_suffix
+                        fragments[column] = (style, text)
+
+            return Transformation(fragments)
+        else:
+            return Transformation(fragments)
+
+
+class BeforeInput(Processor):
+    """
+    Insert text before the input.
+
+    :param text: This can be either plain text or formatted text
+        (or a callable that returns any of those).
+    :param style: style to be applied to this prompt/prefix.
+    """
+
+    def __init__(self, text: AnyFormattedText, style: str = "") -> None:
+        self.text = text
+        self.style = style
+
+    def apply_transformation(self, ti: TransformationInput) -> Transformation:
+        source_to_display: Optional[SourceToDisplay]
+        display_to_source: Optional[DisplayToSource]
+
+        if ti.lineno == 0:
+            # Get fragments.
+            fragments_before = to_formatted_text(self.text, self.style)
+            fragments = fragments_before + ti.fragments
+
+            shift_position = fragment_list_len(fragments_before)
+            source_to_display = lambda i: i + shift_position
+            display_to_source = lambda i: i - shift_position
+        else:
+            fragments = ti.fragments
+            source_to_display = None
+            display_to_source = None
+
+        return Transformation(
+            fragments,
+            source_to_display=source_to_display,
+            display_to_source=display_to_source,
+        )
+
+    def __repr__(self) -> str:
+        return f"BeforeInput({self.text!r}, {self.style!r})"
+
+
+class ShowArg(BeforeInput):
+    """
+    Display the 'arg' in front of the input.
+
+    This was used by the `PromptSession`, but now it uses the
+    `Window.get_line_prefix` function instead.
+    """
+
+    def __init__(self) -> None:
+        super().__init__(self._get_text_fragments)
+
+    def _get_text_fragments(self) -> StyleAndTextTuples:
+        app = get_app()
+        if app.key_processor.arg is None:
+            return []
+        else:
+            arg = app.key_processor.arg
+
+            return [
+                ("class:prompt.arg", "(arg: "),
+                ("class:prompt.arg.text", str(arg)),
+                ("class:prompt.arg", ") "),
+            ]
+
+    def __repr__(self) -> str:
+        return "ShowArg()"
+
+
+class AfterInput(Processor):
+    """
+    Insert text after the input.
+
+    :param text: This can be either plain text or formatted text
+        (or a callable that returns any of those).
+    :param style: style to be applied to this prompt/prefix.
+    """
+
+    def __init__(self, text: AnyFormattedText, style: str = "") -> None:
+        self.text = text
+        self.style = style
+
+    def apply_transformation(self, ti: TransformationInput) -> Transformation:
+        # Insert fragments after the last line.
+        if ti.lineno == ti.document.line_count - 1:
+            # Get fragments.
+            fragments_after = to_formatted_text(self.text, self.style)
+            return Transformation(fragments=ti.fragments + fragments_after)
+        else:
+            return Transformation(fragments=ti.fragments)
+
+    def __repr__(self) -> str:
+        return f"{self.__class__.__name__}({self.text!r}, style={self.style!r})"
+
+
+class AppendAutoSuggestion(Processor):
+    """
+    Append the auto suggestion to the input.
+    (The user can then press the right arrow the insert the suggestion.)
+    """
+
+    def __init__(self, style: str = "class:auto-suggestion") -> None:
+        self.style = style
+
+    def apply_transformation(self, ti: TransformationInput) -> Transformation:
+        # Insert fragments after the last line.
+        if ti.lineno == ti.document.line_count - 1:
+            buffer = ti.buffer_control.buffer
+
+            if buffer.suggestion and ti.document.is_cursor_at_the_end:
+                suggestion = buffer.suggestion.text
+            else:
+                suggestion = ""
+
+            return Transformation(fragments=ti.fragments + [(self.style, suggestion)])
+        else:
+            return Transformation(fragments=ti.fragments)
+
+
+class ShowLeadingWhiteSpaceProcessor(Processor):
+    """
+    Make leading whitespace visible.
+
+    :param get_char: Callable that returns one character.
+    """
+
+    def __init__(
+        self,
+        get_char: Optional[Callable[[], str]] = None,
+        style: str = "class:leading-whitespace",
+    ) -> None:
+        def default_get_char() -> str:
+            if "\xb7".encode(get_app().output.encoding(), "replace") == b"?":
+                return "."
+            else:
+                return "\xb7"
+
+        self.style = style
+        self.get_char = get_char or default_get_char
+
+    def apply_transformation(self, ti: TransformationInput) -> Transformation:
+        fragments = ti.fragments
+
+        # Walk through all te fragments.
+        if fragments and fragment_list_to_text(fragments).startswith(" "):
+            t = (self.style, self.get_char())
+            fragments = explode_text_fragments(fragments)
+
+            for i in range(len(fragments)):
+                if fragments[i][1] == " ":
+                    fragments[i] = t
+                else:
+                    break
+
+        return Transformation(fragments)
+
+
+class ShowTrailingWhiteSpaceProcessor(Processor):
+    """
+    Make trailing whitespace visible.
+
+    :param get_char: Callable that returns one character.
+    """
+
+    def __init__(
+        self,
+        get_char: Optional[Callable[[], str]] = None,
+        style: str = "class:training-whitespace",
+    ) -> None:
+        def default_get_char() -> str:
+            if "\xb7".encode(get_app().output.encoding(), "replace") == b"?":
+                return "."
+            else:
+                return "\xb7"
+
+        self.style = style
+        self.get_char = get_char or default_get_char
+
+    def apply_transformation(self, ti: TransformationInput) -> Transformation:
+        fragments = ti.fragments
+
+        if fragments and fragments[-1][1].endswith(" "):
+            t = (self.style, self.get_char())
+            fragments = explode_text_fragments(fragments)
+
+            # Walk backwards through all te fragments and replace whitespace.
+            for i in range(len(fragments) - 1, -1, -1):
+                char = fragments[i][1]
+                if char == " ":
+                    fragments[i] = t
+                else:
+                    break
+
+        return Transformation(fragments)
+
+
+class TabsProcessor(Processor):
+    """
+    Render tabs as spaces (instead of ^I) or make them visible (for instance,
+    by replacing them with dots.)
+
+    :param tabstop: Horizontal space taken by a tab. (`int` or callable that
+        returns an `int`).
+    :param char1: Character or callable that returns a character (text of
+        length one). This one is used for the first space taken by the tab.
+    :param char2: Like `char1`, but for the rest of the space.
+    """
+
+    def __init__(
+        self,
+        tabstop: Union[int, Callable[[], int]] = 4,
+        char1: Union[str, Callable[[], str]] = "|",
+        char2: Union[str, Callable[[], str]] = "\u2508",
+        style: str = "class:tab",
+    ) -> None:
+
+        self.char1 = char1
+        self.char2 = char2
+        self.tabstop = tabstop
+        self.style = style
+
+    def apply_transformation(self, ti: TransformationInput) -> Transformation:
+        tabstop = to_int(self.tabstop)
+        style = self.style
+
+        # Create separator for tabs.
+        separator1 = to_str(self.char1)
+        separator2 = to_str(self.char2)
+
+        # Transform fragments.
+        fragments = explode_text_fragments(ti.fragments)
+
+        position_mappings = {}
+        result_fragments: StyleAndTextTuples = []
+        pos = 0
+
+        for i, fragment_and_text in enumerate(fragments):
+            position_mappings[i] = pos
+
+            if fragment_and_text[1] == "\t":
+                # Calculate how many characters we have to insert.
+                count = tabstop - (pos % tabstop)
+                if count == 0:
+                    count = tabstop
+
+                # Insert tab.
+                result_fragments.append((style, separator1))
+                result_fragments.append((style, separator2 * (count - 1)))
+                pos += count
+            else:
+                result_fragments.append(fragment_and_text)
+                pos += 1
+
+        position_mappings[len(fragments)] = pos
+        # Add `pos+1` to mapping, because the cursor can be right after the
+        # line as well.
+        position_mappings[len(fragments) + 1] = pos + 1
+
+        def source_to_display(from_position: int) -> int:
+            "Maps original cursor position to the new one."
+            return position_mappings[from_position]
+
+        def display_to_source(display_pos: int) -> int:
+            "Maps display cursor position to the original one."
+            position_mappings_reversed = {v: k for k, v in position_mappings.items()}
+
+            while display_pos >= 0:
+                try:
+                    return position_mappings_reversed[display_pos]
+                except KeyError:
+                    display_pos -= 1
+            return 0
+
+        return Transformation(
+            result_fragments,
+            source_to_display=source_to_display,
+            display_to_source=display_to_source,
+        )
+
+
+class ReverseSearchProcessor(Processor):
+    """
+    Process to display the "(reverse-i-search)`...`:..." stuff around
+    the search buffer.
+
+    Note: This processor is meant to be applied to the BufferControl that
+    contains the search buffer, it's not meant for the original input.
+    """
+
+    _excluded_input_processors: List[Type[Processor]] = [
+        HighlightSearchProcessor,
+        HighlightSelectionProcessor,
+        BeforeInput,
+        AfterInput,
+    ]
+
+    def _get_main_buffer(
+        self, buffer_control: "BufferControl"
+    ) -> Optional["BufferControl"]:
+        from prompt_toolkit.layout.controls import BufferControl
+
+        prev_control = get_app().layout.search_target_buffer_control
+        if (
+            isinstance(prev_control, BufferControl)
+            and prev_control.search_buffer_control == buffer_control
+        ):
+            return prev_control
+        return None
+
+    def _content(
+        self, main_control: "BufferControl", ti: TransformationInput
+    ) -> "UIContent":
+        from prompt_toolkit.layout.controls import BufferControl
+
+        # Emulate the BufferControl through which we are searching.
+        # For this we filter out some of the input processors.
+        excluded_processors = tuple(self._excluded_input_processors)
+
+        def filter_processor(item: Processor) -> Optional[Processor]:
+            """Filter processors from the main control that we want to disable
+            here. This returns either an accepted processor or None."""
+            # For a `_MergedProcessor`, check each individual processor, recursively.
+            if isinstance(item, _MergedProcessor):
+                accepted_processors = [filter_processor(p) for p in item.processors]
+                return merge_processors(
+                    [p for p in accepted_processors if p is not None]
+                )
+
+            # For a `ConditionalProcessor`, check the body.
+            elif isinstance(item, ConditionalProcessor):
+                p = filter_processor(item.processor)
+                if p:
+                    return ConditionalProcessor(p, item.filter)
+
+            # Otherwise, check the processor itself.
+            else:
+                if not isinstance(item, excluded_processors):
+                    return item
+
+            return None
+
+        filtered_processor = filter_processor(
+            merge_processors(main_control.input_processors or [])
+        )
+        highlight_processor = HighlightIncrementalSearchProcessor()
+
+        if filtered_processor:
+            new_processors = [filtered_processor, highlight_processor]
+        else:
+            new_processors = [highlight_processor]
+
+        from .controls import SearchBufferControl
+
+        assert isinstance(ti.buffer_control, SearchBufferControl)
+
+        buffer_control = BufferControl(
+            buffer=main_control.buffer,
+            input_processors=new_processors,
+            include_default_input_processors=False,
+            lexer=main_control.lexer,
+            preview_search=True,
+            search_buffer_control=ti.buffer_control,
+        )
+
+        return buffer_control.create_content(ti.width, ti.height, preview_search=True)
+
+    def apply_transformation(self, ti: TransformationInput) -> Transformation:
+        from .controls import SearchBufferControl
+
+        assert isinstance(
+            ti.buffer_control, SearchBufferControl
+        ), "`ReverseSearchProcessor` should be applied to a `SearchBufferControl` only."
+
+        source_to_display: Optional[SourceToDisplay]
+        display_to_source: Optional[DisplayToSource]
+
+        main_control = self._get_main_buffer(ti.buffer_control)
+
+        if ti.lineno == 0 and main_control:
+            content = self._content(main_control, ti)
+
+            # Get the line from the original document for this search.
+            line_fragments = content.get_line(content.cursor_position.y)
+
+            if main_control.search_state.direction == SearchDirection.FORWARD:
+                direction_text = "i-search"
+            else:
+                direction_text = "reverse-i-search"
+
+            fragments_before: StyleAndTextTuples = [
+                ("class:prompt.search", "("),
+                ("class:prompt.search", direction_text),
+                ("class:prompt.search", ")`"),
+            ]
+
+            fragments = (
+                fragments_before
+                + [
+                    ("class:prompt.search.text", fragment_list_to_text(ti.fragments)),
+                    ("", "': "),
+                ]
+                + line_fragments
+            )
+
+            shift_position = fragment_list_len(fragments_before)
+            source_to_display = lambda i: i + shift_position
+            display_to_source = lambda i: i - shift_position
+        else:
+            source_to_display = None
+            display_to_source = None
+            fragments = ti.fragments
+
+        return Transformation(
+            fragments,
+            source_to_display=source_to_display,
+            display_to_source=display_to_source,
+        )
+
+
+class ConditionalProcessor(Processor):
+    """
+    Processor that applies another processor, according to a certain condition.
+    Example::
+
+        # Create a function that returns whether or not the processor should
+        # currently be applied.
+        def highlight_enabled():
+            return true_or_false
+
+        # Wrapped it in a `ConditionalProcessor` for usage in a `BufferControl`.
+        BufferControl(input_processors=[
+            ConditionalProcessor(HighlightSearchProcessor(),
+                                 Condition(highlight_enabled))])
+
+    :param processor: :class:`.Processor` instance.
+    :param filter: :class:`~prompt_toolkit.filters.Filter` instance.
+    """
+
+    def __init__(self, processor: Processor, filter: FilterOrBool) -> None:
+        self.processor = processor
+        self.filter = to_filter(filter)
+
+    def apply_transformation(
+        self, transformation_input: TransformationInput
+    ) -> Transformation:
+        # Run processor when enabled.
+        if self.filter():
+            return self.processor.apply_transformation(transformation_input)
+        else:
+            return Transformation(transformation_input.fragments)
+
+    def __repr__(self) -> str:
+        return "{}(processor={!r}, filter={!r})".format(
+            self.__class__.__name__,
+            self.processor,
+            self.filter,
+        )
+
+
+class DynamicProcessor(Processor):
+    """
+    Processor class that dynamically returns any Processor.
+
+    :param get_processor: Callable that returns a :class:`.Processor` instance.
+    """
+
+    def __init__(self, get_processor: Callable[[], Optional[Processor]]) -> None:
+        self.get_processor = get_processor
+
+    def apply_transformation(self, ti: TransformationInput) -> Transformation:
+        processor = self.get_processor() or DummyProcessor()
+        return processor.apply_transformation(ti)
+
+
+def merge_processors(processors: List[Processor]) -> Processor:
+    """
+    Merge multiple `Processor` objects into one.
+    """
+    if len(processors) == 0:
+        return DummyProcessor()
+
+    if len(processors) == 1:
+        return processors[0]  # Nothing to merge.
+
+    return _MergedProcessor(processors)
+
+
+class _MergedProcessor(Processor):
+    """
+    Processor that groups multiple other `Processor` objects, but exposes an
+    API as if it is one `Processor`.
+    """
+
+    def __init__(self, processors: List[Processor]):
+        self.processors = processors
+
+    def apply_transformation(self, ti: TransformationInput) -> Transformation:
+        source_to_display_functions = [ti.source_to_display]
+        display_to_source_functions = []
+        fragments = ti.fragments
+
+        def source_to_display(i: int) -> int:
+            """Translate x position from the buffer to the x position in the
+            processor fragments list."""
+            for f in source_to_display_functions:
+                i = f(i)
+            return i
+
+        for p in self.processors:
+            transformation = p.apply_transformation(
+                TransformationInput(
+                    ti.buffer_control,
+                    ti.document,
+                    ti.lineno,
+                    source_to_display,
+                    fragments,
+                    ti.width,
+                    ti.height,
+                )
+            )
+            fragments = transformation.fragments
+            display_to_source_functions.append(transformation.display_to_source)
+            source_to_display_functions.append(transformation.source_to_display)
+
+        def display_to_source(i: int) -> int:
+            for f in reversed(display_to_source_functions):
+                i = f(i)
+            return i
+
+        # In the case of a nested _MergedProcessor, each processor wants to
+        # receive a 'source_to_display' function (as part of the
+        # TransformationInput) that has everything in the chain before
+        # included, because it can be called as part of the
+        # `apply_transformation` function. However, this first
+        # `source_to_display` should not be part of the output that we are
+        # returning. (This is the most consistent with `display_to_source`.)
+        del source_to_display_functions[:1]
+
+        return Transformation(fragments, source_to_display, display_to_source)
diff --git a/src/prompt_toolkit/layout/screen.py b/src/prompt_toolkit/layout/screen.py
new file mode 100644
index 0000000..2b58272
--- /dev/null
+++ b/src/prompt_toolkit/layout/screen.py
@@ -0,0 +1,328 @@
+from collections import defaultdict
+from typing import TYPE_CHECKING, Callable, DefaultDict, Dict, List, Optional, Tuple
+
+from prompt_toolkit.cache import FastDictCache
+from prompt_toolkit.data_structures import Point
+from prompt_toolkit.utils import get_cwidth
+
+if TYPE_CHECKING:
+    from .containers import Window
+
+
+__all__ = [
+    "Screen",
+    "Char",
+]
+
+
+class Char:
+    """
+    Represent a single character in a :class:`.Screen`.
+
+    This should be considered immutable.
+
+    :param char: A single character (can be a double-width character).
+    :param style: A style string. (Can contain classnames.)
+    """
+
+    __slots__ = ("char", "style", "width")
+
+    # If we end up having one of these special control sequences in the input string,
+    # we should display them as follows:
+    # Usually this happens after a "quoted insert".
+    display_mappings: Dict[str, str] = {
+        "\x00": "^@",  # Control space
+        "\x01": "^A",
+        "\x02": "^B",
+        "\x03": "^C",
+        "\x04": "^D",
+        "\x05": "^E",
+        "\x06": "^F",
+        "\x07": "^G",
+        "\x08": "^H",
+        "\x09": "^I",
+        "\x0a": "^J",
+        "\x0b": "^K",
+        "\x0c": "^L",
+        "\x0d": "^M",
+        "\x0e": "^N",
+        "\x0f": "^O",
+        "\x10": "^P",
+        "\x11": "^Q",
+        "\x12": "^R",
+        "\x13": "^S",
+        "\x14": "^T",
+        "\x15": "^U",
+        "\x16": "^V",
+        "\x17": "^W",
+        "\x18": "^X",
+        "\x19": "^Y",
+        "\x1a": "^Z",
+        "\x1b": "^[",  # Escape
+        "\x1c": "^\\",
+        "\x1d": "^]",
+        "\x1e": "^^",
+        "\x1f": "^_",
+        "\x7f": "^?",  # ASCII Delete (backspace).
+        # Special characters. All visualized like Vim does.
+        "\x80": "<80>",
+        "\x81": "<81>",
+        "\x82": "<82>",
+        "\x83": "<83>",
+        "\x84": "<84>",
+        "\x85": "<85>",
+        "\x86": "<86>",
+        "\x87": "<87>",
+        "\x88": "<88>",
+        "\x89": "<89>",
+        "\x8a": "<8a>",
+        "\x8b": "<8b>",
+        "\x8c": "<8c>",
+        "\x8d": "<8d>",
+        "\x8e": "<8e>",
+        "\x8f": "<8f>",
+        "\x90": "<90>",
+        "\x91": "<91>",
+        "\x92": "<92>",
+        "\x93": "<93>",
+        "\x94": "<94>",
+        "\x95": "<95>",
+        "\x96": "<96>",
+        "\x97": "<97>",
+        "\x98": "<98>",
+        "\x99": "<99>",
+        "\x9a": "<9a>",
+        "\x9b": "<9b>",
+        "\x9c": "<9c>",
+        "\x9d": "<9d>",
+        "\x9e": "<9e>",
+        "\x9f": "<9f>",
+        # For the non-breaking space: visualize like Emacs does by default.
+        # (Print a space, but attach the 'nbsp' class that applies the
+        # underline style.)
+        "\xa0": " ",
+    }
+
+    def __init__(self, char: str = " ", style: str = "") -> None:
+        # If this character has to be displayed otherwise, take that one.
+        if char in self.display_mappings:
+            if char == "\xa0":
+                style += " class:nbsp "  # Will be underlined.
+            else:
+                style += " class:control-character "
+
+            char = self.display_mappings[char]
+
+        self.char = char
+        self.style = style
+
+        # Calculate width. (We always need this, so better to store it directly
+        # as a member for performance.)
+        self.width = get_cwidth(char)
+
+    # In theory, `other` can be any type of object, but because of performance
+    # we don't want to do an `isinstance` check every time. We assume "other"
+    # is always a "Char".
+    def _equal(self, other: "Char") -> bool:
+        return self.char == other.char and self.style == other.style
+
+    def _not_equal(self, other: "Char") -> bool:
+        # Not equal: We don't do `not char.__eq__` here, because of the
+        # performance of calling yet another function.
+        return self.char != other.char or self.style != other.style
+
+    if not TYPE_CHECKING:
+        __eq__ = _equal
+        __ne__ = _not_equal
+
+    def __repr__(self) -> str:
+        return f"{self.__class__.__name__}({self.char!r}, {self.style!r})"
+
+
+_CHAR_CACHE: FastDictCache[Tuple[str, str], Char] = FastDictCache(
+    Char, size=1000 * 1000
+)
+Transparent = "[transparent]"
+
+
+class Screen:
+    """
+    Two dimensional buffer of :class:`.Char` instances.
+    """
+
+    def __init__(
+        self,
+        default_char: Optional[Char] = None,
+        initial_width: int = 0,
+        initial_height: int = 0,
+    ) -> None:
+
+        if default_char is None:
+            default_char2 = _CHAR_CACHE[" ", Transparent]
+        else:
+            default_char2 = default_char
+
+        self.data_buffer: DefaultDict[int, DefaultDict[int, Char]] = defaultdict(
+            lambda: defaultdict(lambda: default_char2)
+        )
+
+        #: Escape sequences to be injected.
+        self.zero_width_escapes: DefaultDict[int, DefaultDict[int, str]] = defaultdict(
+            lambda: defaultdict(lambda: "")
+        )
+
+        #: Position of the cursor.
+        self.cursor_positions: Dict[
+            "Window", Point
+        ] = {}  # Map `Window` objects to `Point` objects.
+
+        #: Visibility of the cursor.
+        self.show_cursor = True
+
+        #: (Optional) Where to position the menu. E.g. at the start of a completion.
+        #: (We can't use the cursor position, because we don't want the
+        #: completion menu to change its position when we browse through all the
+        #: completions.)
+        self.menu_positions: Dict[
+            "Window", Point
+        ] = {}  # Map `Window` objects to `Point` objects.
+
+        #: Currently used width/height of the screen. This will increase when
+        #: data is written to the screen.
+        self.width = initial_width or 0
+        self.height = initial_height or 0
+
+        # Windows that have been drawn. (Each `Window` class will add itself to
+        # this list.)
+        self.visible_windows_to_write_positions: Dict["Window", "WritePosition"] = {}
+
+        # List of (z_index, draw_func)
+        self._draw_float_functions: List[Tuple[int, Callable[[], None]]] = []
+
+    @property
+    def visible_windows(self) -> List["Window"]:
+        return list(self.visible_windows_to_write_positions.keys())
+
+    def set_cursor_position(self, window: "Window", position: Point) -> None:
+        """
+        Set the cursor position for a given window.
+        """
+        self.cursor_positions[window] = position
+
+    def set_menu_position(self, window: "Window", position: Point) -> None:
+        """
+        Set the cursor position for a given window.
+        """
+        self.menu_positions[window] = position
+
+    def get_cursor_position(self, window: "Window") -> Point:
+        """
+        Get the cursor position for a given window.
+        Returns a `Point`.
+        """
+        try:
+            return self.cursor_positions[window]
+        except KeyError:
+            return Point(x=0, y=0)
+
+    def get_menu_position(self, window: "Window") -> Point:
+        """
+        Get the menu position for a given window.
+        (This falls back to the cursor position if no menu position was set.)
+        """
+        try:
+            return self.menu_positions[window]
+        except KeyError:
+            try:
+                return self.cursor_positions[window]
+            except KeyError:
+                return Point(x=0, y=0)
+
+    def draw_with_z_index(self, z_index: int, draw_func: Callable[[], None]) -> None:
+        """
+        Add a draw-function for a `Window` which has a >= 0 z_index.
+        This will be postponed until `draw_all_floats` is called.
+        """
+        self._draw_float_functions.append((z_index, draw_func))
+
+    def draw_all_floats(self) -> None:
+        """
+        Draw all float functions in order of z-index.
+        """
+        # We keep looping because some draw functions could add new functions
+        # to this list. See `FloatContainer`.
+        while self._draw_float_functions:
+            # Sort the floats that we have so far by z_index.
+            functions = sorted(self._draw_float_functions, key=lambda item: item[0])
+
+            # Draw only one at a time, then sort everything again. Now floats
+            # might have been added.
+            self._draw_float_functions = functions[1:]
+            functions[0][1]()
+
+    def append_style_to_content(self, style_str: str) -> None:
+        """
+        For all the characters in the screen.
+        Set the style string to the given `style_str`.
+        """
+        b = self.data_buffer
+        char_cache = _CHAR_CACHE
+
+        append_style = " " + style_str
+
+        for y, row in b.items():
+            for x, char in row.items():
+                row[x] = char_cache[char.char, char.style + append_style]
+
+    def fill_area(
+        self, write_position: "WritePosition", style: str = "", after: bool = False
+    ) -> None:
+        """
+        Fill the content of this area, using the given `style`.
+        The style is prepended before whatever was here before.
+        """
+        if not style.strip():
+            return
+
+        xmin = write_position.xpos
+        xmax = write_position.xpos + write_position.width
+        char_cache = _CHAR_CACHE
+        data_buffer = self.data_buffer
+
+        if after:
+            append_style = " " + style
+            prepend_style = ""
+        else:
+            append_style = ""
+            prepend_style = style + " "
+
+        for y in range(
+            write_position.ypos, write_position.ypos + write_position.height
+        ):
+            row = data_buffer[y]
+            for x in range(xmin, xmax):
+                cell = row[x]
+                row[x] = char_cache[
+                    cell.char, prepend_style + cell.style + append_style
+                ]
+
+
+class WritePosition:
+    def __init__(self, xpos: int, ypos: int, width: int, height: int) -> None:
+        assert height >= 0
+        assert width >= 0
+        # xpos and ypos can be negative. (A float can be partially visible.)
+
+        self.xpos = xpos
+        self.ypos = ypos
+        self.width = width
+        self.height = height
+
+    def __repr__(self) -> str:
+        return "{}(x={!r}, y={!r}, width={!r}, height={!r})".format(
+            self.__class__.__name__,
+            self.xpos,
+            self.ypos,
+            self.width,
+            self.height,
+        )
diff --git a/src/prompt_toolkit/layout/scrollable_pane.py b/src/prompt_toolkit/layout/scrollable_pane.py
new file mode 100644
index 0000000..810e405
--- /dev/null
+++ b/src/prompt_toolkit/layout/scrollable_pane.py
@@ -0,0 +1,494 @@
+from typing import Dict, List, Optional
+
+from prompt_toolkit.data_structures import Point
+from prompt_toolkit.filters import FilterOrBool, to_filter
+from prompt_toolkit.key_binding import KeyBindingsBase
+from prompt_toolkit.mouse_events import MouseEvent
+
+from .containers import Container, ScrollOffsets
+from .dimension import AnyDimension, Dimension, sum_layout_dimensions, to_dimension
+from .mouse_handlers import MouseHandler, MouseHandlers
+from .screen import Char, Screen, WritePosition
+
+__all__ = ["ScrollablePane"]
+
+# Never go beyond this height, because performance will degrade.
+MAX_AVAILABLE_HEIGHT = 10_000
+
+
+class ScrollablePane(Container):
+    """
+    Container widget that exposes a larger virtual screen to its content and
+    displays it in a vertical scrollbale region.
+
+    Typically this is wrapped in a large `HSplit` container. Make sure in that
+    case to not specify a `height` dimension of the `HSplit`, so that it will
+    scale according to the content.
+
+    .. note::
+
+        If you want to display a completion menu for widgets in this
+        `ScrollablePane`, then it's still a good practice to use a
+        `FloatContainer` with a `CompletionsMenu` in a `Float` at the top-level
+        of the layout hierarchy, rather then nesting a `FloatContainer` in this
+        `ScrollablePane`. (Otherwise, it's possible that the completion menu
+        is clipped.)
+
+    :param content: The content container.
+    :param scrolloffset: Try to keep the cursor within this distance from the
+        top/bottom (left/right offset is not used).
+    :param keep_cursor_visible: When `True`, automatically scroll the pane so
+        that the cursor (of the focused window) is always visible.
+    :param keep_focused_window_visible: When `True`, automatically scroll th e
+        pane so that the focused window is visible, or as much visible as
+        possible if it doen't completely fit the screen.
+    :param max_available_height: Always constraint the height to this amount
+        for performance reasons.
+    :param width: When given, use this width instead of looking at the children.
+    :param height: When given, use this height instead of looking at the children.
+    :param show_scrollbar: When `True` display a scrollbar on the right.
+    """
+
+    def __init__(
+        self,
+        content: Container,
+        scroll_offsets: Optional[ScrollOffsets] = None,
+        keep_cursor_visible: FilterOrBool = True,
+        keep_focused_window_visible: FilterOrBool = True,
+        max_available_height: int = MAX_AVAILABLE_HEIGHT,
+        width: AnyDimension = None,
+        height: AnyDimension = None,
+        show_scrollbar: FilterOrBool = True,
+        display_arrows: FilterOrBool = True,
+        up_arrow_symbol: str = "^",
+        down_arrow_symbol: str = "v",
+    ) -> None:
+        self.content = content
+        self.scroll_offsets = scroll_offsets or ScrollOffsets(top=1, bottom=1)
+        self.keep_cursor_visible = to_filter(keep_cursor_visible)
+        self.keep_focused_window_visible = to_filter(keep_focused_window_visible)
+        self.max_available_height = max_available_height
+        self.width = width
+        self.height = height
+        self.show_scrollbar = to_filter(show_scrollbar)
+        self.display_arrows = to_filter(display_arrows)
+        self.up_arrow_symbol = up_arrow_symbol
+        self.down_arrow_symbol = down_arrow_symbol
+
+        self.vertical_scroll = 0
+
+    def __repr__(self) -> str:
+        return f"ScrollablePane({self.content!r})"
+
+    def reset(self) -> None:
+        self.content.reset()
+
+    def preferred_width(self, max_available_width: int) -> Dimension:
+        if self.width is not None:
+            return to_dimension(self.width)
+
+        # We're only scrolling vertical. So the preferred width is equal to
+        # that of the content.
+        content_width = self.content.preferred_width(max_available_width)
+
+        # If a scrollbar needs to be displayed, add +1 to the content width.
+        if self.show_scrollbar():
+            return sum_layout_dimensions([Dimension.exact(1), content_width])
+
+        return content_width
+
+    def preferred_height(self, width: int, max_available_height: int) -> Dimension:
+        if self.height is not None:
+            return to_dimension(self.height)
+
+        # Prefer a height large enough so that it fits all the content. If not,
+        # we'll make the pane scrollable.
+        if self.show_scrollbar():
+            # If `show_scrollbar` is set. Always reserve space for the scrollbar.
+            width -= 1
+
+        dimension = self.content.preferred_height(width, self.max_available_height)
+
+        # Only take 'preferred' into account. Min/max can be anything.
+        return Dimension(min=0, preferred=dimension.preferred)
+
+    def write_to_screen(
+        self,
+        screen: Screen,
+        mouse_handlers: MouseHandlers,
+        write_position: WritePosition,
+        parent_style: str,
+        erase_bg: bool,
+        z_index: Optional[int],
+    ) -> None:
+        """
+        Render scrollable pane content.
+
+        This works by rendering on an off-screen canvas, and copying over the
+        visible region.
+        """
+        show_scrollbar = self.show_scrollbar()
+
+        if show_scrollbar:
+            virtual_width = write_position.width - 1
+        else:
+            virtual_width = write_position.width
+
+        # Compute preferred height again.
+        virtual_height = self.content.preferred_height(
+            virtual_width, self.max_available_height
+        ).preferred
+
+        # Ensure virtual height is at least the available height.
+        virtual_height = max(virtual_height, write_position.height)
+        virtual_height = min(virtual_height, self.max_available_height)
+
+        # First, write the content to a virtual screen, then copy over the
+        # visible part to the real screen.
+        temp_screen = Screen(default_char=Char(char=" ", style=parent_style))
+        temp_screen.show_cursor = screen.show_cursor
+        temp_write_position = WritePosition(
+            xpos=0, ypos=0, width=virtual_width, height=virtual_height
+        )
+
+        temp_mouse_handlers = MouseHandlers()
+
+        self.content.write_to_screen(
+            temp_screen,
+            temp_mouse_handlers,
+            temp_write_position,
+            parent_style,
+            erase_bg,
+            z_index,
+        )
+        temp_screen.draw_all_floats()
+
+        # If anything in the virtual screen is focused, move vertical scroll to
+        from prompt_toolkit.application import get_app
+
+        focused_window = get_app().layout.current_window
+
+        try:
+            visible_win_write_pos = temp_screen.visible_windows_to_write_positions[
+                focused_window
+            ]
+        except KeyError:
+            pass  # No window focused here. Don't scroll.
+        else:
+            # Make sure this window is visible.
+            self._make_window_visible(
+                write_position.height,
+                virtual_height,
+                visible_win_write_pos,
+                temp_screen.cursor_positions.get(focused_window),
+            )
+
+        # Copy over virtual screen and zero width escapes to real screen.
+        self._copy_over_screen(screen, temp_screen, write_position, virtual_width)
+
+        # Copy over mouse handlers.
+        self._copy_over_mouse_handlers(
+            mouse_handlers, temp_mouse_handlers, write_position, virtual_width
+        )
+
+        # Set screen.width/height.
+        ypos = write_position.ypos
+        xpos = write_position.xpos
+
+        screen.width = max(screen.width, xpos + virtual_width)
+        screen.height = max(screen.height, ypos + write_position.height)
+
+        # Copy over window write positions.
+        self._copy_over_write_positions(screen, temp_screen, write_position)
+
+        if temp_screen.show_cursor:
+            screen.show_cursor = True
+
+        # Copy over cursor positions, if they are visible.
+        for window, point in temp_screen.cursor_positions.items():
+            if (
+                0 <= point.x < write_position.width
+                and self.vertical_scroll
+                <= point.y
+                < write_position.height + self.vertical_scroll
+            ):
+                screen.cursor_positions[window] = Point(
+                    x=point.x + xpos, y=point.y + ypos - self.vertical_scroll
+                )
+
+        # Copy over menu positions, but clip them to the visible area.
+        for window, point in temp_screen.menu_positions.items():
+            screen.menu_positions[window] = self._clip_point_to_visible_area(
+                Point(x=point.x + xpos, y=point.y + ypos - self.vertical_scroll),
+                write_position,
+            )
+
+        # Draw scrollbar.
+        if show_scrollbar:
+            self._draw_scrollbar(
+                write_position,
+                virtual_height,
+                screen,
+            )
+
+    def _clip_point_to_visible_area(
+        self, point: Point, write_position: WritePosition
+    ) -> Point:
+        """
+        Ensure that the cursor and menu positions always are always reported
+        """
+        if point.x < write_position.xpos:
+            point = point._replace(x=write_position.xpos)
+        if point.y < write_position.ypos:
+            point = point._replace(y=write_position.ypos)
+        if point.x >= write_position.xpos + write_position.width:
+            point = point._replace(x=write_position.xpos + write_position.width - 1)
+        if point.y >= write_position.ypos + write_position.height:
+            point = point._replace(y=write_position.ypos + write_position.height - 1)
+
+        return point
+
+    def _copy_over_screen(
+        self,
+        screen: Screen,
+        temp_screen: Screen,
+        write_position: WritePosition,
+        virtual_width: int,
+    ) -> None:
+        """
+        Copy over visible screen content and "zero width escape sequences".
+        """
+        ypos = write_position.ypos
+        xpos = write_position.xpos
+
+        for y in range(write_position.height):
+            temp_row = temp_screen.data_buffer[y + self.vertical_scroll]
+            row = screen.data_buffer[y + ypos]
+            temp_zero_width_escapes = temp_screen.zero_width_escapes[
+                y + self.vertical_scroll
+            ]
+            zero_width_escapes = screen.zero_width_escapes[y + ypos]
+
+            for x in range(virtual_width):
+                row[x + xpos] = temp_row[x]
+
+                if x in temp_zero_width_escapes:
+                    zero_width_escapes[x + xpos] = temp_zero_width_escapes[x]
+
+    def _copy_over_mouse_handlers(
+        self,
+        mouse_handlers: MouseHandlers,
+        temp_mouse_handlers: MouseHandlers,
+        write_position: WritePosition,
+        virtual_width: int,
+    ) -> None:
+        """
+        Copy over mouse handlers from virtual screen to real screen.
+
+        Note: we take `virtual_width` because we don't want to copy over mouse
+              handlers that we possibly have behind the scrollbar.
+        """
+        ypos = write_position.ypos
+        xpos = write_position.xpos
+
+        # Cache mouse handlers when wrapping them. Very often the same mouse
+        # handler is registered for many positions.
+        mouse_handler_wrappers: Dict[MouseHandler, MouseHandler] = {}
+
+        def wrap_mouse_handler(handler: MouseHandler) -> MouseHandler:
+            "Wrap mouse handler. Translate coordinates in `MouseEvent`."
+            if handler not in mouse_handler_wrappers:
+
+                def new_handler(event: MouseEvent) -> None:
+                    new_event = MouseEvent(
+                        position=Point(
+                            x=event.position.x - xpos,
+                            y=event.position.y + self.vertical_scroll - ypos,
+                        ),
+                        event_type=event.event_type,
+                        button=event.button,
+                        modifiers=event.modifiers,
+                    )
+                    handler(new_event)
+
+                mouse_handler_wrappers[handler] = new_handler
+            return mouse_handler_wrappers[handler]
+
+        # Copy handlers.
+        mouse_handlers_dict = mouse_handlers.mouse_handlers
+        temp_mouse_handlers_dict = temp_mouse_handlers.mouse_handlers
+
+        for y in range(write_position.height):
+            if y in temp_mouse_handlers_dict:
+                temp_mouse_row = temp_mouse_handlers_dict[y + self.vertical_scroll]
+                mouse_row = mouse_handlers_dict[y + ypos]
+                for x in range(virtual_width):
+                    if x in temp_mouse_row:
+                        mouse_row[x + xpos] = wrap_mouse_handler(temp_mouse_row[x])
+
+    def _copy_over_write_positions(
+        self, screen: Screen, temp_screen: Screen, write_position: WritePosition
+    ) -> None:
+        """
+        Copy over window write positions.
+        """
+        ypos = write_position.ypos
+        xpos = write_position.xpos
+
+        for win, write_pos in temp_screen.visible_windows_to_write_positions.items():
+            screen.visible_windows_to_write_positions[win] = WritePosition(
+                xpos=write_pos.xpos + xpos,
+                ypos=write_pos.ypos + ypos - self.vertical_scroll,
+                # TODO: if the window is only partly visible, then truncate width/height.
+                #       This could be important if we have nested ScrollablePanes.
+                height=write_pos.height,
+                width=write_pos.width,
+            )
+
+    def is_modal(self) -> bool:
+        return self.content.is_modal()
+
+    def get_key_bindings(self) -> Optional[KeyBindingsBase]:
+        return self.content.get_key_bindings()
+
+    def get_children(self) -> List["Container"]:
+        return [self.content]
+
+    def _make_window_visible(
+        self,
+        visible_height: int,
+        virtual_height: int,
+        visible_win_write_pos: WritePosition,
+        cursor_position: Optional[Point],
+    ) -> None:
+        """
+        Scroll the scrollable pane, so that this window becomes visible.
+
+        :param visible_height: Height of this `ScrollablePane` that is rendered.
+        :param virtual_height: Height of the virtual, temp screen.
+        :param visible_win_write_pos: `WritePosition` of the nested window on the
+            temp screen.
+        :param cursor_position: The location of the cursor position of this
+            window on the temp screen.
+        """
+        # Start with maximum allowed scroll range, and then reduce according to
+        # the focused window and cursor position.
+        min_scroll = 0
+        max_scroll = virtual_height - visible_height
+
+        if self.keep_cursor_visible():
+            # Reduce min/max scroll according to the cursor in the focused window.
+            if cursor_position is not None:
+                offsets = self.scroll_offsets
+                cpos_min_scroll = (
+                    cursor_position.y - visible_height + 1 + offsets.bottom
+                )
+                cpos_max_scroll = cursor_position.y - offsets.top
+                min_scroll = max(min_scroll, cpos_min_scroll)
+                max_scroll = max(0, min(max_scroll, cpos_max_scroll))
+
+        if self.keep_focused_window_visible():
+            # Reduce min/max scroll according to focused window position.
+            # If the window is small enough, bot the top and bottom of the window
+            # should be visible.
+            if visible_win_write_pos.height <= visible_height:
+                window_min_scroll = (
+                    visible_win_write_pos.ypos
+                    + visible_win_write_pos.height
+                    - visible_height
+                )
+                window_max_scroll = visible_win_write_pos.ypos
+            else:
+                # Window does not fit on the screen. Make sure at least the whole
+                # screen is occupied with this window, and nothing else is shown.
+                window_min_scroll = visible_win_write_pos.ypos
+                window_max_scroll = (
+                    visible_win_write_pos.ypos
+                    + visible_win_write_pos.height
+                    - visible_height
+                )
+
+            min_scroll = max(min_scroll, window_min_scroll)
+            max_scroll = min(max_scroll, window_max_scroll)
+
+        if min_scroll > max_scroll:
+            min_scroll = max_scroll  # Should not happen.
+
+        # Finally, properly clip the vertical scroll.
+        if self.vertical_scroll > max_scroll:
+            self.vertical_scroll = max_scroll
+        if self.vertical_scroll < min_scroll:
+            self.vertical_scroll = min_scroll
+
+    def _draw_scrollbar(
+        self, write_position: WritePosition, content_height: int, screen: Screen
+    ) -> None:
+        """
+        Draw the scrollbar on the screen.
+
+        Note: There is some code duplication with the `ScrollbarMargin`
+              implementation.
+        """
+
+        window_height = write_position.height
+        display_arrows = self.display_arrows()
+
+        if display_arrows:
+            window_height -= 2
+
+        try:
+            fraction_visible = write_position.height / float(content_height)
+            fraction_above = self.vertical_scroll / float(content_height)
+
+            scrollbar_height = int(
+                min(window_height, max(1, window_height * fraction_visible))
+            )
+            scrollbar_top = int(window_height * fraction_above)
+        except ZeroDivisionError:
+            return
+        else:
+
+            def is_scroll_button(row: int) -> bool:
+                "True if we should display a button on this row."
+                return scrollbar_top <= row <= scrollbar_top + scrollbar_height
+
+            xpos = write_position.xpos + write_position.width - 1
+            ypos = write_position.ypos
+            data_buffer = screen.data_buffer
+
+            # Up arrow.
+            if display_arrows:
+                data_buffer[ypos][xpos] = Char(
+                    self.up_arrow_symbol, "class:scrollbar.arrow"
+                )
+                ypos += 1
+
+            # Scrollbar body.
+            scrollbar_background = "class:scrollbar.background"
+            scrollbar_background_start = "class:scrollbar.background,scrollbar.start"
+            scrollbar_button = "class:scrollbar.button"
+            scrollbar_button_end = "class:scrollbar.button,scrollbar.end"
+
+            for i in range(window_height):
+                style = ""
+                if is_scroll_button(i):
+                    if not is_scroll_button(i + 1):
+                        # Give the last cell a different style, because we want
+                        # to underline this.
+                        style = scrollbar_button_end
+                    else:
+                        style = scrollbar_button
+                else:
+                    if is_scroll_button(i + 1):
+                        style = scrollbar_background_start
+                    else:
+                        style = scrollbar_background
+
+                data_buffer[ypos][xpos] = Char(" ", style)
+                ypos += 1
+
+            # Down arrow
+            if display_arrows:
+                data_buffer[ypos][xpos] = Char(
+                    self.down_arrow_symbol, "class:scrollbar.arrow"
+                )
diff --git a/src/prompt_toolkit/layout/utils.py b/src/prompt_toolkit/layout/utils.py
new file mode 100644
index 0000000..cebdf28
--- /dev/null
+++ b/src/prompt_toolkit/layout/utils.py
@@ -0,0 +1,80 @@
+from typing import TYPE_CHECKING, Iterable, List, TypeVar, Union, cast, overload
+
+from prompt_toolkit.formatted_text.base import OneStyleAndTextTuple
+
+if TYPE_CHECKING:
+    from typing_extensions import SupportsIndex
+
+__all__ = [
+    "explode_text_fragments",
+]
+
+_T = TypeVar("_T", bound=OneStyleAndTextTuple)
+
+
+class _ExplodedList(List[_T]):
+    """
+    Wrapper around a list, that marks it as 'exploded'.
+
+    As soon as items are added or the list is extended, the new items are
+    automatically exploded as well.
+    """
+
+    exploded = True
+
+    def append(self, item: _T) -> None:
+        self.extend([item])
+
+    def extend(self, lst: Iterable[_T]) -> None:
+        super().extend(explode_text_fragments(lst))
+
+    def insert(self, index: "SupportsIndex", item: _T) -> None:
+        raise NotImplementedError  # TODO
+
+    # TODO: When creating a copy() or [:], return also an _ExplodedList.
+
+    @overload
+    def __setitem__(self, index: "SupportsIndex", value: _T) -> None:
+        ...
+
+    @overload
+    def __setitem__(self, index: slice, value: Iterable[_T]) -> None:
+        ...
+
+    def __setitem__(
+        self, index: Union["SupportsIndex", slice], value: Union[_T, Iterable[_T]]
+    ) -> None:
+        """
+        Ensure that when `(style_str, 'long string')` is set, the string will be
+        exploded.
+        """
+        if not isinstance(index, slice):
+            int_index = index.__index__()
+            index = slice(int_index, int_index + 1)
+        if isinstance(value, tuple):  # In case of `OneStyleAndTextTuple`.
+            value = cast("List[_T]", [value])
+
+        super().__setitem__(index, explode_text_fragments(cast("Iterable[_T]", value)))
+
+
+def explode_text_fragments(fragments: Iterable[_T]) -> _ExplodedList[_T]:
+    """
+    Turn a list of (style_str, text) tuples into another list where each string is
+    exactly one character.
+
+    It should be fine to call this function several times. Calling this on a
+    list that is already exploded, is a null operation.
+
+    :param fragments: List of (style, text) tuples.
+    """
+    # When the fragments is already exploded, don't explode again.
+    if isinstance(fragments, _ExplodedList):
+        return fragments
+
+    result: List[_T] = []
+
+    for style, string, *rest in fragments:
+        for c in string:
+            result.append((style, c, *rest))  # type: ignore
+
+    return _ExplodedList(result)