Adding upstream version 1:115.7.0.upstream/1%115.7.0upstream

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

1 files changed, 6654 insertions, 0 deletions

diff --git a/gfx/layers/apz/src/AsyncPanZoomController.cpp b/gfx/layers/apz/src/AsyncPanZoomController.cpp
new file mode 100644
index 0000000000..802d5b9dbf
--- /dev/null
+++ b/gfx/layers/apz/src/AsyncPanZoomController.cpp
@@ -0,0 +1,6654 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "AsyncPanZoomController.h"  // for AsyncPanZoomController, etc
+
+#include <math.h>       // for fabsf, fabs, atan2
+#include <stdint.h>     // for uint32_t, uint64_t
+#include <sys/types.h>  // for int32_t
+#include <algorithm>    // for max, min
+#include <utility>      // for std::make_pair
+
+#include "APZCTreeManager.h"            // for APZCTreeManager
+#include "AsyncPanZoomAnimation.h"      // for AsyncPanZoomAnimation
+#include "AutoDirWheelDeltaAdjuster.h"  // for APZAutoDirWheelDeltaAdjuster
+#include "AutoscrollAnimation.h"        // for AutoscrollAnimation
+#include "Axis.h"                       // for AxisX, AxisY, Axis, etc
+#include "CheckerboardEvent.h"          // for CheckerboardEvent
+#include "Compositor.h"                 // for Compositor
+#include "DesktopFlingPhysics.h"        // for DesktopFlingPhysics
+#include "FrameMetrics.h"               // for FrameMetrics, etc
+#include "GenericFlingAnimation.h"      // for GenericFlingAnimation
+#include "GestureEventListener.h"       // for GestureEventListener
+#include "HitTestingTreeNode.h"         // for HitTestingTreeNode
+#include "InputData.h"                  // for MultiTouchInput, etc
+#include "InputBlockState.h"            // for InputBlockState, TouchBlockState
+#include "InputQueue.h"                 // for InputQueue
+#include "Overscroll.h"                 // for OverscrollAnimation
+#include "OverscrollHandoffState.h"     // for OverscrollHandoffState
+#include "SimpleVelocityTracker.h"      // for SimpleVelocityTracker
+#include "Units.h"                      // for CSSRect, CSSPoint, etc
+#include "UnitTransforms.h"             // for TransformTo
+#include "base/message_loop.h"          // for MessageLoop
+#include "base/task.h"                  // for NewRunnableMethod, etc
+#include "gfxTypes.h"                   // for gfxFloat
+#include "mozilla/Assertions.h"         // for MOZ_ASSERT, etc
+#include "mozilla/BasicEvents.h"        // for Modifiers, MODIFIER_*
+#include "mozilla/ClearOnShutdown.h"    // for ClearOnShutdown
+#include "mozilla/ServoStyleConsts.h"   // for StyleComputedTimingFunction
+#include "mozilla/EventForwards.h"      // for nsEventStatus_*
+#include "mozilla/EventStateManager.h"  // for EventStateManager
+#include "mozilla/MouseEvents.h"        // for WidgetWheelEvent
+#include "mozilla/Preferences.h"        // for Preferences
+#include "mozilla/RecursiveMutex.h"     // for RecursiveMutexAutoLock, etc
+#include "mozilla/RefPtr.h"             // for RefPtr
+#include "mozilla/ScrollTypes.h"
+#include "mozilla/StaticPrefs_apz.h"
+#include "mozilla/StaticPrefs_general.h"
+#include "mozilla/StaticPrefs_gfx.h"
+#include "mozilla/StaticPrefs_mousewheel.h"
+#include "mozilla/StaticPrefs_layers.h"
+#include "mozilla/StaticPrefs_layout.h"
+#include "mozilla/StaticPrefs_slider.h"
+#include "mozilla/StaticPrefs_test.h"
+#include "mozilla/StaticPrefs_toolkit.h"
+#include "mozilla/Telemetry.h"  // for Telemetry
+#include "mozilla/TimeStamp.h"  // for TimeDuration, TimeStamp
+#include "mozilla/dom/CheckerboardReportService.h"  // for CheckerboardEventStorage
+// note: CheckerboardReportService.h actually lives in gfx/layers/apz/util/
+#include "mozilla/dom/Touch.h"              // for Touch
+#include "mozilla/gfx/gfxVars.h"            // for gfxVars
+#include "mozilla/gfx/BasePoint.h"          // for BasePoint
+#include "mozilla/gfx/BaseRect.h"           // for BaseRect
+#include "mozilla/gfx/Point.h"              // for Point, RoundedToInt, etc
+#include "mozilla/gfx/Rect.h"               // for RoundedIn
+#include "mozilla/gfx/ScaleFactor.h"        // for ScaleFactor
+#include "mozilla/layers/APZThreadUtils.h"  // for AssertOnControllerThread, etc
+#include "mozilla/layers/APZUtils.h"        // for AsyncTransform
+#include "mozilla/layers/CompositorController.h"  // for CompositorController
+#include "mozilla/layers/DirectionUtils.h"  // for GetAxis{Start,End,Length,Scale}
+#include "mozilla/layers/APZPublicUtils.h"  // for GetScrollMode
+#include "mozilla/mozalloc.h"               // for operator new, etc
+#include "mozilla/Unused.h"                 // for unused
+#include "nsAlgorithm.h"                    // for clamped
+#include "nsCOMPtr.h"                       // for already_AddRefed
+#include "nsDebug.h"                        // for NS_WARNING
+#include "nsLayoutUtils.h"
+#include "nsMathUtils.h"  // for NS_hypot
+#include "nsPoint.h"      // for nsIntPoint
+#include "nsStyleConsts.h"
+#include "nsTArray.h"        // for nsTArray, nsTArray_Impl, etc
+#include "nsThreadUtils.h"   // for NS_IsMainThread
+#include "nsViewportInfo.h"  // for ViewportMinScale(), ViewportMaxScale()
+#include "prsystem.h"        // for PR_GetPhysicalMemorySize
+#include "mozilla/ipc/SharedMemoryBasic.h"  // for SharedMemoryBasic
+#include "ScrollSnap.h"                     // for ScrollSnapUtils
+#include "ScrollAnimationPhysics.h"         // for ComputeAcceleratedWheelDelta
+#include "SmoothMsdScrollAnimation.h"
+#include "SmoothScrollAnimation.h"
+#include "WheelScrollAnimation.h"
+#if defined(MOZ_WIDGET_ANDROID)
+#  include "AndroidAPZ.h"
+#endif  // defined(MOZ_WIDGET_ANDROID)
+
+static mozilla::LazyLogModule sApzCtlLog("apz.controller");
+#define APZC_LOG(...) MOZ_LOG(sApzCtlLog, LogLevel::Debug, (__VA_ARGS__))
+#define APZC_LOGV(...) MOZ_LOG(sApzCtlLog, LogLevel::Verbose, (__VA_ARGS__))
+
+// Log to the apz.controller log with additional info from the APZC
+#define APZC_LOG_DETAIL(fmt, apzc, ...)                   \
+  APZC_LOG("%p(%s scrollId=%" PRIu64 "): " fmt, (apzc),   \
+           (apzc)->IsRootContent() ? "root" : "subframe", \
+           (apzc)->GetScrollId(), ##__VA_ARGS__)
+
+#define APZC_LOG_FM_COMMON(fm, prefix, level, ...)                 \
+  if (MOZ_LOG_TEST(sApzCtlLog, level)) {                           \
+    std::stringstream ss;                                          \
+    ss << nsPrintfCString(prefix, __VA_ARGS__).get() << ":" << fm; \
+    MOZ_LOG(sApzCtlLog, level, ("%s\n", ss.str().c_str()));        \
+  }
+#define APZC_LOG_FM(fm, prefix, ...) \
+  APZC_LOG_FM_COMMON(fm, prefix, LogLevel::Debug, __VA_ARGS__)
+#define APZC_LOGV_FM(fm, prefix, ...) \
+  APZC_LOG_FM_COMMON(fm, prefix, LogLevel::Verbose, __VA_ARGS__)
+
+namespace mozilla {
+namespace layers {
+
+typedef mozilla::layers::AllowedTouchBehavior AllowedTouchBehavior;
+typedef GeckoContentController::APZStateChange APZStateChange;
+typedef GeckoContentController::TapType TapType;
+typedef mozilla::gfx::Point Point;
+typedef mozilla::gfx::Matrix4x4 Matrix4x4;
+
+// Choose between platform-specific implementations.
+#ifdef MOZ_WIDGET_ANDROID
+typedef WidgetOverscrollEffect OverscrollEffect;
+typedef AndroidSpecificState PlatformSpecificState;
+#else
+typedef GenericOverscrollEffect OverscrollEffect;
+typedef PlatformSpecificStateBase
+    PlatformSpecificState;  // no extra state, just use the base class
+#endif
+
+/**
+ * \page APZCPrefs APZ preferences
+ *
+ * The following prefs are used to control the behaviour of the APZC.
+ * The default values are provided in StaticPrefList.yaml.
+ *
+ * \li\b apz.allow_double_tap_zooming
+ * Pref that allows or disallows double tap to zoom
+ *
+ * \li\b apz.allow_immediate_handoff
+ * If set to true, scroll can be handed off from one APZC to another within
+ * a single input block. If set to false, a single input block can only
+ * scroll one APZC.
+ *
+ * \li\b apz.allow_zooming_out
+ * If set to true, APZ will allow zooming out past the initial scale on
+ * desktop. This is false by default to match Chrome's behaviour.
+ *
+ * \li\b apz.android.chrome_fling_physics.friction
+ * A tunable parameter for Chrome fling physics on Android that governs
+ * how quickly a fling animation slows down due to friction (and therefore
+ * also how far it reaches). Should be in the range [0-1].
+ *
+ * \li\b apz.android.chrome_fling_physics.inflexion
+ * A tunable parameter for Chrome fling physics on Android that governs
+ * the shape of the fling curve. Should be in the range [0-1].
+ *
+ * \li\b apz.android.chrome_fling_physics.stop_threshold
+ * A tunable parameter for Chrome fling physics on Android that governs
+ * how close the fling animation has to get to its target destination
+ * before it stops.
+ * Units: ParentLayer pixels
+ *
+ * \li\b apz.autoscroll.enabled
+ * If set to true, autoscrolling is driven by APZ rather than the content
+ * process main thread.
+ *
+ * \li\b apz.axis_lock.mode
+ * The preferred axis locking style. See AxisLockMode for possible values.
+ *
+ * \li\b apz.axis_lock.lock_angle
+ * Angle from axis within which we stay axis-locked.\n
+ * Units: radians
+ *
+ * \li\b apz.axis_lock.breakout_threshold
+ * Distance in inches the user must pan before axis lock can be broken.\n
+ * Units: (real-world, i.e. screen) inches
+ *
+ * \li\b apz.axis_lock.breakout_angle
+ * Angle at which axis lock can be broken.\n
+ * Units: radians
+ *
+ * \li\b apz.axis_lock.direct_pan_angle
+ * If the angle from an axis to the line drawn by a pan move is less than
+ * this value, we can assume that panning can be done in the allowed direction
+ * (horizontal or vertical).\n
+ * Currently used only for touch-action css property stuff and was addded to
+ * keep behaviour consistent with IE.\n
+ * Units: radians
+ *
+ * \li\b apz.content_response_timeout
+ * Amount of time before we timeout response from content. For example, if
+ * content is being unruly/slow and we don't get a response back within this
+ * time, we will just pretend that content did not preventDefault any touch
+ * events we dispatched to it.\n
+ * Units: milliseconds
+ *
+ * \li\b apz.danger_zone_x
+ * \li\b apz.danger_zone_y
+ * When drawing high-res tiles, we drop down to drawing low-res tiles
+ * when we know we can't keep up with the scrolling. The way we determine
+ * this is by checking if we are entering the "danger zone", which is the
+ * boundary of the painted content. For example, if the painted content
+ * goes from y=0...1000 and the visible portion is y=250...750 then
+ * we're far from checkerboarding. If we get to y=490...990 though then we're
+ * only 10 pixels away from showing checkerboarding so we are probably in
+ * a state where we can't keep up with scrolling. The danger zone prefs specify
+ * how wide this margin is; in the above example a y-axis danger zone of 10
+ * pixels would make us drop to low-res at y=490...990.\n
+ * This value is in screen pixels.
+ *
+ * \li\b apz.disable_for_scroll_linked_effects
+ * Setting this pref to true will disable APZ scrolling on documents where
+ * scroll-linked effects are detected. A scroll linked effect is detected if
+ * positioning or transform properties are updated inside a scroll event
+ * dispatch; we assume that such an update is in response to the scroll event
+ * and is therefore a scroll-linked effect which will be laggy with APZ
+ * scrolling.
+ *
+ * \li\b apz.displayport_expiry_ms
+ * While a scrollable frame is scrolling async, we set a displayport on it
+ * to make sure it is layerized. However this takes up memory, so once the
+ * scrolling stops we want to remove the displayport. This pref controls how
+ * long after scrolling stops the displayport is removed. A value of 0 will
+ * disable the expiry behavior entirely.
+ * Units: milliseconds
+ *
+ * \li\b apz.drag.enabled
+ * Setting this pref to true will cause APZ to handle mouse-dragging of
+ * scrollbar thumbs.
+ *
+ * \li\b apz.drag.initial.enabled
+ * Setting this pref to true will cause APZ to try to handle mouse-dragging
+ * of scrollbar thumbs without an initial round-trip to content to start it
+ * if possible. Only has an effect if apz.drag.enabled is also true.
+ *
+ * \li\b apz.drag.touch.enabled
+ * Setting this pref to true will cause APZ to handle touch-dragging of
+ * scrollbar thumbs. Only has an effect if apz.drag.enabled is also true.
+ *
+ * \li\b apz.enlarge_displayport_when_clipped
+ * Pref that enables enlarging of the displayport along one axis when the
+ * generated displayport's size is beyond that of the scrollable rect on the
+ * opposite axis.
+ *
+ * \li\b apz.fling_accel_min_fling_velocity
+ * The minimum velocity of the second fling, and the minimum velocity of the
+ * previous fling animation at the point of interruption, for the new fling to
+ * be considered for fling acceleration.
+ * Units: screen pixels per milliseconds
+ *
+ * \li\b apz.fling_accel_min_pan_velocity
+ * The minimum velocity during the pan gesture that causes a fling for that
+ * fling to be considered for fling acceleration.
+ * Units: screen pixels per milliseconds
+ *
+ * \li\b apz.fling_accel_max_pause_interval_ms
+ * The maximum time that is allowed to elapse between the touch start event that
+ * interrupts the previous fling, and the touch move that initiates panning for
+ * the current fling, for that fling to be considered for fling acceleration.
+ * Units: milliseconds
+ *
+ * \li\b apz.fling_accel_base_mult
+ * \li\b apz.fling_accel_supplemental_mult
+ * When applying an acceleration on a fling, the new computed velocity is
+ * (new_fling_velocity * base_mult) + (old_velocity * supplemental_mult).
+ * The base_mult and supplemental_mult multiplier values are controlled by
+ * these prefs. Note that "old_velocity" here is the initial velocity of the
+ * previous fling _after_ acceleration was applied to it (if applicable).
+ *
+ * \li\b apz.fling_curve_function_x1
+ * \li\b apz.fling_curve_function_y1
+ * \li\b apz.fling_curve_function_x2
+ * \li\b apz.fling_curve_function_y2
+ * \li\b apz.fling_curve_threshold_inches_per_ms
+ * These five parameters define a Bezier curve function and threshold used to
+ * increase the actual velocity relative to the user's finger velocity. When the
+ * finger velocity is below the threshold (or if the threshold is not positive),
+ * the velocity is used as-is. If the finger velocity exceeds the threshold
+ * velocity, then the function defined by the curve is applied on the part of
+ * the velocity that exceeds the threshold. Note that the upper bound of the
+ * velocity is still specified by the \b apz.max_velocity_inches_per_ms pref,
+ * and the function will smoothly curve the velocity from the threshold to the
+ * max. In general the function parameters chosen should define an ease-out
+ * curve in order to increase the velocity in this range, or an ease-in curve to
+ * decrease the velocity. A straight-line curve is equivalent to disabling the
+ * curve entirely by setting the threshold to -1. The max velocity pref must
+ * also be set in order for the curving to take effect, as it defines the upper
+ * bound of the velocity curve.\n
+ * The points (x1, y1) and (x2, y2) used as the two intermediate control points
+ * in the cubic bezier curve; the first and last points are (0,0) and (1,1).\n
+ * Some example values for these prefs can be found at\n
+ * https://searchfox.org/mozilla-central/rev/f82d5c549f046cb64ce5602bfd894b7ae807c8f8/dom/animation/ComputedTimingFunction.cpp#27-33
+ *
+ * \li\b apz.fling_friction
+ * Amount of friction applied during flings. This is used in the following
+ * formula: v(t1) = v(t0) * (1 - f)^(t1 - t0), where v(t1) is the velocity
+ * for a new sample, v(t0) is the velocity at the previous sample, f is the
+ * value of this pref, and (t1 - t0) is the amount of time, in milliseconds,
+ * that has elapsed between the two samples.\n
+ * NOTE: Not currently used in Android fling calculations.
+ *
+ * \li\b apz.fling_min_velocity_threshold
+ * Minimum velocity for a fling to actually kick off. If the user pans and lifts
+ * their finger such that the velocity is smaller than or equal to this amount,
+ * no fling is initiated.\n
+ * Units: screen pixels per millisecond
+ *
+ * \li\b apz.fling_stop_on_tap_threshold
+ * When flinging, if the velocity is above this number, then a tap on the
+ * screen will stop the fling without dispatching a tap to content. If the
+ * velocity is below this threshold a tap will also be dispatched.
+ * Note: when modifying this pref be sure to run the APZC gtests as some of
+ * them depend on the value of this pref.\n
+ * Units: screen pixels per millisecond
+ *
+ * \li\b apz.fling_stopped_threshold
+ * When flinging, if the velocity goes below this number, we just stop the
+ * animation completely. This is to prevent asymptotically approaching 0
+ * velocity and rerendering unnecessarily.\n
+ * Units: screen pixels per millisecond.\n
+ * NOTE: Should not be set to anything
+ * other than 0.0 for Android except for tests to disable flings.
+ *
+ * \li\b apz.keyboard.enabled
+ * Determines whether scrolling with the keyboard will be allowed to be handled
+ * by APZ.
+ *
+ * \li\b apz.keyboard.passive-listeners
+ * When enabled, APZ will interpret the passive event listener flag to mean
+ * that the event listener won't change the focused element or selection of
+ * the page. With this, web content can use passive key listeners and not have
+ * keyboard APZ disabled.
+ *
+ * \li\b apz.max_tap_time
+ * Maximum time for a touch on the screen and corresponding lift of the finger
+ * to be considered a tap. This also applies to double taps, except that it is
+ * used both for the interval between the first touchdown and first touchup,
+ * and for the interval between the first touchup and the second touchdown.\n
+ * Units: milliseconds.
+ *
+ * \li\b apz.max_velocity_inches_per_ms
+ * Maximum velocity.  Velocity will be capped at this value if a faster fling
+ * occurs.  Negative values indicate unlimited velocity.\n
+ * Units: (real-world, i.e. screen) inches per millisecond
+ *
+ * \li\b apz.max_velocity_queue_size
+ * Maximum size of velocity queue. The queue contains last N velocity records.
+ * On touch end we calculate the average velocity in order to compensate
+ * touch/mouse drivers misbehaviour.
+ *
+ * \li\b apz.min_skate_speed
+ * Minimum amount of speed along an axis before we switch to "skate" multipliers
+ * rather than using the "stationary" multipliers.\n
+ * Units: CSS pixels per millisecond
+ *
+ * \li\b apz.one_touch_pinch.enabled
+ * Whether or not the "one-touch-pinch" gesture (for zooming with one finger)
+ * is enabled or not.
+ *
+ * \li\b apz.overscroll.enabled
+ * Pref that enables overscrolling. If this is disabled, excess scroll that
+ * cannot be handed off is discarded.
+ *
+ * \li\b apz.overscroll.min_pan_distance_ratio
+ * The minimum ratio of the pan distance along one axis to the pan distance
+ * along the other axis needed to initiate overscroll along the first axis
+ * during panning.
+ *
+ * \li\b apz.overscroll.stretch_factor
+ * How much overscrolling can stretch content along an axis.
+ * The maximum stretch along an axis is a factor of (1 + kStretchFactor).
+ * (So if kStretchFactor is 0, you can't stretch at all; if kStretchFactor
+ * is 1, you can stretch at most by a factor of 2).
+ *
+ * \li\b apz.overscroll.stop_distance_threshold
+ * \li\b apz.overscroll.stop_velocity_threshold
+ * Thresholds for stopping the overscroll animation. When both the distance
+ * and the velocity fall below their thresholds, we stop oscillating.\n
+ * Units: screen pixels (for distance)
+ *        screen pixels per millisecond (for velocity)
+ *
+ * \li\b apz.overscroll.spring_stiffness
+ * The spring stiffness constant for the overscroll mass-spring-damper model.
+ *
+ * \li\b apz.overscroll.damping
+ * The damping constant for the overscroll mass-spring-damper model.
+ *
+ * \li\b apz.overscroll.max_velocity
+ * The maximum velocity (in ParentLayerPixels per millisecond) allowed when
+ * initiating the overscroll snap-back animation.
+ *
+ * \li\b apz.paint_skipping.enabled
+ * When APZ is scrolling and sending repaint requests to the main thread, often
+ * the main thread doesn't actually need to do a repaint. This pref allows the
+ * main thread to skip doing those repaints in cases where it doesn't need to.
+ *
+ * \li\b apz.pinch_lock.mode
+ * The preferred pinch locking style. See PinchLockMode for possible values.
+ *
+ * \li\b apz.pinch_lock.scroll_lock_threshold
+ * Pinch locking is triggered if the user scrolls more than this distance
+ * and pinches less than apz.pinch_lock.span_lock_threshold.\n
+ * Units: (real-world, i.e. screen) inches
+ *
+ * \li\b apz.pinch_lock.span_breakout_threshold
+ * Distance in inches the user must pinch before lock can be broken.\n
+ * Units: (real-world, i.e. screen) inches measured between two touch points
+ *
+ * \li\b apz.pinch_lock.span_lock_threshold
+ * Pinch locking is triggered if the user pinches less than this distance
+ * and scrolls more than apz.pinch_lock.scroll_lock_threshold.\n
+ * Units: (real-world, i.e. screen) inches measured between two touch points
+ *
+ * \li\b apz.pinch_lock.buffer_max_age
+ * To ensure that pinch locking threshold calculations are not affected by
+ * variations in touch screen sensitivity, calculations draw from a buffer of
+ * recent events. This preference specifies the maximum time that events are
+ * held in this buffer.
+ * Units: milliseconds
+ *
+ * \li\b apz.popups.enabled
+ * Determines whether APZ is used for XUL popup widgets with remote content.
+ * Ideally, this should always be true, but it is currently not well tested, and
+ * has known issues, so needs to be prefable.
+ *
+ * \li\b apz.record_checkerboarding
+ * Whether or not to record detailed info on checkerboarding events.
+ *
+ * \li\b apz.second_tap_tolerance
+ * Constant describing the tolerance in distance we use, multiplied by the
+ * device DPI, within which a second tap is counted as part of a gesture
+ * continuing from the first tap. Making this larger allows the user more
+ * distance between the first and second taps in a "double tap" or "one touch
+ * pinch" gesture.\n
+ * Units: (real-world, i.e. screen) inches
+ *
+ * \li\b apz.test.logging_enabled
+ * Enable logging of APZ test data (see bug 961289).
+ *
+ * \li\b apz.touch_move_tolerance
+ * See the description for apz.touch_start_tolerance below. This is a similar
+ * threshold, except it is used to suppress touchmove events from being
+ * delivered to content for NON-scrollable frames (or more precisely, for APZCs
+ * where ArePointerEventsConsumable returns false).\n Units: (real-world, i.e.
+ * screen) inches
+ *
+ * \li\b apz.touch_start_tolerance
+ * Constant describing the tolerance in distance we use, multiplied by the
+ * device DPI, before we start panning the screen. This is to prevent us from
+ * accidentally processing taps as touch moves, and from very short/accidental
+ * touches moving the screen. touchmove events are also not delivered to content
+ * within this distance on scrollable frames.\n
+ * Units: (real-world, i.e. screen) inches
+ *
+ * \li\b apz.velocity_bias
+ * How much to adjust the displayport in the direction of scrolling. This value
+ * is multiplied by the velocity and added to the displayport offset.
+ *
+ * \li\b apz.velocity_relevance_time_ms
+ * When computing a fling velocity from the most recently stored velocity
+ * information, only velocities within the most X milliseconds are used.
+ * This pref controls the value of X.\n
+ * Units: ms
+ *
+ * \li\b apz.x_skate_size_multiplier
+ * \li\b apz.y_skate_size_multiplier
+ * The multiplier we apply to the displayport size if it is skating (current
+ * velocity is above \b apz.min_skate_speed). We prefer to increase the size of
+ * the Y axis because it is more natural in the case that a user is reading a
+ * page page that scrolls up/down. Note that one, both or neither of these may
+ * be used at any instant.\n In general we want \b
+ * apz.[xy]_skate_size_multiplier to be smaller than the corresponding
+ * stationary size multiplier because when panning fast we would like to paint
+ * less and get faster, more predictable paint times. When panning slowly we
+ * can afford to paint more even though it's slower.
+ *
+ * \li\b apz.x_stationary_size_multiplier
+ * \li\b apz.y_stationary_size_multiplier
+ * The multiplier we apply to the displayport size if it is not skating (see
+ * documentation for the skate size multipliers above).
+ *
+ * \li\b apz.x_skate_highmem_adjust
+ * \li\b apz.y_skate_highmem_adjust
+ * On high memory systems, we adjust the displayport during skating
+ * to be larger so we can reduce checkerboarding.
+ *
+ * \li\b apz.zoom_animation_duration_ms
+ * This controls how long the zoom-to-rect animation takes.\n
+ * Units: ms
+ *
+ * \li\b apz.scale_repaint_delay_ms
+ * How long to delay between repaint requests during a scale.
+ * A negative number prevents repaint requests during a scale.\n
+ * Units: ms
+ */
+
+/**
+ * Computed time function used for sampling frames of a zoom to animation.
+ */
+StaticAutoPtr<StyleComputedTimingFunction> gZoomAnimationFunction;
+
+/**
+ * Computed time function used for curving up velocity when it gets high.
+ */
+StaticAutoPtr<StyleComputedTimingFunction> gVelocityCurveFunction;
+
+/**
+ * The estimated duration of a paint for the purposes of calculating a new
+ * displayport, in milliseconds.
+ */
+static const double kDefaultEstimatedPaintDurationMs = 50;
+
+/**
+ * Returns true if this is a high memory system and we can use
+ * extra memory for a larger displayport to reduce checkerboarding.
+ */
+static bool gIsHighMemSystem = false;
+static bool IsHighMemSystem() { return gIsHighMemSystem; }
+
+AsyncPanZoomAnimation* PlatformSpecificStateBase::CreateFlingAnimation(
+    AsyncPanZoomController& aApzc, const FlingHandoffState& aHandoffState,
+    float aPLPPI) {
+  return new GenericFlingAnimation<DesktopFlingPhysics>(aApzc, aHandoffState,
+                                                        aPLPPI);
+}
+
+UniquePtr<VelocityTracker> PlatformSpecificStateBase::CreateVelocityTracker(
+    Axis* aAxis) {
+  return MakeUnique<SimpleVelocityTracker>(aAxis);
+}
+
+SampleTime AsyncPanZoomController::GetFrameTime() const {
+  APZCTreeManager* treeManagerLocal = GetApzcTreeManager();
+  return treeManagerLocal ? treeManagerLocal->GetFrameTime()
+                          : SampleTime::FromNow();
+}
+
+bool AsyncPanZoomController::IsZero(const ParentLayerPoint& aPoint) const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+  const auto zoom = Metrics().GetZoom();
+
+  if (zoom == CSSToParentLayerScale(0)) {
+    return true;
+  }
+
+  return layers::IsZero(aPoint / zoom);
+}
+
+bool AsyncPanZoomController::IsZero(ParentLayerCoord aCoord) const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+  const auto zoom = Metrics().GetZoom();
+
+  if (zoom == CSSToParentLayerScale(0)) {
+    return true;
+  }
+
+  return FuzzyEqualsAdditive((aCoord / zoom), CSSCoord(), COORDINATE_EPSILON);
+}
+
+bool AsyncPanZoomController::FuzzyGreater(ParentLayerCoord aCoord1,
+                                          ParentLayerCoord aCoord2) const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+  const auto zoom = Metrics().GetZoom();
+
+  if (zoom == CSSToParentLayerScale(0)) {
+    return false;
+  }
+
+  return (aCoord1 - aCoord2) / zoom > COORDINATE_EPSILON;
+}
+
+class MOZ_STACK_CLASS StateChangeNotificationBlocker final {
+ public:
+  explicit StateChangeNotificationBlocker(AsyncPanZoomController* aApzc)
+      : mApzc(aApzc) {
+    RecursiveMutexAutoLock lock(mApzc->mRecursiveMutex);
+    mInitialState = mApzc->mState;
+    mApzc->mNotificationBlockers++;
+  }
+
+  ~StateChangeNotificationBlocker() {
+    AsyncPanZoomController::PanZoomState newState;
+    {
+      RecursiveMutexAutoLock lock(mApzc->mRecursiveMutex);
+      mApzc->mNotificationBlockers--;
+      newState = mApzc->mState;
+    }
+    mApzc->DispatchStateChangeNotification(mInitialState, newState);
+  }
+
+ private:
+  AsyncPanZoomController* mApzc;
+  AsyncPanZoomController::PanZoomState mInitialState;
+};
+
+/**
+ * An RAII class to temporarily apply async test attributes to the provided
+ * AsyncPanZoomController.
+ *
+ * This class should be used in the implementation of any AsyncPanZoomController
+ * method that queries the async scroll offset or async zoom (this includes
+ * the async layout viewport offset, since modifying the async scroll offset
+ * may result in the layout viewport moving as well).
+ */
+class MOZ_RAII AutoApplyAsyncTestAttributes final {
+ public:
+  explicit AutoApplyAsyncTestAttributes(
+      const AsyncPanZoomController*,
+      const RecursiveMutexAutoLock& aProofOfLock);
+  ~AutoApplyAsyncTestAttributes();
+
+ private:
+  AsyncPanZoomController* mApzc;
+  FrameMetrics mPrevFrameMetrics;
+  ParentLayerPoint mPrevOverscroll;
+  const RecursiveMutexAutoLock& mProofOfLock;
+};
+
+AutoApplyAsyncTestAttributes::AutoApplyAsyncTestAttributes(
+    const AsyncPanZoomController* aApzc,
+    const RecursiveMutexAutoLock& aProofOfLock)
+    // Having to use const_cast here seems less ugly than the alternatives
+    // of making several members of AsyncPanZoomController that
+    // ApplyAsyncTestAttributes() modifies |mutable|, or several methods that
+    // query the async transforms non-const.
+    : mApzc(const_cast<AsyncPanZoomController*>(aApzc)),
+      mPrevFrameMetrics(aApzc->Metrics()),
+      mPrevOverscroll(aApzc->GetOverscrollAmountInternal()),
+      mProofOfLock(aProofOfLock) {
+  mApzc->ApplyAsyncTestAttributes(aProofOfLock);
+}
+
+AutoApplyAsyncTestAttributes::~AutoApplyAsyncTestAttributes() {
+  mApzc->UnapplyAsyncTestAttributes(mProofOfLock, mPrevFrameMetrics,
+                                    mPrevOverscroll);
+}
+
+class ZoomAnimation : public AsyncPanZoomAnimation {
+ public:
+  ZoomAnimation(AsyncPanZoomController& aApzc, const CSSPoint& aStartOffset,
+                const CSSToParentLayerScale& aStartZoom,
+                const CSSPoint& aEndOffset,
+                const CSSToParentLayerScale& aEndZoom)
+      : mApzc(aApzc),
+        mTotalDuration(TimeDuration::FromMilliseconds(
+            StaticPrefs::apz_zoom_animation_duration_ms())),
+        mStartOffset(aStartOffset),
+        mStartZoom(aStartZoom),
+        mEndOffset(aEndOffset),
+        mEndZoom(aEndZoom) {}
+
+  virtual bool DoSample(FrameMetrics& aFrameMetrics,
+                        const TimeDuration& aDelta) override {
+    mDuration += aDelta;
+    double animPosition = mDuration / mTotalDuration;
+
+    if (animPosition >= 1.0) {
+      aFrameMetrics.SetZoom(mEndZoom);
+      mApzc.SetVisualScrollOffset(mEndOffset);
+      return false;
+    }
+
+    // Sample the zoom at the current time point.  The sampled zoom
+    // will affect the final computed resolution.
+    float sampledPosition =
+        gZoomAnimationFunction->At(animPosition, /* aBeforeFlag = */ false);
+
+    // We scale the scrollOffset linearly with sampledPosition, so the zoom
+    // needs to scale inversely to match.
+    if (mStartZoom == CSSToParentLayerScale(0) ||
+        mEndZoom == CSSToParentLayerScale(0)) {
+      return false;
+    }
+
+    aFrameMetrics.SetZoom(
+        CSSToParentLayerScale(1 / (sampledPosition / mEndZoom.scale +
+                                   (1 - sampledPosition) / mStartZoom.scale)));
+
+    mApzc.SetVisualScrollOffset(CSSPoint::FromUnknownPoint(gfx::Point(
+        mEndOffset.x * sampledPosition + mStartOffset.x * (1 - sampledPosition),
+        mEndOffset.y * sampledPosition +
+            mStartOffset.y * (1 - sampledPosition))));
+    return true;
+  }
+
+  virtual bool WantsRepaints() override { return true; }
+
+ private:
+  AsyncPanZoomController& mApzc;
+
+  TimeDuration mDuration;
+  const TimeDuration mTotalDuration;
+
+  // Old metrics from before we started a zoom animation. This is only valid
+  // when we are in the "ANIMATED_ZOOM" state. This is used so that we can
+  // interpolate between the start and end frames. We only use the
+  // |mViewportScrollOffset| and |mResolution| fields on this.
+  CSSPoint mStartOffset;
+  CSSToParentLayerScale mStartZoom;
+
+  // Target metrics for a zoom to animation. This is only valid when we are in
+  // the "ANIMATED_ZOOM" state. We only use the |mViewportScrollOffset| and
+  // |mResolution| fields on this.
+  CSSPoint mEndOffset;
+  CSSToParentLayerScale mEndZoom;
+};
+
+/*static*/
+void AsyncPanZoomController::InitializeGlobalState() {
+  static bool sInitialized = false;
+  if (sInitialized) return;
+  sInitialized = true;
+
+  MOZ_ASSERT(NS_IsMainThread());
+
+  gZoomAnimationFunction = new StyleComputedTimingFunction(
+      StyleComputedTimingFunction::Keyword(StyleTimingKeyword::Ease));
+  ClearOnShutdown(&gZoomAnimationFunction);
+  gVelocityCurveFunction =
+      new StyleComputedTimingFunction(StyleComputedTimingFunction::CubicBezier(
+          StaticPrefs::apz_fling_curve_function_x1_AtStartup(),
+          StaticPrefs::apz_fling_curve_function_y1_AtStartup(),
+          StaticPrefs::apz_fling_curve_function_x2_AtStartup(),
+          StaticPrefs::apz_fling_curve_function_y2_AtStartup()));
+  ClearOnShutdown(&gVelocityCurveFunction);
+
+  uint64_t sysmem = PR_GetPhysicalMemorySize();
+  uint64_t threshold = 1LL << 32;  // 4 GB in bytes
+  gIsHighMemSystem = sysmem >= threshold;
+
+  PlatformSpecificState::InitializeGlobalState();
+}
+
+AsyncPanZoomController::AsyncPanZoomController(
+    LayersId aLayersId, APZCTreeManager* aTreeManager,
+    const RefPtr<InputQueue>& aInputQueue,
+    GeckoContentController* aGeckoContentController, GestureBehavior aGestures)
+    : mLayersId(aLayersId),
+      mGeckoContentController(aGeckoContentController),
+      mRefPtrMonitor("RefPtrMonitor"),
+      // mTreeManager must be initialized before GetFrameTime() is called
+      mTreeManager(aTreeManager),
+      mRecursiveMutex("AsyncPanZoomController"),
+      mLastContentPaintMetrics(mLastContentPaintMetadata.GetMetrics()),
+      mPanDirRestricted(false),
+      mPinchLocked(false),
+      mPinchEventBuffer(TimeDuration::FromMilliseconds(
+          StaticPrefs::apz_pinch_lock_buffer_max_age_AtStartup())),
+      mZoomConstraints(false, false,
+                       mScrollMetadata.GetMetrics().GetDevPixelsPerCSSPixel() *
+                           ViewportMinScale() / ParentLayerToScreenScale(1),
+                       mScrollMetadata.GetMetrics().GetDevPixelsPerCSSPixel() *
+                           ViewportMaxScale() / ParentLayerToScreenScale(1)),
+      mLastSampleTime(GetFrameTime()),
+      mLastCheckerboardReport(GetFrameTime()),
+      mLastNotifiedZoom(),
+      mOverscrollEffect(MakeUnique<OverscrollEffect>(*this)),
+      mState(NOTHING),
+      mX(this),
+      mY(this),
+      mNotificationBlockers(0),
+      mInputQueue(aInputQueue),
+      mPinchPaintTimerSet(false),
+      mDelayedTransformEnd(false),
+      mTestAttributeAppliers(0),
+      mTestHasAsyncKeyScrolled(false),
+      mCheckerboardEventLock("APZCBELock") {
+  if (aGestures == USE_GESTURE_DETECTOR) {
+    mGestureEventListener = new GestureEventListener(this);
+  }
+  // Put one default-constructed sampled state in the queue.
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  mSampledState.emplace_back();
+}
+
+AsyncPanZoomController::~AsyncPanZoomController() { MOZ_ASSERT(IsDestroyed()); }
+
+PlatformSpecificStateBase* AsyncPanZoomController::GetPlatformSpecificState() {
+  if (!mPlatformSpecificState) {
+    mPlatformSpecificState = MakeUnique<PlatformSpecificState>();
+  }
+  return mPlatformSpecificState.get();
+}
+
+already_AddRefed<GeckoContentController>
+AsyncPanZoomController::GetGeckoContentController() const {
+  MonitorAutoLock lock(mRefPtrMonitor);
+  RefPtr<GeckoContentController> controller = mGeckoContentController;
+  return controller.forget();
+}
+
+already_AddRefed<GestureEventListener>
+AsyncPanZoomController::GetGestureEventListener() const {
+  MonitorAutoLock lock(mRefPtrMonitor);
+  RefPtr<GestureEventListener> listener = mGestureEventListener;
+  return listener.forget();
+}
+
+const RefPtr<InputQueue>& AsyncPanZoomController::GetInputQueue() const {
+  return mInputQueue;
+}
+
+void AsyncPanZoomController::Destroy() {
+  AssertOnUpdaterThread();
+
+  CancelAnimation(CancelAnimationFlags::ScrollSnap);
+
+  {  // scope the lock
+    MonitorAutoLock lock(mRefPtrMonitor);
+    mGeckoContentController = nullptr;
+    mGestureEventListener = nullptr;
+  }
+  mParent = nullptr;
+  mTreeManager = nullptr;
+}
+
+bool AsyncPanZoomController::IsDestroyed() const {
+  return mTreeManager == nullptr;
+}
+
+float AsyncPanZoomController::GetDPI() const {
+  if (APZCTreeManager* localPtr = mTreeManager) {
+    return localPtr->GetDPI();
+  }
+  // If this APZC has been destroyed then this value is not going to be
+  // used for anything that the user will end up seeing, so we can just
+  // return 0.
+  return 0.0;
+}
+
+ScreenCoord AsyncPanZoomController::GetTouchStartTolerance() const {
+  return (StaticPrefs::apz_touch_start_tolerance() * GetDPI());
+}
+
+ScreenCoord AsyncPanZoomController::GetTouchMoveTolerance() const {
+  return (StaticPrefs::apz_touch_move_tolerance() * GetDPI());
+}
+
+ScreenCoord AsyncPanZoomController::GetSecondTapTolerance() const {
+  return (StaticPrefs::apz_second_tap_tolerance() * GetDPI());
+}
+
+/* static */ AsyncPanZoomController::AxisLockMode
+AsyncPanZoomController::GetAxisLockMode() {
+  return static_cast<AxisLockMode>(StaticPrefs::apz_axis_lock_mode());
+}
+
+bool AsyncPanZoomController::UsingStatefulAxisLock() const {
+  return (GetAxisLockMode() == STANDARD || GetAxisLockMode() == STICKY);
+}
+
+/* static */ AsyncPanZoomController::PinchLockMode
+AsyncPanZoomController::GetPinchLockMode() {
+  return static_cast<PinchLockMode>(StaticPrefs::apz_pinch_lock_mode());
+}
+
+PointerEventsConsumableFlags AsyncPanZoomController::ArePointerEventsConsumable(
+    TouchBlockState* aBlock, const MultiTouchInput& aInput) {
+  uint32_t touchPoints = aInput.mTouches.Length();
+  if (touchPoints == 0) {
+    // Cant' do anything with zero touch points
+    return {false, false};
+  }
+
+  // This logic is simplified, erring on the side of returning true if we're
+  // not sure. It's safer to pretend that we can consume the event and then
+  // not be able to than vice-versa. But at the same time, we should try hard
+  // to return an accurate result, because returning true can trigger a
+  // pointercancel event to web content, which can break certain features
+  // that are using touch-action and handling the pointermove events.
+  //
+  // Note that in particular this function can return true if APZ is waiting on
+  // the main thread for touch-action information. In this scenario, the
+  // APZEventState::MainThreadAgreesEventsAreConsumableByAPZ() function tries
+  // to use the main-thread touch-action information to filter out false
+  // positives.
+  //
+  // We could probably enhance this logic to determine things like "we're
+  // not pannable, so we can only zoom in, and the zoom is already maxed
+  // out, so we're not zoomable either" but no need for that at this point.
+
+  bool pannableX = aBlock->GetOverscrollHandoffChain()->CanScrollInDirection(
+      this, ScrollDirection::eHorizontal);
+  bool touchActionAllowsX = aBlock->TouchActionAllowsPanningX();
+  bool pannableY = (aBlock->GetOverscrollHandoffChain()->CanScrollInDirection(
+                        this, ScrollDirection::eVertical) ||
+                    // In the case of the root APZC with any dynamic toolbar, it
+                    // shoule be pannable if there is room moving the dynamic
+                    // toolbar.
+                    (IsRootContent() && CanVerticalScrollWithDynamicToolbar()));
+  bool touchActionAllowsY = aBlock->TouchActionAllowsPanningY();
+
+  bool pannable;
+  bool touchActionAllowsPanning;
+
+  Maybe<ScrollDirection> panDirection =
+      aBlock->GetBestGuessPanDirection(aInput);
+  if (panDirection == Some(ScrollDirection::eVertical)) {
+    pannable = pannableY;
+    touchActionAllowsPanning = touchActionAllowsY;
+  } else if (panDirection == Some(ScrollDirection::eHorizontal)) {
+    pannable = pannableX;
+    touchActionAllowsPanning = touchActionAllowsX;
+  } else {
+    // If we don't have a guessed pan direction, err on the side of returning
+    // true.
+    pannable = pannableX || pannableY;
+    touchActionAllowsPanning = touchActionAllowsX || touchActionAllowsY;
+  }
+
+  if (touchPoints == 1) {
+    return {pannable, touchActionAllowsPanning};
+  }
+
+  bool zoomable = ZoomConstraintsAllowZoom();
+  bool touchActionAllowsZoom = aBlock->TouchActionAllowsPinchZoom();
+
+  return {pannable || zoomable,
+          touchActionAllowsPanning || touchActionAllowsZoom};
+}
+
+nsEventStatus AsyncPanZoomController::HandleDragEvent(
+    const MouseInput& aEvent, const AsyncDragMetrics& aDragMetrics,
+    OuterCSSCoord aInitialThumbPos) {
+  // RDM is a special case where touch events will be synthesized in response
+  // to mouse events, and APZ will receive both even though RDM prevent-defaults
+  // the mouse events. This is because mouse events don't opt into APZ waiting
+  // to check if the event has been prevent-defaulted and are still processed
+  // as a result. To handle this, have APZ ignore mouse events when RDM and
+  // touch simulation are active.
+  bool isRDMTouchSimulationActive = false;
+  {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    isRDMTouchSimulationActive =
+        mScrollMetadata.GetIsRDMTouchSimulationActive();
+  }
+
+  if (!StaticPrefs::apz_drag_enabled() || isRDMTouchSimulationActive) {
+    return nsEventStatus_eIgnore;
+  }
+
+  if (!GetApzcTreeManager()) {
+    return nsEventStatus_eConsumeNoDefault;
+  }
+
+  {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+    if (aEvent.mType == MouseInput::MouseType::MOUSE_UP) {
+      if (mState == SCROLLBAR_DRAG) {
+        APZC_LOG("%p ending drag\n", this);
+        SetState(NOTHING);
+      }
+
+      SnapBackIfOverscrolled();
+
+      return nsEventStatus_eConsumeNoDefault;
+    }
+  }
+
+  HitTestingTreeNodeAutoLock node;
+  GetApzcTreeManager()->FindScrollThumbNode(aDragMetrics, mLayersId, node);
+  if (!node) {
+    APZC_LOG("%p unable to find scrollthumb node with viewid %" PRIu64 "\n",
+             this, aDragMetrics.mViewId);
+    return nsEventStatus_eConsumeNoDefault;
+  }
+
+  if (aEvent.mType == MouseInput::MouseType::MOUSE_DOWN) {
+    APZC_LOG("%p starting scrollbar drag\n", this);
+    SetState(SCROLLBAR_DRAG);
+  }
+
+  if (aEvent.mType != MouseInput::MouseType::MOUSE_MOVE) {
+    APZC_LOG("%p discarding event of type %d\n", this, aEvent.mType);
+    return nsEventStatus_eConsumeNoDefault;
+  }
+
+  const ScrollbarData& scrollbarData = node->GetScrollbarData();
+  MOZ_ASSERT(scrollbarData.mScrollbarLayerType ==
+             layers::ScrollbarLayerType::Thumb);
+  MOZ_ASSERT(scrollbarData.mDirection.isSome());
+  ScrollDirection direction = *scrollbarData.mDirection;
+
+  bool isMouseAwayFromThumb = false;
+  if (int snapMultiplier = StaticPrefs::slider_snapMultiplier_AtStartup()) {
+    // It's fine to ignore the async component of the thumb's transform,
+    // because any async transform of the thumb will be in the direction of
+    // scrolling, but here we're interested in the other direction.
+    ParentLayerRect thumbRect =
+        (node->GetTransform() * AsyncTransformMatrix())
+            .TransformBounds(LayerRect(node->GetVisibleRegion().GetBounds()));
+    ScrollDirection otherDirection = GetPerpendicularDirection(direction);
+    ParentLayerCoord distance =
+        GetAxisStart(otherDirection, thumbRect.DistanceTo(aEvent.mLocalOrigin));
+    ParentLayerCoord thumbWidth = GetAxisLength(otherDirection, thumbRect);
+    // Avoid triggering this condition spuriously when the thumb is
+    // offscreen and its visible region is therefore empty.
+    if (thumbWidth > 0 && thumbWidth * snapMultiplier < distance) {
+      isMouseAwayFromThumb = true;
+      APZC_LOG("%p determined mouse is away from thumb, will snap\n", this);
+    }
+  }
+
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  OuterCSSCoord thumbPosition;
+  if (isMouseAwayFromThumb) {
+    thumbPosition = aInitialThumbPos;
+  } else {
+    thumbPosition = ConvertScrollbarPoint(aEvent.mLocalOrigin, scrollbarData) -
+                    aDragMetrics.mScrollbarDragOffset;
+  }
+
+  OuterCSSCoord maxThumbPos = scrollbarData.mScrollTrackLength;
+  maxThumbPos -= scrollbarData.mThumbLength;
+
+  float scrollPercent =
+      maxThumbPos.value == 0.0f ? 0.0f : (float)(thumbPosition / maxThumbPos);
+  APZC_LOG("%p scrollbar dragged to %f percent\n", this, scrollPercent);
+
+  CSSCoord minScrollPosition =
+      GetAxisStart(direction, Metrics().GetScrollableRect().TopLeft());
+  CSSCoord maxScrollPosition =
+      GetAxisStart(direction, Metrics().GetScrollableRect().BottomRight()) -
+      GetAxisLength(direction, Metrics().CalculateCompositedSizeInCssPixels());
+  CSSCoord scrollPosition =
+      minScrollPosition +
+      (scrollPercent * (maxScrollPosition - minScrollPosition));
+
+  scrollPosition = std::max(scrollPosition, minScrollPosition);
+  scrollPosition = std::min(scrollPosition, maxScrollPosition);
+
+  CSSPoint scrollOffset = Metrics().GetVisualScrollOffset();
+  if (direction == ScrollDirection::eHorizontal) {
+    scrollOffset.x = scrollPosition;
+  } else {
+    scrollOffset.y = scrollPosition;
+  }
+  APZC_LOG("%p set scroll offset to %s from scrollbar drag\n", this,
+           ToString(scrollOffset).c_str());
+  SetVisualScrollOffset(scrollOffset);
+  ScheduleCompositeAndMaybeRepaint();
+
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+nsEventStatus AsyncPanZoomController::HandleInputEvent(
+    const InputData& aEvent,
+    const ScreenToParentLayerMatrix4x4& aTransformToApzc) {
+  APZThreadUtils::AssertOnControllerThread();
+
+  nsEventStatus rv = nsEventStatus_eIgnore;
+
+  switch (aEvent.mInputType) {
+    case MULTITOUCH_INPUT: {
+      MultiTouchInput multiTouchInput = aEvent.AsMultiTouchInput();
+      RefPtr<GestureEventListener> listener = GetGestureEventListener();
+      if (listener) {
+        // We only care about screen coordinates in the gesture listener,
+        // so we don't bother transforming the event to parent layer coordinates
+        rv = listener->HandleInputEvent(multiTouchInput);
+        if (rv == nsEventStatus_eConsumeNoDefault) {
+          return rv;
+        }
+      }
+
+      if (!multiTouchInput.TransformToLocal(aTransformToApzc)) {
+        return rv;
+      }
+
+      switch (multiTouchInput.mType) {
+        case MultiTouchInput::MULTITOUCH_START:
+          rv = OnTouchStart(multiTouchInput);
+          break;
+        case MultiTouchInput::MULTITOUCH_MOVE:
+          rv = OnTouchMove(multiTouchInput);
+          break;
+        case MultiTouchInput::MULTITOUCH_END:
+          rv = OnTouchEnd(multiTouchInput);
+          break;
+        case MultiTouchInput::MULTITOUCH_CANCEL:
+          rv = OnTouchCancel(multiTouchInput);
+          break;
+      }
+      break;
+    }
+    case PANGESTURE_INPUT: {
+      PanGestureInput panGestureInput = aEvent.AsPanGestureInput();
+      if (!panGestureInput.TransformToLocal(aTransformToApzc)) {
+        return rv;
+      }
+
+      switch (panGestureInput.mType) {
+        case PanGestureInput::PANGESTURE_MAYSTART:
+          rv = OnPanMayBegin(panGestureInput);
+          break;
+        case PanGestureInput::PANGESTURE_CANCELLED:
+          rv = OnPanCancelled(panGestureInput);
+          break;
+        case PanGestureInput::PANGESTURE_START:
+          rv = OnPanBegin(panGestureInput);
+          break;
+        case PanGestureInput::PANGESTURE_PAN:
+          rv = OnPan(panGestureInput, FingersOnTouchpad::Yes);
+          break;
+        case PanGestureInput::PANGESTURE_END:
+          rv = OnPanEnd(panGestureInput);
+          break;
+        case PanGestureInput::PANGESTURE_MOMENTUMSTART:
+          rv = OnPanMomentumStart(panGestureInput);
+          break;
+        case PanGestureInput::PANGESTURE_MOMENTUMPAN:
+          rv = OnPan(panGestureInput, FingersOnTouchpad::No);
+          break;
+        case PanGestureInput::PANGESTURE_MOMENTUMEND:
+          rv = OnPanMomentumEnd(panGestureInput);
+          break;
+        case PanGestureInput::PANGESTURE_INTERRUPTED:
+          rv = OnPanInterrupted(panGestureInput);
+          break;
+      }
+      break;
+    }
+    case MOUSE_INPUT: {
+      MouseInput mouseInput = aEvent.AsMouseInput();
+      if (!mouseInput.TransformToLocal(aTransformToApzc)) {
+        return rv;
+      }
+      break;
+    }
+    case SCROLLWHEEL_INPUT: {
+      ScrollWheelInput scrollInput = aEvent.AsScrollWheelInput();
+      if (!scrollInput.TransformToLocal(aTransformToApzc)) {
+        return rv;
+      }
+
+      rv = OnScrollWheel(scrollInput);
+      break;
+    }
+    case PINCHGESTURE_INPUT: {
+      // The APZCTreeManager should take care of ensuring that only root-content
+      // APZCs get pinch inputs.
+      MOZ_ASSERT(IsRootContent());
+      PinchGestureInput pinchInput = aEvent.AsPinchGestureInput();
+      if (!pinchInput.TransformToLocal(aTransformToApzc)) {
+        return rv;
+      }
+
+      rv = HandleGestureEvent(pinchInput);
+      break;
+    }
+    case TAPGESTURE_INPUT: {
+      TapGestureInput tapInput = aEvent.AsTapGestureInput();
+      if (!tapInput.TransformToLocal(aTransformToApzc)) {
+        return rv;
+      }
+
+      rv = HandleGestureEvent(tapInput);
+      break;
+    }
+    case KEYBOARD_INPUT: {
+      const KeyboardInput& keyInput = aEvent.AsKeyboardInput();
+      rv = OnKeyboard(keyInput);
+      break;
+    }
+  }
+
+  return rv;
+}
+
+nsEventStatus AsyncPanZoomController::HandleGestureEvent(
+    const InputData& aEvent) {
+  APZThreadUtils::AssertOnControllerThread();
+
+  nsEventStatus rv = nsEventStatus_eIgnore;
+
+  switch (aEvent.mInputType) {
+    case PINCHGESTURE_INPUT: {
+      // This may be invoked via a one-touch-pinch gesture from
+      // GestureEventListener. In that case we want redirect it to the enclosing
+      // root-content APZC.
+      if (!IsRootContent()) {
+        if (APZCTreeManager* treeManagerLocal = GetApzcTreeManager()) {
+          if (RefPtr<AsyncPanZoomController> root =
+                  treeManagerLocal->FindZoomableApzc(this)) {
+            rv = root->HandleGestureEvent(aEvent);
+          }
+        }
+        break;
+      }
+      PinchGestureInput pinchGestureInput = aEvent.AsPinchGestureInput();
+      pinchGestureInput.TransformToLocal(GetTransformToThis());
+      switch (pinchGestureInput.mType) {
+        case PinchGestureInput::PINCHGESTURE_START:
+          rv = OnScaleBegin(pinchGestureInput);
+          break;
+        case PinchGestureInput::PINCHGESTURE_SCALE:
+          rv = OnScale(pinchGestureInput);
+          break;
+        case PinchGestureInput::PINCHGESTURE_FINGERLIFTED:
+        case PinchGestureInput::PINCHGESTURE_END:
+          rv = OnScaleEnd(pinchGestureInput);
+          break;
+      }
+      break;
+    }
+    case TAPGESTURE_INPUT: {
+      TapGestureInput tapGestureInput = aEvent.AsTapGestureInput();
+      tapGestureInput.TransformToLocal(GetTransformToThis());
+      switch (tapGestureInput.mType) {
+        case TapGestureInput::TAPGESTURE_LONG:
+          rv = OnLongPress(tapGestureInput);
+          break;
+        case TapGestureInput::TAPGESTURE_LONG_UP:
+          rv = OnLongPressUp(tapGestureInput);
+          break;
+        case TapGestureInput::TAPGESTURE_UP:
+          rv = OnSingleTapUp(tapGestureInput);
+          break;
+        case TapGestureInput::TAPGESTURE_CONFIRMED:
+          rv = OnSingleTapConfirmed(tapGestureInput);
+          break;
+        case TapGestureInput::TAPGESTURE_DOUBLE:
+          // This means that double tapping on an oop iframe "works" in that we
+          // don't try (and fail) to zoom the oop iframe. But it also means it
+          // is impossible to zoom to some content inside that oop iframe.
+          // Instead the best we can do is zoom to the oop iframe itself. This
+          // is consistent with what Chrome and Safari currently do. Allowing
+          // zooming to content inside an oop iframe would be decently
+          // complicated and it doesn't seem worth it. Bug 1715179 is on file
+          // for this.
+          if (!IsRootContent()) {
+            if (APZCTreeManager* treeManagerLocal = GetApzcTreeManager()) {
+              if (RefPtr<AsyncPanZoomController> root =
+                      treeManagerLocal->FindZoomableApzc(this)) {
+                rv = root->OnDoubleTap(tapGestureInput);
+              }
+            }
+            break;
+          }
+          rv = OnDoubleTap(tapGestureInput);
+          break;
+        case TapGestureInput::TAPGESTURE_SECOND:
+          rv = OnSecondTap(tapGestureInput);
+          break;
+        case TapGestureInput::TAPGESTURE_CANCEL:
+          rv = OnCancelTap(tapGestureInput);
+          break;
+      }
+      break;
+    }
+    default:
+      MOZ_ASSERT_UNREACHABLE("Unhandled input event");
+      break;
+  }
+
+  return rv;
+}
+
+void AsyncPanZoomController::StartAutoscroll(const ScreenPoint& aPoint) {
+  // Cancel any existing animation.
+  CancelAnimation();
+
+  SetState(AUTOSCROLL);
+  StartAnimation(new AutoscrollAnimation(*this, aPoint));
+}
+
+void AsyncPanZoomController::StopAutoscroll() {
+  if (mState == AUTOSCROLL) {
+    CancelAnimation(TriggeredExternally);
+  }
+}
+
+nsEventStatus AsyncPanZoomController::OnTouchStart(
+    const MultiTouchInput& aEvent) {
+  APZC_LOG_DETAIL("got a touch-start in state %s\n", this,
+                  ToString(mState).c_str());
+  mPanDirRestricted = false;
+
+  switch (mState) {
+    case FLING:
+    case ANIMATING_ZOOM:
+    case SMOOTH_SCROLL:
+    case SMOOTHMSD_SCROLL:
+    case OVERSCROLL_ANIMATION:
+    case WHEEL_SCROLL:
+    case KEYBOARD_SCROLL:
+    case PAN_MOMENTUM:
+    case AUTOSCROLL:
+      MOZ_ASSERT(GetCurrentTouchBlock());
+      GetCurrentTouchBlock()->GetOverscrollHandoffChain()->CancelAnimations(
+          ExcludeOverscroll);
+      [[fallthrough]];
+    case SCROLLBAR_DRAG:
+    case NOTHING: {
+      ParentLayerPoint point = GetFirstTouchPoint(aEvent);
+      mLastTouch.mPosition = mStartTouch = GetFirstExternalTouchPoint(aEvent);
+      StartTouch(point, aEvent.mTimeStamp);
+      if (RefPtr<GeckoContentController> controller =
+              GetGeckoContentController()) {
+        MOZ_ASSERT(GetCurrentTouchBlock());
+        controller->NotifyAPZStateChange(
+            GetGuid(), APZStateChange::eStartTouch,
+            GetCurrentTouchBlock()->GetOverscrollHandoffChain()->CanBePanned(
+                this),
+            Some(GetCurrentTouchBlock()->GetBlockId()));
+      }
+      mLastTouch.mTimeStamp = mTouchStartTime = aEvent.mTimeStamp;
+      SetState(TOUCHING);
+      break;
+    }
+    case TOUCHING:
+    case PANNING:
+    case PANNING_LOCKED_X:
+    case PANNING_LOCKED_Y:
+    case PINCHING:
+      NS_WARNING("Received impossible touch in OnTouchStart");
+      break;
+  }
+
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+nsEventStatus AsyncPanZoomController::OnTouchMove(
+    const MultiTouchInput& aEvent) {
+  APZC_LOG_DETAIL("got a touch-move in state %s\n", this,
+                  ToString(mState).c_str());
+  switch (mState) {
+    case FLING:
+    case SMOOTHMSD_SCROLL:
+    case NOTHING:
+    case ANIMATING_ZOOM:
+      // May happen if the user double-taps and drags without lifting after the
+      // second tap. Ignore the move if this happens.
+      return nsEventStatus_eIgnore;
+
+    case TOUCHING: {
+      ScreenCoord panThreshold = GetTouchStartTolerance();
+      ExternalPoint extPoint = GetFirstExternalTouchPoint(aEvent);
+      Maybe<std::pair<MultiTouchInput, MultiTouchInput>> splitEvent;
+
+      // We intentionally skip the UpdateWithTouchAtDevicePoint call when the
+      // panThreshold is zero. This ensures more deterministic behaviour during
+      // testing. If we call that, Axis::mPos gets updated to the point of this
+      // touchmove event, but we "consume" the move to overcome the
+      // panThreshold, so it's hard to pan a specific amount reliably from a
+      // mochitest.
+      if (panThreshold > 0.0f) {
+        const float vectorLength = PanVector(extPoint).Length();
+
+        if (vectorLength < panThreshold) {
+          UpdateWithTouchAtDevicePoint(aEvent);
+          mLastTouch = {extPoint, aEvent.mTimeStamp};
+
+          return nsEventStatus_eIgnore;
+        }
+
+        splitEvent = MaybeSplitTouchMoveEvent(aEvent, panThreshold,
+                                              vectorLength, extPoint);
+
+        UpdateWithTouchAtDevicePoint(splitEvent ? splitEvent->first : aEvent);
+      }
+
+      nsEventStatus result;
+      const MultiTouchInput& firstEvent =
+          splitEvent ? splitEvent->first : aEvent;
+
+      MOZ_ASSERT(GetCurrentTouchBlock());
+      if (GetCurrentTouchBlock()->TouchActionAllowsPanningXY()) {
+        // In the calls to StartPanning() below, the first argument needs to be
+        // the External position of |firstEvent|.
+        // However, instead of computing that using
+        // GetFirstExternalTouchPoint(firstEvent), we pass |extPoint| which
+        // has been modified by MaybeSplitTouchMoveEvent() to the desired
+        // value. This is a workaround for the fact that recomputing the
+        // External point would require a round-trip through |mScreenPoint|
+        // which is an integer.
+
+        // User tries to trigger a touch behavior. If allowed touch behavior is
+        // vertical pan + horizontal pan (touch-action value is equal to AUTO)
+        // we can return ConsumeNoDefault status immediately to trigger cancel
+        // event further.
+        // It should happen independent of the parent type (whether it is
+        // scrolling or not).
+        StartPanning(extPoint, firstEvent.mTimeStamp);
+        result = nsEventStatus_eConsumeNoDefault;
+      } else {
+        result = StartPanning(extPoint, firstEvent.mTimeStamp);
+      }
+
+      if (splitEvent && IsInPanningState()) {
+        TrackTouch(splitEvent->second);
+        return nsEventStatus_eConsumeNoDefault;
+      }
+
+      return result;
+    }
+
+    case PANNING:
+    case PANNING_LOCKED_X:
+    case PANNING_LOCKED_Y:
+    case PAN_MOMENTUM:
+      TrackTouch(aEvent);
+      return nsEventStatus_eConsumeNoDefault;
+
+    case PINCHING:
+      // The scale gesture listener should have handled this.
+      NS_WARNING(
+          "Gesture listener should have handled pinching in OnTouchMove.");
+      return nsEventStatus_eIgnore;
+
+    case SMOOTH_SCROLL:
+    case WHEEL_SCROLL:
+    case KEYBOARD_SCROLL:
+    case OVERSCROLL_ANIMATION:
+    case AUTOSCROLL:
+    case SCROLLBAR_DRAG:
+      // Should not receive a touch-move in the OVERSCROLL_ANIMATION state
+      // as touch blocks that begin in an overscrolled state cancel the
+      // animation. The same is true for wheel scroll animations.
+      NS_WARNING("Received impossible touch in OnTouchMove");
+      break;
+  }
+
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+nsEventStatus AsyncPanZoomController::OnTouchEnd(
+    const MultiTouchInput& aEvent) {
+  APZC_LOG_DETAIL("got a touch-end in state %s\n", this,
+                  ToString(mState).c_str());
+  OnTouchEndOrCancel();
+
+  // In case no touch behavior triggered previously we can avoid sending
+  // scroll events or requesting content repaint. This condition is added
+  // to make tests consistent - in case touch-action is NONE (and therefore
+  // no pans/zooms can be performed) we expected neither scroll or repaint
+  // events.
+  if (mState != NOTHING) {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+  }
+
+  switch (mState) {
+    case FLING:
+      // Should never happen.
+      NS_WARNING("Received impossible touch end in OnTouchEnd.");
+      [[fallthrough]];
+    case ANIMATING_ZOOM:
+    case SMOOTHMSD_SCROLL:
+    case NOTHING:
+      // May happen if the user double-taps and drags without lifting after the
+      // second tap. Ignore if this happens.
+      return nsEventStatus_eIgnore;
+
+    case TOUCHING:
+      // We may have some velocity stored on the axis from move events
+      // that were not big enough to trigger scrolling. Clear that out.
+      SetVelocityVector(ParentLayerPoint(0, 0));
+      MOZ_ASSERT(GetCurrentTouchBlock());
+      APZC_LOG("%p still has %u touch points active\n", this,
+               GetCurrentTouchBlock()->GetActiveTouchCount());
+      // In cases where the user is panning, then taps the second finger without
+      // entering a pinch, we will arrive here when the second finger is lifted.
+      // However the first finger is still down so we want to remain in state
+      // TOUCHING.
+      if (GetCurrentTouchBlock()->GetActiveTouchCount() == 0) {
+        // It's possible we may be overscrolled if the user tapped during a
+        // previous overscroll pan. Make sure to snap back in this situation.
+        // An ancestor APZC could be overscrolled instead of this APZC, so
+        // walk the handoff chain as well.
+        GetCurrentTouchBlock()
+            ->GetOverscrollHandoffChain()
+            ->SnapBackOverscrolledApzc(this);
+        mFlingAccelerator.Reset();
+        // SnapBackOverscrolledApzc() will put any APZC it causes to snap back
+        // into the OVERSCROLL_ANIMATION state. If that's not us, since we're
+        // done TOUCHING enter the NOTHING state.
+        if (mState != OVERSCROLL_ANIMATION) {
+          SetState(NOTHING);
+        }
+      }
+      return nsEventStatus_eIgnore;
+
+    case PANNING:
+    case PANNING_LOCKED_X:
+    case PANNING_LOCKED_Y:
+    case PAN_MOMENTUM: {
+      MOZ_ASSERT(GetCurrentTouchBlock());
+      EndTouch(aEvent.mTimeStamp, Axis::ClearAxisLock::Yes);
+      return HandleEndOfPan();
+    }
+    case PINCHING:
+      SetState(NOTHING);
+      // Scale gesture listener should have handled this.
+      NS_WARNING(
+          "Gesture listener should have handled pinching in OnTouchEnd.");
+      return nsEventStatus_eIgnore;
+
+    case SMOOTH_SCROLL:
+    case WHEEL_SCROLL:
+    case KEYBOARD_SCROLL:
+    case OVERSCROLL_ANIMATION:
+    case AUTOSCROLL:
+    case SCROLLBAR_DRAG:
+      // Should not receive a touch-end in the OVERSCROLL_ANIMATION state
+      // as touch blocks that begin in an overscrolled state cancel the
+      // animation. The same is true for WHEEL_SCROLL.
+      NS_WARNING("Received impossible touch in OnTouchEnd");
+      break;
+  }
+
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+nsEventStatus AsyncPanZoomController::OnTouchCancel(
+    const MultiTouchInput& aEvent) {
+  APZC_LOG_DETAIL("got a touch-cancel in state %s\n", this,
+                  ToString(mState).c_str());
+  OnTouchEndOrCancel();
+  CancelAnimationAndGestureState();
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+nsEventStatus AsyncPanZoomController::OnScaleBegin(
+    const PinchGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a scale-begin in state %s\n", this,
+                  ToString(mState).c_str());
+
+  mPinchLocked = false;
+  mPinchPaintTimerSet = false;
+  // Note that there may not be a touch block at this point, if we received the
+  // PinchGestureEvent directly from widget code without any touch events.
+  if (HasReadyTouchBlock() &&
+      !GetCurrentTouchBlock()->TouchActionAllowsPinchZoom()) {
+    return nsEventStatus_eIgnore;
+  }
+
+  // For platforms that don't support APZ zooming, dispatch a message to the
+  // content controller, it may want to do something else with this gesture.
+  // FIXME: bug 1525793 -- this may need to handle zooming or not on a
+  // per-document basis.
+  if (!StaticPrefs::apz_allow_zooming()) {
+    if (RefPtr<GeckoContentController> controller =
+            GetGeckoContentController()) {
+      APZC_LOG("%p notifying controller of pinch gesture start\n", this);
+      controller->NotifyPinchGesture(
+          aEvent.mType, GetGuid(),
+          ViewAs<LayoutDevicePixel>(
+              aEvent.mFocusPoint,
+              PixelCastJustification::
+                  LayoutDeviceIsScreenForUntransformedEvent),
+          0, aEvent.modifiers);
+    }
+  }
+
+  SetState(PINCHING);
+  Telemetry::Accumulate(Telemetry::APZ_ZOOM_PINCHSOURCE, (int)aEvent.mSource);
+  SetVelocityVector(ParentLayerPoint(0, 0));
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  mLastZoomFocus =
+      aEvent.mLocalFocusPoint - Metrics().GetCompositionBounds().TopLeft();
+
+  mPinchEventBuffer.push(aEvent);
+
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+nsEventStatus AsyncPanZoomController::OnScale(const PinchGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a scale in state %s\n", this, ToString(mState).c_str());
+
+  if (HasReadyTouchBlock() &&
+      !GetCurrentTouchBlock()->TouchActionAllowsPinchZoom()) {
+    return nsEventStatus_eIgnore;
+  }
+
+  if (mState != PINCHING) {
+    return nsEventStatus_eConsumeNoDefault;
+  }
+
+  mPinchEventBuffer.push(aEvent);
+  HandlePinchLocking(aEvent);
+  bool allowZoom = ZoomConstraintsAllowZoom() && !mPinchLocked;
+
+  // If we are pinch-locked, this is a two-finger pan.
+  // Tracking panning distance and velocity.
+  // UpdateWithTouchAtDevicePoint() acquires the tree lock, so
+  // it cannot be called while the mRecursiveMutex lock is held.
+  if (mPinchLocked) {
+    mX.UpdateWithTouchAtDevicePoint(aEvent.mLocalFocusPoint.x,
+                                    aEvent.mTimeStamp);
+    mY.UpdateWithTouchAtDevicePoint(aEvent.mLocalFocusPoint.y,
+                                    aEvent.mTimeStamp);
+  }
+
+  // FIXME: bug 1525793 -- this may need to handle zooming or not on a
+  // per-document basis.
+  if (!StaticPrefs::apz_allow_zooming()) {
+    if (RefPtr<GeckoContentController> controller =
+            GetGeckoContentController()) {
+      APZC_LOG("%p notifying controller of pinch gesture\n", this);
+      controller->NotifyPinchGesture(
+          aEvent.mType, GetGuid(),
+          ViewAs<LayoutDevicePixel>(
+              aEvent.mFocusPoint,
+              PixelCastJustification::
+                  LayoutDeviceIsScreenForUntransformedEvent),
+          ViewAs<LayoutDevicePixel>(
+              aEvent.mCurrentSpan - aEvent.mPreviousSpan,
+              PixelCastJustification::
+                  LayoutDeviceIsScreenForUntransformedEvent),
+          aEvent.modifiers);
+    }
+  }
+
+  {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    // Only the root APZC is zoomable, and the root APZC is not allowed to have
+    // different x and y scales. If it did, the calculations in this function
+    // would have to be adjusted (as e.g. it would no longer be valid to take
+    // the minimum or maximum of the ratios of the widths and heights of the
+    // page rect and the composition bounds).
+    MOZ_ASSERT(Metrics().IsRootContent());
+
+    CSSToParentLayerScale userZoom = Metrics().GetZoom();
+    ParentLayerPoint focusPoint =
+        aEvent.mLocalFocusPoint - Metrics().GetCompositionBounds().TopLeft();
+    CSSPoint cssFocusPoint;
+    if (Metrics().GetZoom() != CSSToParentLayerScale(0)) {
+      cssFocusPoint = focusPoint / Metrics().GetZoom();
+    }
+
+    ParentLayerPoint focusChange = mLastZoomFocus - focusPoint;
+    mLastZoomFocus = focusPoint;
+    // If displacing by the change in focus point will take us off page bounds,
+    // then reduce the displacement such that it doesn't.
+    focusChange.x -= mX.DisplacementWillOverscrollAmount(focusChange.x);
+    focusChange.y -= mY.DisplacementWillOverscrollAmount(focusChange.y);
+    if (userZoom != CSSToParentLayerScale(0)) {
+      ScrollBy(focusChange / userZoom);
+    }
+
+    // If the span is zero or close to it, we don't want to process this zoom
+    // change because we're going to get wonky numbers for the spanRatio. So
+    // let's bail out here. Note that we do this after the focus-change-scroll
+    // above, so that if we have a pinch with zero span but changing focus,
+    // such as generated by some Synaptics touchpads on Windows, we still
+    // scroll properly.
+    float prevSpan = aEvent.mPreviousSpan;
+    if (fabsf(prevSpan) <= EPSILON || fabsf(aEvent.mCurrentSpan) <= EPSILON) {
+      // We might have done a nonzero ScrollBy above, so update metrics and
+      // repaint/recomposite
+      ScheduleCompositeAndMaybeRepaint();
+      return nsEventStatus_eConsumeNoDefault;
+    }
+    float spanRatio = aEvent.mCurrentSpan / aEvent.mPreviousSpan;
+
+    // When we zoom in with focus, we can zoom too much towards the boundaries
+    // that we actually go over them. These are the needed displacements along
+    // either axis such that we don't overscroll the boundaries when zooming.
+    CSSPoint neededDisplacement;
+
+    CSSToParentLayerScale realMinZoom = mZoomConstraints.mMinZoom;
+    CSSToParentLayerScale realMaxZoom = mZoomConstraints.mMaxZoom;
+    realMinZoom.scale =
+        std::max(realMinZoom.scale, Metrics().GetCompositionBounds().Width() /
+                                        Metrics().GetScrollableRect().Width());
+    realMinZoom.scale =
+        std::max(realMinZoom.scale, Metrics().GetCompositionBounds().Height() /
+                                        Metrics().GetScrollableRect().Height());
+    if (realMaxZoom < realMinZoom) {
+      realMaxZoom = realMinZoom;
+    }
+
+    bool doScale = allowZoom && ((spanRatio > 1.0 && userZoom < realMaxZoom) ||
+                                 (spanRatio < 1.0 && userZoom > realMinZoom));
+
+    if (doScale) {
+      spanRatio = clamped(spanRatio, realMinZoom.scale / userZoom.scale,
+                          realMaxZoom.scale / userZoom.scale);
+
+      // Note that the spanRatio here should never put us into OVERSCROLL_BOTH
+      // because up above we clamped it.
+      neededDisplacement.x =
+          -mX.ScaleWillOverscrollAmount(spanRatio, cssFocusPoint.x);
+      neededDisplacement.y =
+          -mY.ScaleWillOverscrollAmount(spanRatio, cssFocusPoint.y);
+
+      ScaleWithFocus(spanRatio, cssFocusPoint);
+
+      if (neededDisplacement != CSSPoint()) {
+        ScrollBy(neededDisplacement);
+      }
+
+      // We don't want to redraw on every scale, so throttle it.
+      if (!mPinchPaintTimerSet) {
+        const int delay = StaticPrefs::apz_scale_repaint_delay_ms();
+        if (delay >= 0) {
+          if (RefPtr<GeckoContentController> controller =
+                  GetGeckoContentController()) {
+            mPinchPaintTimerSet = true;
+            controller->PostDelayedTask(
+                NewRunnableMethod(
+                    "layers::AsyncPanZoomController::"
+                    "DoDelayedRequestContentRepaint",
+                    this,
+                    &AsyncPanZoomController::DoDelayedRequestContentRepaint),
+                delay);
+          }
+        }
+      } else if (apz::AboutToCheckerboard(mLastContentPaintMetrics,
+                                          Metrics())) {
+        // If we already scheduled a throttled repaint request but are also
+        // in danger of checkerboarding soon, trigger the repaint request to
+        // go out immediately. This should reduce the amount of time we spend
+        // checkerboarding.
+        //
+        // Note that if we remain in this "about to
+        // checkerboard" state over a period of time with multiple pinch input
+        // events (which is quite likely), then we will flip-flop between taking
+        // the above branch (!mPinchPaintTimerSet) and this branch (which will
+        // flush the repaint request and reset mPinchPaintTimerSet to false).
+        // This is sort of desirable because it halves the number of repaint
+        // requests we send, and therefore reduces IPC traffic.
+        // Keep in mind that many of these repaint requests will be ignored on
+        // the main-thread anyway due to the resolution mismatch - the first
+        // repaint request will be honored because APZ's notion of the painted
+        // resolution matches the actual main thread resolution, but that first
+        // repaint request will change the resolution on the main thread.
+        // Subsequent repaint requests will be ignored in APZCCallbackHelper, at
+        // https://searchfox.org/mozilla-central/rev/e0eb861a187f0bb6d994228f2e0e49b2c9ee455e/gfx/layers/apz/util/APZCCallbackHelper.cpp#331-338,
+        // until we receive a NotifyLayersUpdated call that re-syncs APZ's
+        // notion of the painted resolution to the main thread. These ignored
+        // repaint requests are contributing to IPC traffic needlessly, and so
+        // halving the number of repaint requests (as mentioned above) seems
+        // desirable.
+        DoDelayedRequestContentRepaint();
+      }
+    } else {
+      // Trigger a repaint request after scrolling.
+      RequestContentRepaint();
+    }
+
+    // We did a ScrollBy call above even if we didn't do a scale, so we
+    // should composite for that.
+    ScheduleComposite();
+  }
+
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+nsEventStatus AsyncPanZoomController::OnScaleEnd(
+    const PinchGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a scale-end in state %s\n", this,
+                  ToString(mState).c_str());
+
+  mPinchPaintTimerSet = false;
+
+  if (HasReadyTouchBlock() &&
+      !GetCurrentTouchBlock()->TouchActionAllowsPinchZoom()) {
+    return nsEventStatus_eIgnore;
+  }
+
+  // FIXME: bug 1525793 -- this may need to handle zooming or not on a
+  // per-document basis.
+  if (!StaticPrefs::apz_allow_zooming()) {
+    if (RefPtr<GeckoContentController> controller =
+            GetGeckoContentController()) {
+      controller->NotifyPinchGesture(
+          aEvent.mType, GetGuid(),
+          ViewAs<LayoutDevicePixel>(
+              aEvent.mFocusPoint,
+              PixelCastJustification::
+                  LayoutDeviceIsScreenForUntransformedEvent),
+          0, aEvent.modifiers);
+    }
+  }
+
+  {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    ScheduleComposite();
+    RequestContentRepaint();
+  }
+
+  mPinchEventBuffer.clear();
+
+  if (aEvent.mType == PinchGestureInput::PINCHGESTURE_FINGERLIFTED) {
+    // One finger is still down, so transition to a TOUCHING state
+    if (!mPinchLocked) {
+      mPanDirRestricted = false;
+      mLastTouch.mPosition = mStartTouch =
+          ToExternalPoint(aEvent.mScreenOffset, aEvent.mFocusPoint);
+      mLastTouch.mTimeStamp = mTouchStartTime = aEvent.mTimeStamp;
+      StartTouch(aEvent.mLocalFocusPoint, aEvent.mTimeStamp);
+      SetState(TOUCHING);
+    } else {
+      // If we are pinch locked, StartTouch() was already called
+      // when we entered the pinch lock.
+      StartPanning(ToExternalPoint(aEvent.mScreenOffset, aEvent.mFocusPoint),
+                   aEvent.mTimeStamp);
+    }
+  } else {
+    // Otherwise, handle the gesture being completely done.
+
+    // Some of the code paths below, like ScrollSnap() or HandleEndOfPan(),
+    // may start an animation, but otherwise we want to end up in the NOTHING
+    // state. To avoid state change notification churn, we use a
+    // notification blocker.
+    bool stateWasPinching = (mState == PINCHING);
+    StateChangeNotificationBlocker blocker(this);
+    SetState(NOTHING);
+
+    if (ZoomConstraintsAllowZoom()) {
+      RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+      // We can get into a situation where we are overscrolled at the end of a
+      // pinch if we go into overscroll with a two-finger pan, and then turn
+      // that into a pinch by increasing the span sufficiently. In such a case,
+      // there is no snap-back animation to get us out of overscroll, so we need
+      // to get out of it somehow.
+      // Moreover, in cases of scroll handoff, the overscroll can be on an APZC
+      // further up in the handoff chain rather than on the current APZC, so
+      // we need to clear overscroll along the entire handoff chain.
+      if (HasReadyTouchBlock()) {
+        GetCurrentTouchBlock()->GetOverscrollHandoffChain()->ClearOverscroll();
+      } else {
+        ClearOverscroll();
+      }
+      // Along with clearing the overscroll, we also want to snap to the nearest
+      // snap point as appropriate.
+      ScrollSnap(ScrollSnapFlags::IntendedEndPosition);
+    } else {
+      // when zoom is not allowed
+      EndTouch(aEvent.mTimeStamp, Axis::ClearAxisLock::Yes);
+      if (stateWasPinching) {
+        // still pinching
+        if (HasReadyTouchBlock()) {
+          return HandleEndOfPan();
+        }
+      }
+    }
+  }
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+nsEventStatus AsyncPanZoomController::HandleEndOfPan() {
+  MOZ_ASSERT(!mAnimation);
+  MOZ_ASSERT(GetCurrentTouchBlock() || GetCurrentPanGestureBlock());
+  GetCurrentInputBlock()->GetOverscrollHandoffChain()->FlushRepaints();
+  ParentLayerPoint flingVelocity = GetVelocityVector();
+
+  // Clear our velocities; if DispatchFling() gives the fling to us,
+  // the fling velocity gets *added* to our existing velocity in
+  // AcceptFling().
+  SetVelocityVector(ParentLayerPoint(0, 0));
+  // Clear our state so that we don't stay in the PANNING state
+  // if DispatchFling() gives the fling to somone else. However,
+  // don't send the state change notification until we've determined
+  // what our final state is to avoid notification churn.
+  StateChangeNotificationBlocker blocker(this);
+  SetState(NOTHING);
+
+  APZC_LOG("%p starting a fling animation if %f > %f\n", this,
+           flingVelocity.Length().value,
+           StaticPrefs::apz_fling_min_velocity_threshold());
+
+  if (flingVelocity.Length() <=
+      StaticPrefs::apz_fling_min_velocity_threshold()) {
+    // Relieve overscroll now if needed, since we will not transition to a fling
+    // animation and then an overscroll animation, and relieve it then.
+    GetCurrentInputBlock()
+        ->GetOverscrollHandoffChain()
+        ->SnapBackOverscrolledApzc(this);
+    mFlingAccelerator.Reset();
+    return nsEventStatus_eConsumeNoDefault;
+  }
+
+  // Make a local copy of the tree manager pointer and check that it's not
+  // null before calling DispatchFling(). This is necessary because Destroy(),
+  // which nulls out mTreeManager, could be called concurrently.
+  if (APZCTreeManager* treeManagerLocal = GetApzcTreeManager()) {
+    const FlingHandoffState handoffState{
+        flingVelocity,
+        GetCurrentInputBlock()->GetOverscrollHandoffChain(),
+        Some(mTouchStartRestingTimeBeforePan),
+        mMinimumVelocityDuringPan.valueOr(0),
+        false /* not handoff */,
+        GetCurrentInputBlock()->GetScrolledApzc()};
+    treeManagerLocal->DispatchFling(this, handoffState);
+  }
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+Maybe<LayoutDevicePoint> AsyncPanZoomController::ConvertToGecko(
+    const ScreenIntPoint& aPoint) {
+  if (APZCTreeManager* treeManagerLocal = GetApzcTreeManager()) {
+    if (Maybe<ScreenIntPoint> layoutPoint =
+            treeManagerLocal->ConvertToGecko(aPoint, this)) {
+      return Some(LayoutDevicePoint(ViewAs<LayoutDevicePixel>(
+          *layoutPoint,
+          PixelCastJustification::LayoutDeviceIsScreenForUntransformedEvent)));
+    }
+  }
+  return Nothing();
+}
+
+OuterCSSCoord AsyncPanZoomController::ConvertScrollbarPoint(
+    const ParentLayerPoint& aScrollbarPoint,
+    const ScrollbarData& aThumbData) const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+  CSSPoint scrollbarPoint;
+  if (Metrics().GetZoom() != CSSToParentLayerScale(0)) {
+    // First, get it into the right coordinate space.
+    scrollbarPoint = aScrollbarPoint / Metrics().GetZoom();
+  }
+
+  // The scrollbar can be transformed with the frame but the pres shell
+  // resolution is only applied to the scroll frame.
+  OuterCSSPoint outerScrollbarPoint =
+      scrollbarPoint * Metrics().GetCSSToOuterCSSScale();
+
+  // Now, get it to be relative to the beginning of the scroll track.
+  OuterCSSRect cssCompositionBound =
+      Metrics().CalculateCompositionBoundsInOuterCssPixels();
+  return GetAxisStart(*aThumbData.mDirection, outerScrollbarPoint) -
+         GetAxisStart(*aThumbData.mDirection, cssCompositionBound) -
+         aThumbData.mScrollTrackStart;
+}
+
+static bool AllowsScrollingMoreThanOnePage(double aMultiplier) {
+  return Abs(aMultiplier) >=
+         EventStateManager::MIN_MULTIPLIER_VALUE_ALLOWING_OVER_ONE_PAGE_SCROLL;
+}
+
+ParentLayerPoint AsyncPanZoomController::GetScrollWheelDelta(
+    const ScrollWheelInput& aEvent) const {
+  return GetScrollWheelDelta(aEvent, aEvent.mDeltaX, aEvent.mDeltaY,
+                             aEvent.mUserDeltaMultiplierX,
+                             aEvent.mUserDeltaMultiplierY);
+}
+
+ParentLayerPoint AsyncPanZoomController::GetScrollWheelDelta(
+    const ScrollWheelInput& aEvent, double aDeltaX, double aDeltaY,
+    double aMultiplierX, double aMultiplierY) const {
+  ParentLayerSize scrollAmount;
+  ParentLayerSize pageScrollSize;
+
+  {
+    // Grab the lock to access the frame metrics.
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    LayoutDeviceIntSize scrollAmountLD = mScrollMetadata.GetLineScrollAmount();
+    LayoutDeviceIntSize pageScrollSizeLD =
+        mScrollMetadata.GetPageScrollAmount();
+    scrollAmount = scrollAmountLD / Metrics().GetDevPixelsPerCSSPixel() *
+                   Metrics().GetZoom();
+    pageScrollSize = pageScrollSizeLD / Metrics().GetDevPixelsPerCSSPixel() *
+                     Metrics().GetZoom();
+  }
+
+  ParentLayerPoint delta;
+  switch (aEvent.mDeltaType) {
+    case ScrollWheelInput::SCROLLDELTA_LINE: {
+      delta.x = aDeltaX * scrollAmount.width;
+      delta.y = aDeltaY * scrollAmount.height;
+      break;
+    }
+    case ScrollWheelInput::SCROLLDELTA_PAGE: {
+      delta.x = aDeltaX * pageScrollSize.width;
+      delta.y = aDeltaY * pageScrollSize.height;
+      break;
+    }
+    case ScrollWheelInput::SCROLLDELTA_PIXEL: {
+      delta = ToParentLayerCoordinates(ScreenPoint(aDeltaX, aDeltaY),
+                                       aEvent.mOrigin);
+      break;
+    }
+  }
+
+  // Apply user-set multipliers.
+  delta.x *= aMultiplierX;
+  delta.y *= aMultiplierY;
+
+  // For the conditions under which we allow system scroll overrides, see
+  // WidgetWheelEvent::OverriddenDelta{X,Y}.
+  // Note that we do *not* restrict this to the root content, see bug 1217715
+  // for discussion on this.
+  if (StaticPrefs::mousewheel_system_scroll_override_enabled() &&
+      !aEvent.IsCustomizedByUserPrefs() &&
+      aEvent.mDeltaType == ScrollWheelInput::SCROLLDELTA_LINE &&
+      aEvent.mAllowToOverrideSystemScrollSpeed) {
+    delta.x = WidgetWheelEvent::ComputeOverriddenDelta(delta.x, false);
+    delta.y = WidgetWheelEvent::ComputeOverriddenDelta(delta.y, true);
+  }
+
+  // If this is a line scroll, and this event was part of a scroll series, then
+  // it might need extra acceleration. See WheelHandlingHelper.cpp.
+  if (aEvent.mDeltaType == ScrollWheelInput::SCROLLDELTA_LINE &&
+      aEvent.mScrollSeriesNumber > 0) {
+    int32_t start = StaticPrefs::mousewheel_acceleration_start();
+    if (start >= 0 && aEvent.mScrollSeriesNumber >= uint32_t(start)) {
+      int32_t factor = StaticPrefs::mousewheel_acceleration_factor();
+      if (factor > 0) {
+        delta.x = ComputeAcceleratedWheelDelta(
+            delta.x, aEvent.mScrollSeriesNumber, factor);
+        delta.y = ComputeAcceleratedWheelDelta(
+            delta.y, aEvent.mScrollSeriesNumber, factor);
+      }
+    }
+  }
+
+  // We shouldn't scroll more than one page at once except when the
+  // user preference is large.
+  if (!AllowsScrollingMoreThanOnePage(aMultiplierX) &&
+      Abs(delta.x) > pageScrollSize.width) {
+    delta.x = (delta.x >= 0) ? pageScrollSize.width : -pageScrollSize.width;
+  }
+  if (!AllowsScrollingMoreThanOnePage(aMultiplierY) &&
+      Abs(delta.y) > pageScrollSize.height) {
+    delta.y = (delta.y >= 0) ? pageScrollSize.height : -pageScrollSize.height;
+  }
+
+  return delta;
+}
+
+nsEventStatus AsyncPanZoomController::OnKeyboard(const KeyboardInput& aEvent) {
+  // Mark that this APZC has async key scrolled
+  mTestHasAsyncKeyScrolled = true;
+
+  // Calculate the destination for this keyboard scroll action
+  CSSPoint destination = GetKeyboardDestination(aEvent.mAction);
+  ScrollOrigin scrollOrigin =
+      SmoothScrollAnimation::GetScrollOriginForAction(aEvent.mAction.mType);
+  Maybe<CSSSnapTarget> snapTarget = MaybeAdjustDestinationForScrollSnapping(
+      aEvent, destination, GetScrollSnapFlagsForKeyboardAction(aEvent.mAction));
+  ScrollMode scrollMode = apz::GetScrollModeForOrigin(scrollOrigin);
+
+  RecordScrollPayload(aEvent.mTimeStamp);
+  // If the scrolling is instant, then scroll immediately to the destination
+  if (scrollMode == ScrollMode::Instant) {
+    CancelAnimation();
+
+    ParentLayerPoint startPoint, endPoint;
+
+    {
+      RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+      // CallDispatchScroll interprets the start and end points as the start and
+      // end of a touch scroll so they need to be reversed.
+      startPoint = destination * Metrics().GetZoom();
+      endPoint = Metrics().GetVisualScrollOffset() * Metrics().GetZoom();
+    }
+
+    ParentLayerPoint delta = endPoint - startPoint;
+
+    ScreenPoint distance = ToScreenCoordinates(
+        ParentLayerPoint(fabs(delta.x), fabs(delta.y)), startPoint);
+
+    OverscrollHandoffState handoffState(
+        *mInputQueue->GetCurrentKeyboardBlock()->GetOverscrollHandoffChain(),
+        distance, ScrollSource::Keyboard);
+
+    CallDispatchScroll(startPoint, endPoint, handoffState);
+    ParentLayerPoint remainingDelta = endPoint - startPoint;
+    if (remainingDelta != delta) {
+      // If any scrolling happened, set KEYBOARD_SCROLL explicitly so that it
+      // will trigger a TransformEnd notification.
+      SetState(KEYBOARD_SCROLL);
+    }
+
+    if (snapTarget) {
+      {
+        RecursiveMutexAutoLock lock(mRecursiveMutex);
+        mLastSnapTargetIds = std::move(snapTarget->mTargetIds);
+      }
+    }
+    SetState(NOTHING);
+
+    return nsEventStatus_eConsumeDoDefault;
+  }
+
+  // The lock must be held across the entire update operation, so the
+  // compositor doesn't end the animation before we get a chance to
+  // update it.
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+  if (snapTarget) {
+    // If we're scroll snapping, use a smooth scroll animation to get
+    // the desired physics. Note that SmoothMsdScrollTo() will re-use an
+    // existing smooth scroll animation if there is one.
+    APZC_LOG("%p keyboard scrolling to snap point %s\n", this,
+             ToString(destination).c_str());
+    SmoothMsdScrollTo(std::move(*snapTarget), ScrollTriggeredByScript::No);
+    return nsEventStatus_eConsumeDoDefault;
+  }
+
+  // Use a keyboard scroll animation to scroll, reusing an existing one if it
+  // exists
+  if (mState != KEYBOARD_SCROLL) {
+    CancelAnimation();
+    SetState(KEYBOARD_SCROLL);
+
+    nsPoint initialPosition =
+        CSSPoint::ToAppUnits(Metrics().GetVisualScrollOffset());
+    StartAnimation(
+        new SmoothScrollAnimation(*this, initialPosition, scrollOrigin));
+  }
+
+  // Convert velocity from ParentLayerPoints/ms to ParentLayerPoints/s and then
+  // to appunits/second.
+  nsPoint velocity;
+  if (Metrics().GetZoom() != CSSToParentLayerScale(0)) {
+    velocity =
+        CSSPoint::ToAppUnits(ParentLayerPoint(mX.GetVelocity() * 1000.0f,
+                                              mY.GetVelocity() * 1000.0f) /
+                             Metrics().GetZoom());
+  }
+
+  SmoothScrollAnimation* animation = mAnimation->AsSmoothScrollAnimation();
+  MOZ_ASSERT(animation);
+
+  animation->UpdateDestination(aEvent.mTimeStamp,
+                               CSSPixel::ToAppUnits(destination),
+                               nsSize(velocity.x, velocity.y));
+
+  return nsEventStatus_eConsumeDoDefault;
+}
+
+CSSPoint AsyncPanZoomController::GetKeyboardDestination(
+    const KeyboardScrollAction& aAction) const {
+  CSSSize lineScrollSize;
+  CSSSize pageScrollSize;
+  CSSPoint scrollOffset;
+  CSSRect scrollRect;
+  ParentLayerRect compositionBounds;
+
+  {
+    // Grab the lock to access the frame metrics.
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+    lineScrollSize = mScrollMetadata.GetLineScrollAmount() /
+                     Metrics().GetDevPixelsPerCSSPixel();
+    pageScrollSize = mScrollMetadata.GetPageScrollAmount() /
+                     Metrics().GetDevPixelsPerCSSPixel();
+
+    scrollOffset = GetCurrentAnimationDestination(lock).valueOr(
+        Metrics().GetVisualScrollOffset());
+
+    scrollRect = Metrics().GetScrollableRect();
+    compositionBounds = Metrics().GetCompositionBounds();
+  }
+
+  // Calculate the scroll destination based off of the scroll type and direction
+  CSSPoint scrollDestination = scrollOffset;
+
+  switch (aAction.mType) {
+    case KeyboardScrollAction::eScrollCharacter: {
+      int32_t scrollDistance =
+          StaticPrefs::toolkit_scrollbox_horizontalScrollDistance();
+
+      if (aAction.mForward) {
+        scrollDestination.x += scrollDistance * lineScrollSize.width;
+      } else {
+        scrollDestination.x -= scrollDistance * lineScrollSize.width;
+      }
+      break;
+    }
+    case KeyboardScrollAction::eScrollLine: {
+      int32_t scrollDistance =
+          StaticPrefs::toolkit_scrollbox_verticalScrollDistance();
+      if (scrollDistance * lineScrollSize.height <=
+          compositionBounds.Height()) {
+        if (aAction.mForward) {
+          scrollDestination.y += scrollDistance * lineScrollSize.height;
+        } else {
+          scrollDestination.y -= scrollDistance * lineScrollSize.height;
+        }
+        break;
+      }
+      [[fallthrough]];
+    }
+    case KeyboardScrollAction::eScrollPage: {
+      if (aAction.mForward) {
+        scrollDestination.y += pageScrollSize.height;
+      } else {
+        scrollDestination.y -= pageScrollSize.height;
+      }
+      break;
+    }
+    case KeyboardScrollAction::eScrollComplete: {
+      if (aAction.mForward) {
+        scrollDestination.y = scrollRect.YMost();
+      } else {
+        scrollDestination.y = scrollRect.Y();
+      }
+      break;
+    }
+  }
+
+  return scrollDestination;
+}
+
+ScrollSnapFlags AsyncPanZoomController::GetScrollSnapFlagsForKeyboardAction(
+    const KeyboardScrollAction& aAction) const {
+  switch (aAction.mType) {
+    case KeyboardScrollAction::eScrollCharacter:
+    case KeyboardScrollAction::eScrollLine:
+      return ScrollSnapFlags::IntendedDirection;
+    case KeyboardScrollAction::eScrollPage:
+      return ScrollSnapFlags::IntendedDirection |
+             ScrollSnapFlags::IntendedEndPosition;
+    case KeyboardScrollAction::eScrollComplete:
+      return ScrollSnapFlags::IntendedEndPosition;
+  }
+  return ScrollSnapFlags::Disabled;
+}
+
+ParentLayerPoint AsyncPanZoomController::GetDeltaForEvent(
+    const InputData& aEvent) const {
+  ParentLayerPoint delta;
+  if (aEvent.mInputType == SCROLLWHEEL_INPUT) {
+    delta = GetScrollWheelDelta(aEvent.AsScrollWheelInput());
+  } else if (aEvent.mInputType == PANGESTURE_INPUT) {
+    const PanGestureInput& panInput = aEvent.AsPanGestureInput();
+    delta = ToParentLayerCoordinates(panInput.UserMultipliedPanDisplacement(),
+                                     panInput.mPanStartPoint);
+  }
+  return delta;
+}
+
+CSSRect AsyncPanZoomController::GetCurrentScrollRangeInCssPixels() const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return Metrics().CalculateScrollRange();
+}
+
+// Return whether or not the underlying layer can be scrolled on either axis.
+bool AsyncPanZoomController::CanScroll(const InputData& aEvent) const {
+  ParentLayerPoint delta = GetDeltaForEvent(aEvent);
+  if (!delta.x && !delta.y) {
+    return false;
+  }
+
+  if (SCROLLWHEEL_INPUT == aEvent.mInputType) {
+    const ScrollWheelInput& scrollWheelInput = aEvent.AsScrollWheelInput();
+    // If it's a wheel scroll, we first check if it is an auto-dir scroll.
+    // 1. For an auto-dir scroll, check if it's delta should be adjusted, if it
+    //    is, then we can conclude it must be scrollable; otherwise, fall back
+    //    to checking if it is scrollable without adjusting its delta.
+    // 2. For a non-auto-dir scroll, simply check if it is scrollable without
+    //    adjusting its delta.
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    if (scrollWheelInput.IsAutoDir(mScrollMetadata.ForceMousewheelAutodir())) {
+      auto deltaX = scrollWheelInput.mDeltaX;
+      auto deltaY = scrollWheelInput.mDeltaY;
+      bool isRTL =
+          IsContentOfHonouredTargetRightToLeft(scrollWheelInput.HonoursRoot(
+              mScrollMetadata.ForceMousewheelAutodirHonourRoot()));
+      APZAutoDirWheelDeltaAdjuster adjuster(deltaX, deltaY, mX, mY, isRTL);
+      if (adjuster.ShouldBeAdjusted()) {
+        // If we detect that the delta values should be adjusted for an auto-dir
+        // wheel scroll, then it is impossible to be an unscrollable scroll.
+        return true;
+      }
+    }
+    return CanScrollWithWheel(delta);
+  }
+  return CanScroll(delta);
+}
+
+ScrollDirections AsyncPanZoomController::GetAllowedHandoffDirections() const {
+  ScrollDirections result;
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+  // In Fission there can be non-scrollable APZCs. It's unclear whether
+  // overscroll-behavior should be respected for these
+  // (see https://github.com/w3c/csswg-drafts/issues/6523) but
+  // we currently don't, to match existing practice.
+  const bool isScrollable = mX.CanScroll() || mY.CanScroll();
+  const bool isRoot = IsRootContent();
+  if ((!isScrollable && !isRoot) || mX.OverscrollBehaviorAllowsHandoff()) {
+    result += ScrollDirection::eHorizontal;
+  }
+  if ((!isScrollable && !isRoot) || mY.OverscrollBehaviorAllowsHandoff()) {
+    result += ScrollDirection::eVertical;
+  }
+  return result;
+}
+
+bool AsyncPanZoomController::CanScroll(const ParentLayerPoint& aDelta) const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return mX.CanScroll(ParentLayerCoord(aDelta.x)) ||
+         mY.CanScroll(ParentLayerCoord(aDelta.y));
+}
+
+bool AsyncPanZoomController::CanScrollWithWheel(
+    const ParentLayerPoint& aDelta) const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+  // For more details about the concept of a disregarded direction, refer to the
+  // code in struct ScrollMetadata which defines mDisregardedDirection.
+  Maybe<ScrollDirection> disregardedDirection =
+      mScrollMetadata.GetDisregardedDirection();
+  if (mX.CanScroll(ParentLayerCoord(aDelta.x)) &&
+      disregardedDirection != Some(ScrollDirection::eHorizontal)) {
+    return true;
+  }
+  if (mY.CanScroll(ParentLayerCoord(aDelta.y)) &&
+      disregardedDirection != Some(ScrollDirection::eVertical)) {
+    return true;
+  }
+  return false;
+}
+
+bool AsyncPanZoomController::CanScroll(ScrollDirection aDirection) const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  switch (aDirection) {
+    case ScrollDirection::eHorizontal:
+      return mX.CanScroll();
+    case ScrollDirection::eVertical:
+      return mY.CanScroll();
+  }
+  MOZ_ASSERT_UNREACHABLE("Invalid value");
+  return false;
+}
+
+bool AsyncPanZoomController::CanVerticalScrollWithDynamicToolbar() const {
+  MOZ_ASSERT(IsRootContent());
+
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return mY.CanVerticalScrollWithDynamicToolbar();
+}
+
+bool AsyncPanZoomController::CanScrollDownwards() const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return mY.CanScrollTo(eSideBottom);
+}
+
+SideBits AsyncPanZoomController::ScrollableDirections() const {
+  SideBits result;
+  {  // scope lock to respect lock ordering with APZCTreeManager::mTreeLock
+    // which will be acquired in the `GetCompositorFixedLayerMargins` below.
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    result = mX.ScrollableDirections() | mY.ScrollableDirections();
+  }
+
+  if (IsRootContent()) {
+    if (APZCTreeManager* treeManagerLocal = GetApzcTreeManager()) {
+      ScreenMargin fixedLayerMargins =
+          treeManagerLocal->GetCompositorFixedLayerMargins();
+      {
+        RecursiveMutexAutoLock lock(mRecursiveMutex);
+        result |= mY.ScrollableDirectionsWithDynamicToolbar(fixedLayerMargins);
+      }
+    }
+  }
+
+  return result;
+}
+
+bool AsyncPanZoomController::IsContentOfHonouredTargetRightToLeft(
+    bool aHonoursRoot) const {
+  if (aHonoursRoot) {
+    return mScrollMetadata.IsAutoDirRootContentRTL();
+  }
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return Metrics().IsHorizontalContentRightToLeft();
+}
+
+bool AsyncPanZoomController::AllowScrollHandoffInCurrentBlock() const {
+  bool result = mInputQueue->AllowScrollHandoff();
+  if (!StaticPrefs::apz_allow_immediate_handoff()) {
+    if (InputBlockState* currentBlock = GetCurrentInputBlock()) {
+      // Do not allow handoff beyond the first APZC to scroll.
+      if (currentBlock->GetScrolledApzc() == this) {
+        result = false;
+        APZC_LOG("%p dropping handoff; AllowImmediateHandoff=false\n", this);
+      }
+    }
+  }
+  return result;
+}
+
+void AsyncPanZoomController::DoDelayedRequestContentRepaint() {
+  if (!IsDestroyed() && mPinchPaintTimerSet) {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    RequestContentRepaint();
+  }
+  mPinchPaintTimerSet = false;
+}
+
+void AsyncPanZoomController::DoDelayedTransformEndNotification(
+    PanZoomState aOldState) {
+  if (!IsDestroyed() && IsDelayedTransformEndSet()) {
+    DispatchStateChangeNotification(aOldState, NOTHING);
+  }
+  SetDelayedTransformEnd(false);
+}
+
+static void AdjustDeltaForAllowedScrollDirections(
+    ParentLayerPoint& aDelta,
+    const ScrollDirections& aAllowedScrollDirections) {
+  if (!aAllowedScrollDirections.contains(ScrollDirection::eHorizontal)) {
+    aDelta.x = 0;
+  }
+  if (!aAllowedScrollDirections.contains(ScrollDirection::eVertical)) {
+    aDelta.y = 0;
+  }
+}
+
+nsEventStatus AsyncPanZoomController::OnScrollWheel(
+    const ScrollWheelInput& aEvent) {
+  // Get the scroll wheel's delta values in parent-layer pixels. But before
+  // getting the values, we need to check if it is an auto-dir scroll and if it
+  // should be adjusted, if both answers are yes, let's adjust X and Y values
+  // first, and then get the delta values in parent-layer pixels based on the
+  // adjusted values.
+  bool adjustedByAutoDir = false;
+  auto deltaX = aEvent.mDeltaX;
+  auto deltaY = aEvent.mDeltaY;
+  ParentLayerPoint delta;
+  {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    if (aEvent.IsAutoDir(mScrollMetadata.ForceMousewheelAutodir())) {
+      // It's an auto-dir scroll, so check if its delta should be adjusted, if
+      // so, adjust it.
+      bool isRTL = IsContentOfHonouredTargetRightToLeft(aEvent.HonoursRoot(
+          mScrollMetadata.ForceMousewheelAutodirHonourRoot()));
+      APZAutoDirWheelDeltaAdjuster adjuster(deltaX, deltaY, mX, mY, isRTL);
+      if (adjuster.ShouldBeAdjusted()) {
+        adjuster.Adjust();
+        adjustedByAutoDir = true;
+      }
+    }
+  }
+  // Ensure the calls to GetScrollWheelDelta are outside the mRecursiveMutex
+  // lock since these calls may acquire the APZ tree lock. Holding
+  // mRecursiveMutex while acquiring the APZ tree lock is lock ordering
+  // violation.
+  if (adjustedByAutoDir) {
+    // If the original delta values have been adjusted, we pass them to
+    // replace the original delta values in |aEvent| so that the delta values
+    // in parent-layer pixels are caculated based on the adjusted values, not
+    // the original ones.
+    // Pay special attention to the last two parameters. They are in a swaped
+    // order so that they still correspond to their delta after adjustment.
+    delta = GetScrollWheelDelta(aEvent, deltaX, deltaY,
+                                aEvent.mUserDeltaMultiplierY,
+                                aEvent.mUserDeltaMultiplierX);
+  } else {
+    // If the original delta values haven't been adjusted by auto-dir, just pass
+    // the |aEvent| and caculate the delta values in parent-layer pixels based
+    // on the original delta values from |aEvent|.
+    delta = GetScrollWheelDelta(aEvent);
+  }
+
+  APZC_LOG("%p got a scroll-wheel with delta in parent-layer pixels: %s\n",
+           this, ToString(delta).c_str());
+
+  if (adjustedByAutoDir) {
+    MOZ_ASSERT(delta.x || delta.y,
+               "Adjusted auto-dir delta values can never be all-zero.");
+    APZC_LOG("%p got a scroll-wheel with adjusted auto-dir delta values\n",
+             this);
+  } else if ((delta.x || delta.y) && !CanScrollWithWheel(delta)) {
+    // We can't scroll this apz anymore, so we simply drop the event.
+    if (mInputQueue->GetActiveWheelTransaction() &&
+        StaticPrefs::test_mousescroll()) {
+      if (RefPtr<GeckoContentController> controller =
+              GetGeckoContentController()) {
+        controller->NotifyMozMouseScrollEvent(GetScrollId(),
+                                              u"MozMouseScrollFailed"_ns);
+      }
+    }
+    return nsEventStatus_eConsumeNoDefault;
+  }
+
+  MOZ_ASSERT(mInputQueue->GetCurrentWheelBlock());
+  AdjustDeltaForAllowedScrollDirections(
+      delta, mInputQueue->GetCurrentWheelBlock()->GetAllowedScrollDirections());
+
+  if (delta.x == 0 && delta.y == 0) {
+    // Avoid spurious state changes and unnecessary work
+    return nsEventStatus_eIgnore;
+  }
+
+  switch (aEvent.mScrollMode) {
+    case ScrollWheelInput::SCROLLMODE_INSTANT: {
+      // Wheel events from "clicky" mouse wheels trigger scroll snapping to the
+      // next snap point. Check for this, and adjust the delta to take into
+      // account the snap point.
+      CSSPoint startPosition;
+      {
+        RecursiveMutexAutoLock lock(mRecursiveMutex);
+        startPosition = Metrics().GetVisualScrollOffset();
+      }
+      Maybe<CSSSnapTarget> snapTarget =
+          MaybeAdjustDeltaForScrollSnappingOnWheelInput(aEvent, delta,
+                                                        startPosition);
+
+      ScreenPoint distance = ToScreenCoordinates(
+          ParentLayerPoint(fabs(delta.x), fabs(delta.y)), aEvent.mLocalOrigin);
+
+      CancelAnimation();
+
+      OverscrollHandoffState handoffState(
+          *mInputQueue->GetCurrentWheelBlock()->GetOverscrollHandoffChain(),
+          distance, ScrollSource::Wheel);
+      ParentLayerPoint startPoint = aEvent.mLocalOrigin;
+      ParentLayerPoint endPoint = aEvent.mLocalOrigin - delta;
+      RecordScrollPayload(aEvent.mTimeStamp);
+
+      CallDispatchScroll(startPoint, endPoint, handoffState);
+      ParentLayerPoint remainingDelta = endPoint - startPoint;
+      if (remainingDelta != delta) {
+        // If any scrolling happened, set WHEEL_SCROLL explicitly so that it
+        // will trigger a TransformEnd notification.
+        SetState(WHEEL_SCROLL);
+      }
+
+      if (snapTarget) {
+        {
+          RecursiveMutexAutoLock lock(mRecursiveMutex);
+          mLastSnapTargetIds = std::move(snapTarget->mTargetIds);
+        }
+      }
+      SetState(NOTHING);
+
+      // The calls above handle their own locking; moreover,
+      // ToScreenCoordinates() and CallDispatchScroll() can grab the tree lock.
+      RecursiveMutexAutoLock lock(mRecursiveMutex);
+      RequestContentRepaint();
+
+      break;
+    }
+
+    case ScrollWheelInput::SCROLLMODE_SMOOTH: {
+      // The lock must be held across the entire update operation, so the
+      // compositor doesn't end the animation before we get a chance to
+      // update it.
+      RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+      RecordScrollPayload(aEvent.mTimeStamp);
+      // Perform scroll snapping if appropriate.
+      // If we're already in a wheel scroll or smooth scroll animation,
+      // the delta is applied to its destination, not to the current
+      // scroll position. Take this into account when finding a snap point.
+      CSSPoint startPosition = GetCurrentAnimationDestination(lock).valueOr(
+          Metrics().GetVisualScrollOffset());
+
+      if (Maybe<CSSSnapTarget> snapTarget =
+              MaybeAdjustDeltaForScrollSnappingOnWheelInput(aEvent, delta,
+                                                            startPosition)) {
+        // If we're scroll snapping, use a smooth scroll animation to get
+        // the desired physics. Note that SmoothMsdScrollTo() will re-use an
+        // existing smooth scroll animation if there is one.
+        APZC_LOG("%p wheel scrolling to snap point %s\n", this,
+                 ToString(startPosition).c_str());
+        SmoothMsdScrollTo(std::move(*snapTarget), ScrollTriggeredByScript::No);
+        break;
+      }
+
+      // Otherwise, use a wheel scroll animation, also reusing one if possible.
+      if (mState != WHEEL_SCROLL) {
+        CancelAnimation();
+        SetState(WHEEL_SCROLL);
+
+        nsPoint initialPosition =
+            CSSPoint::ToAppUnits(Metrics().GetVisualScrollOffset());
+        StartAnimation(new WheelScrollAnimation(*this, initialPosition,
+                                                aEvent.mDeltaType));
+      }
+      // Convert velocity from ParentLayerPoints/ms to ParentLayerPoints/s and
+      // then to appunits/second.
+
+      nsPoint deltaInAppUnits;
+      nsPoint velocity;
+      if (Metrics().GetZoom() != CSSToParentLayerScale(0)) {
+        deltaInAppUnits = CSSPoint::ToAppUnits(delta / Metrics().GetZoom());
+        velocity =
+            CSSPoint::ToAppUnits(ParentLayerPoint(mX.GetVelocity() * 1000.0f,
+                                                  mY.GetVelocity() * 1000.0f) /
+                                 Metrics().GetZoom());
+      }
+
+      WheelScrollAnimation* animation = mAnimation->AsWheelScrollAnimation();
+      animation->UpdateDelta(aEvent.mTimeStamp, deltaInAppUnits,
+                             nsSize(velocity.x, velocity.y));
+      break;
+    }
+  }
+
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+void AsyncPanZoomController::NotifyMozMouseScrollEvent(
+    const nsString& aString) const {
+  RefPtr<GeckoContentController> controller = GetGeckoContentController();
+  if (!controller) {
+    return;
+  }
+  controller->NotifyMozMouseScrollEvent(GetScrollId(), aString);
+}
+
+nsEventStatus AsyncPanZoomController::OnPanMayBegin(
+    const PanGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a pan-maybegin in state %s\n", this,
+                  ToString(mState).c_str());
+
+  StartTouch(aEvent.mLocalPanStartPoint, aEvent.mTimeStamp);
+  MOZ_ASSERT(GetCurrentPanGestureBlock());
+  GetCurrentPanGestureBlock()->GetOverscrollHandoffChain()->CancelAnimations();
+
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+nsEventStatus AsyncPanZoomController::OnPanCancelled(
+    const PanGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a pan-cancelled in state %s\n", this,
+                  ToString(mState).c_str());
+
+  mX.CancelGesture();
+  mY.CancelGesture();
+
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+nsEventStatus AsyncPanZoomController::OnPanBegin(
+    const PanGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a pan-begin in state %s\n", this,
+                  ToString(mState).c_str());
+
+  if (mState == SMOOTHMSD_SCROLL) {
+    // SMOOTHMSD_SCROLL scrolls are cancelled by pan gestures.
+    CancelAnimation();
+  }
+
+  StartTouch(aEvent.mLocalPanStartPoint, aEvent.mTimeStamp);
+
+  if (!UsingStatefulAxisLock()) {
+    SetState(PANNING);
+  } else {
+    float dx = aEvent.mPanDisplacement.x, dy = aEvent.mPanDisplacement.y;
+
+    if (dx != 0.0f || dy != 0.0f) {
+      double angle = atan2(dy, dx);  // range [-pi, pi]
+      angle = fabs(angle);           // range [0, pi]
+      HandlePanning(angle);
+    } else {
+      SetState(PANNING);
+    }
+  }
+
+  // Call into OnPan in order to process any delta included in this event.
+  OnPan(aEvent, FingersOnTouchpad::Yes);
+
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+std::tuple<ParentLayerPoint, ScreenPoint>
+AsyncPanZoomController::GetDisplacementsForPanGesture(
+    const PanGestureInput& aEvent) {
+  // Note that there is a multiplier that applies onto the "physical" pan
+  // displacement (how much the user's fingers moved) that produces the
+  // "logical" pan displacement (how much the page should move). For some of the
+  // code below it makes more sense to use the physical displacement rather than
+  // the logical displacement, and vice-versa.
+  ScreenPoint physicalPanDisplacement = aEvent.mPanDisplacement;
+  ParentLayerPoint logicalPanDisplacement =
+      aEvent.UserMultipliedLocalPanDisplacement();
+  if (aEvent.mDeltaType == PanGestureInput::PANDELTA_PAGE) {
+    // Pan events with page units are used by Gtk, so this replicates Gtk:
+    // https://gitlab.gnome.org/GNOME/gtk/blob/c734c7e9188b56f56c3a504abee05fa40c5475ac/gtk/gtkrange.c#L3065-3073
+    CSSSize pageScrollSize;
+    CSSToParentLayerScale zoom;
+    {
+      // Grab the lock to access the frame metrics.
+      RecursiveMutexAutoLock lock(mRecursiveMutex);
+      pageScrollSize = mScrollMetadata.GetPageScrollAmount() /
+                       Metrics().GetDevPixelsPerCSSPixel();
+      zoom = Metrics().GetZoom();
+    }
+    // scrollUnit* is in units of "ParentLayer pixels per page proportion"...
+    auto scrollUnitWidth = std::min(std::pow(pageScrollSize.width, 2.0 / 3.0),
+                                    pageScrollSize.width / 2.0) *
+                           zoom.scale;
+    auto scrollUnitHeight = std::min(std::pow(pageScrollSize.height, 2.0 / 3.0),
+                                     pageScrollSize.height / 2.0) *
+                            zoom.scale;
+    // ... and pan displacements are in units of "page proportion count"
+    // here, so the products of them and scrollUnit* are in ParentLayer pixels
+    ParentLayerPoint physicalPanDisplacementPL(
+        physicalPanDisplacement.x * scrollUnitWidth,
+        physicalPanDisplacement.y * scrollUnitHeight);
+    physicalPanDisplacement = ToScreenCoordinates(physicalPanDisplacementPL,
+                                                  aEvent.mLocalPanStartPoint);
+    logicalPanDisplacement.x *= scrollUnitWidth;
+    logicalPanDisplacement.y *= scrollUnitHeight;
+
+    // Accelerate (decelerate) any pans by raising it to a user configurable
+    // power (apz.touch_acceleration_factor_x, apz.touch_acceleration_factor_y)
+    //
+    // Confine input for pow() to greater than or equal to 0 to avoid domain
+    // errors with non-integer exponents
+    if (mX.GetVelocity() != 0) {
+      float absVelocity = std::abs(mX.GetVelocity());
+      logicalPanDisplacement.x *=
+          std::pow(absVelocity,
+                   StaticPrefs::apz_touch_acceleration_factor_x()) /
+          absVelocity;
+    }
+
+    if (mY.GetVelocity() != 0) {
+      float absVelocity = std::abs(mY.GetVelocity());
+      logicalPanDisplacement.y *=
+          std::pow(absVelocity,
+                   StaticPrefs::apz_touch_acceleration_factor_y()) /
+          absVelocity;
+    }
+  }
+
+  MOZ_ASSERT(GetCurrentPanGestureBlock());
+  AdjustDeltaForAllowedScrollDirections(
+      logicalPanDisplacement,
+      GetCurrentPanGestureBlock()->GetAllowedScrollDirections());
+
+  if (GetAxisLockMode() == DOMINANT_AXIS) {
+    // Given a pan gesture and both directions have a delta, implement
+    // dominant axis scrolling and only use the delta for the larger
+    // axis.
+    if (logicalPanDisplacement.y != 0 && logicalPanDisplacement.x != 0) {
+      if (fabs(logicalPanDisplacement.y) >= fabs(logicalPanDisplacement.x)) {
+        logicalPanDisplacement.x = 0;
+        physicalPanDisplacement.x = 0;
+      } else {
+        logicalPanDisplacement.y = 0;
+        physicalPanDisplacement.y = 0;
+      }
+    }
+  }
+
+  return {logicalPanDisplacement, physicalPanDisplacement};
+}
+
+nsEventStatus AsyncPanZoomController::OnPan(
+    const PanGestureInput& aEvent, FingersOnTouchpad aFingersOnTouchpad) {
+  APZC_LOG_DETAIL("got a pan-pan in state %s\n", this,
+                  ToString(GetState()).c_str());
+
+  if (GetState() == SMOOTHMSD_SCROLL) {
+    if (aFingersOnTouchpad == FingersOnTouchpad::No) {
+      // When a SMOOTHMSD_SCROLL scroll is being processed on a frame, mouse
+      // wheel and trackpad momentum scroll position updates will not cancel the
+      // SMOOTHMSD_SCROLL scroll animations, enabling scripts that depend on
+      // them to be responsive without forcing the user to wait for the momentum
+      // scrolling to completely stop.
+      return nsEventStatus_eConsumeNoDefault;
+    }
+
+    // SMOOTHMSD_SCROLL scrolls are cancelled by pan gestures.
+    CancelAnimation();
+  }
+
+  if (GetState() == NOTHING) {
+    // This event block was interrupted by something else. If the user's fingers
+    // are still on on the touchpad we want to resume scrolling, otherwise we
+    // ignore the rest of the scroll gesture.
+    if (aFingersOnTouchpad == FingersOnTouchpad::No) {
+      return nsEventStatus_eConsumeNoDefault;
+    }
+    // Resume / restart the pan.
+    // PanBegin will call back into this function with mState == PANNING.
+    return OnPanBegin(aEvent);
+  }
+
+  auto [logicalPanDisplacement, physicalPanDisplacement] =
+      GetDisplacementsForPanGesture(aEvent);
+
+  {
+    // Grab the lock to protect the animation from being canceled on the updater
+    // thread.
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    MOZ_ASSERT_IF(GetState() == OVERSCROLL_ANIMATION, mAnimation);
+
+    if (GetState() == OVERSCROLL_ANIMATION && mAnimation &&
+        aFingersOnTouchpad == FingersOnTouchpad::No) {
+      // If there is an on-going overscroll animation, we tell the animation
+      // whether the displacements should be handled by the animation or not.
+      MOZ_ASSERT(mAnimation->AsOverscrollAnimation());
+      if (RefPtr<OverscrollAnimation> overscrollAnimation =
+              mAnimation->AsOverscrollAnimation()) {
+        overscrollAnimation->HandlePanMomentum(logicalPanDisplacement);
+        // And then as a result of the above call, if the animation is currently
+        // affecting on the axis, drop the displacement value on the axis so
+        // that we stop further oversrolling on the axis.
+        if (overscrollAnimation->IsManagingXAxis()) {
+          logicalPanDisplacement.x = 0;
+          physicalPanDisplacement.x = 0;
+        }
+        if (overscrollAnimation->IsManagingYAxis()) {
+          logicalPanDisplacement.y = 0;
+          physicalPanDisplacement.y = 0;
+        }
+      }
+    }
+  }
+
+  HandlePanningUpdate(physicalPanDisplacement);
+
+  MOZ_ASSERT(GetCurrentPanGestureBlock());
+  ScreenPoint panDistance(fabs(physicalPanDisplacement.x),
+                          fabs(physicalPanDisplacement.y));
+  OverscrollHandoffState handoffState(
+      *GetCurrentPanGestureBlock()->GetOverscrollHandoffChain(), panDistance,
+      ScrollSource::Touchpad);
+
+  // Create fake "touch" positions that will result in the desired scroll
+  // motion. Note that the pan displacement describes the change in scroll
+  // position: positive displacement values mean that the scroll position
+  // increases. However, an increase in scroll position means that the scrolled
+  // contents are moved to the left / upwards. Since our simulated "touches"
+  // determine the motion of the scrolled contents, not of the scroll position,
+  // they need to move in the opposite direction of the pan displacement.
+  ParentLayerPoint startPoint = aEvent.mLocalPanStartPoint;
+  ParentLayerPoint endPoint =
+      aEvent.mLocalPanStartPoint - logicalPanDisplacement;
+  if (logicalPanDisplacement != ParentLayerPoint()) {
+    // Don't expect a composite to be triggered if the displacement is zero
+    RecordScrollPayload(aEvent.mTimeStamp);
+  }
+
+  const ParentLayerPoint velocity = GetVelocityVector();
+  bool consumed = CallDispatchScroll(startPoint, endPoint, handoffState);
+
+  const ParentLayerPoint visualDisplacement = ToParentLayerCoordinates(
+      handoffState.mTotalMovement, aEvent.mPanStartPoint);
+  // We need to update the axis velocity in order to get a useful display port
+  // size and position. We need to do so even if this is a momentum pan (i.e.
+  // aFingersOnTouchpad == No); in that case the "with touch" part is not
+  // really appropriate, so we may want to rethink this at some point.
+  // Note that we have to make all simulated positions relative to
+  // Axis::GetPos(), because the current position is an invented position, and
+  // because resetting the position to the mouse position (e.g.
+  // aEvent.mLocalStartPoint) would mess up velocity calculation. (This is
+  // the only caller of UpdateWithTouchAtDevicePoint() for pan events, so
+  // there is no risk of other calls resetting the position.)
+  // Also note that if there is an on-going overscroll animation in the axis,
+  // we shouldn't call UpdateWithTouchAtDevicePoint because the call changes
+  // the velocity which should be managed by the overscroll animation.
+  // Finally, note that we do this *after* CallDispatchScroll(), so that the
+  // position we use reflects the actual amount of movement that occurred
+  // (in particular, if we're in overscroll, if reflects the amount of movement
+  // *after* applying resistance). This is important because we want the axis
+  // velocity to track the visual movement speed of the page.
+  if (visualDisplacement.x != 0) {
+    mX.UpdateWithTouchAtDevicePoint(mX.GetPos() - visualDisplacement.x,
+                                    aEvent.mTimeStamp);
+  }
+  if (visualDisplacement.y != 0) {
+    mY.UpdateWithTouchAtDevicePoint(mY.GetPos() - visualDisplacement.y,
+                                    aEvent.mTimeStamp);
+  }
+
+  if (aFingersOnTouchpad == FingersOnTouchpad::No) {
+    if (IsOverscrolled() && GetState() != OVERSCROLL_ANIMATION) {
+      StartOverscrollAnimation(velocity, GetOverscrollSideBits());
+    } else if (!consumed) {
+      // If there is unconsumed scroll and we're in the momentum part of the
+      // pan gesture, terminate the momentum scroll. This prevents momentum
+      // scroll events from unexpectedly causing scrolling later if somehow
+      // the APZC becomes scrollable again in this direction (e.g. if the user
+      // uses some other input method to scroll in the opposite direction).
+      SetState(NOTHING);
+    }
+  }
+
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+nsEventStatus AsyncPanZoomController::OnPanEnd(const PanGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a pan-end in state %s\n", this,
+                  ToString(mState).c_str());
+
+  // This can happen if the OS sends a second pan-end event after the first one
+  // has already started an overscroll animation or entered a fling state.
+  // This has been observed on some Wayland versions.
+  PanZoomState currentState = GetState();
+  if (currentState == OVERSCROLL_ANIMATION || currentState == NOTHING ||
+      currentState == FLING) {
+    return nsEventStatus_eIgnore;
+  }
+
+  if (aEvent.mPanDisplacement != ScreenPoint{}) {
+    // Call into OnPan in order to process the delta included in this event.
+    OnPan(aEvent, FingersOnTouchpad::Yes);
+  }
+
+  // Do not unlock the axis lock at the end of a pan gesture. The axis lock
+  // should extend into the momentum scroll.
+  EndTouch(aEvent.mTimeStamp, Axis::ClearAxisLock::No);
+
+  // Use HandleEndOfPan for fling on platforms that don't
+  // emit momentum events (Gtk).
+  if (aEvent.mSimulateMomentum) {
+    return HandleEndOfPan();
+  }
+
+  MOZ_ASSERT(GetCurrentPanGestureBlock());
+  RefPtr<const OverscrollHandoffChain> overscrollHandoffChain =
+      GetCurrentPanGestureBlock()->GetOverscrollHandoffChain();
+
+  // Call SnapBackOverscrolledApzcForMomentum regardless whether this APZC is
+  // overscrolled or not since overscroll animations for ancestor APZCs in this
+  // overscroll handoff chain might have been cancelled by the current pan
+  // gesture block.
+  overscrollHandoffChain->SnapBackOverscrolledApzcForMomentum(
+      this, GetVelocityVector());
+  // If this APZC is overscrolled, the above SnapBackOverscrolledApzcForMomentum
+  // triggers an overscroll animation. When we're finished with the overscroll
+  // animation, the state will be reset and a TransformEnd will be sent to the
+  // main thread.
+  currentState = GetState();
+  if (currentState != OVERSCROLL_ANIMATION) {
+    // Do not send a state change notification to the content controller here.
+    // Instead queue a delayed task to dispatch the notification if no
+    // momentum pan or scroll snap follows the pan-end.
+    RefPtr<GeckoContentController> controller = GetGeckoContentController();
+    if (controller) {
+      SetDelayedTransformEnd(true);
+      controller->PostDelayedTask(
+          NewRunnableMethod<PanZoomState>(
+              "layers::AsyncPanZoomController::"
+              "DoDelayedTransformEndNotification",
+              this, &AsyncPanZoomController::DoDelayedTransformEndNotification,
+              currentState),
+          StaticPrefs::apz_scrollend_event_content_delay_ms());
+      SetStateNoContentControllerDispatch(NOTHING);
+    } else {
+      SetState(NOTHING);
+    }
+  }
+
+  // Drop any velocity on axes where we don't have room to scroll anyways
+  // (in this APZC, or an APZC further in the handoff chain).
+  // This ensures that we don't enlarge the display port unnecessarily.
+  {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    if (!overscrollHandoffChain->CanScrollInDirection(
+            this, ScrollDirection::eHorizontal)) {
+      mX.SetVelocity(0);
+    }
+    if (!overscrollHandoffChain->CanScrollInDirection(
+            this, ScrollDirection::eVertical)) {
+      mY.SetVelocity(0);
+    }
+  }
+
+  RequestContentRepaint();
+  ScrollSnapToDestination();
+
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+nsEventStatus AsyncPanZoomController::OnPanMomentumStart(
+    const PanGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a pan-momentumstart in state %s\n", this,
+                  ToString(mState).c_str());
+
+  if (mState == SMOOTHMSD_SCROLL || mState == OVERSCROLL_ANIMATION) {
+    return nsEventStatus_eConsumeNoDefault;
+  }
+
+  if (IsDelayedTransformEndSet()) {
+    // Do not send another TransformBegin notification if we have not
+    // delivered a corresponding TransformEnd. Also ensure that any
+    // queued transform-end due to a pan-end is not sent. Instead rely
+    // on the transform-end sent due to the momentum pan.
+    SetDelayedTransformEnd(false);
+    SetStateNoContentControllerDispatch(PAN_MOMENTUM);
+  } else {
+    SetState(PAN_MOMENTUM);
+  }
+
+  // Call into OnPan in order to process any delta included in this event.
+  OnPan(aEvent, FingersOnTouchpad::No);
+
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+nsEventStatus AsyncPanZoomController::OnPanMomentumEnd(
+    const PanGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a pan-momentumend in state %s\n", this,
+                  ToString(mState).c_str());
+
+  if (mState == OVERSCROLL_ANIMATION) {
+    return nsEventStatus_eConsumeNoDefault;
+  }
+
+  // Call into OnPan in order to process any delta included in this event.
+  OnPan(aEvent, FingersOnTouchpad::No);
+
+  // We need to reset the velocity to zero. We don't really have a "touch"
+  // here because the touch has already ended long before the momentum
+  // animation started, but I guess it doesn't really matter for now.
+  mX.CancelGesture();
+  mY.CancelGesture();
+  SetState(NOTHING);
+
+  RequestContentRepaint();
+
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+nsEventStatus AsyncPanZoomController::OnPanInterrupted(
+    const PanGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a pan-interrupted in state %s\n", this,
+                  ToString(mState).c_str());
+
+  CancelAnimation();
+
+  return nsEventStatus_eIgnore;
+}
+
+nsEventStatus AsyncPanZoomController::OnLongPress(
+    const TapGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a long-press in state %s\n", this,
+                  ToString(mState).c_str());
+  RefPtr<GeckoContentController> controller = GetGeckoContentController();
+  if (controller) {
+    if (Maybe<LayoutDevicePoint> geckoScreenPoint =
+            ConvertToGecko(aEvent.mPoint)) {
+      TouchBlockState* touch = GetCurrentTouchBlock();
+      if (!touch) {
+        APZC_LOG(
+            "%p dropping long-press because some non-touch block interrupted "
+            "it\n",
+            this);
+        return nsEventStatus_eIgnore;
+      }
+      if (touch->IsDuringFastFling()) {
+        APZC_LOG("%p dropping long-press because of fast fling\n", this);
+        return nsEventStatus_eIgnore;
+      }
+      uint64_t blockId = GetInputQueue()->InjectNewTouchBlock(this);
+      controller->HandleTap(TapType::eLongTap, *geckoScreenPoint,
+                            aEvent.modifiers, GetGuid(), blockId);
+      return nsEventStatus_eConsumeNoDefault;
+    }
+  }
+  return nsEventStatus_eIgnore;
+}
+
+nsEventStatus AsyncPanZoomController::OnLongPressUp(
+    const TapGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a long-tap-up in state %s\n", this,
+                  ToString(mState).c_str());
+  return GenerateSingleTap(TapType::eLongTapUp, aEvent.mPoint,
+                           aEvent.modifiers);
+}
+
+nsEventStatus AsyncPanZoomController::GenerateSingleTap(
+    TapType aType, const ScreenIntPoint& aPoint,
+    mozilla::Modifiers aModifiers) {
+  RefPtr<GeckoContentController> controller = GetGeckoContentController();
+  if (controller) {
+    if (Maybe<LayoutDevicePoint> geckoScreenPoint = ConvertToGecko(aPoint)) {
+      TouchBlockState* touch = GetCurrentTouchBlock();
+      // |touch| may be null in the case where this function is
+      // invoked by GestureEventListener on a timeout. In that case we already
+      // verified that the single tap is allowed so we let it through.
+      // XXX there is a bug here that in such a case the touch block that
+      // generated this tap will not get its mSingleTapOccurred flag set.
+      // See https://bugzilla.mozilla.org/show_bug.cgi?id=1256344#c6
+      if (touch) {
+        if (touch->IsDuringFastFling()) {
+          APZC_LOG(
+              "%p dropping single-tap because it was during a fast-fling\n",
+              this);
+          return nsEventStatus_eIgnore;
+        }
+        touch->SetSingleTapOccurred();
+      }
+      // Because this may be being running as part of
+      // APZCTreeManager::ReceiveInputEvent, calling controller->HandleTap
+      // directly might mean that content receives the single tap message before
+      // the corresponding touch-up. To avoid that we schedule the singletap
+      // message to run on the next spin of the event loop. See bug 965381 for
+      // the issue this was causing.
+      APZC_LOG("posting runnable for HandleTap from GenerateSingleTap");
+      RefPtr<Runnable> runnable =
+          NewRunnableMethod<TapType, LayoutDevicePoint, mozilla::Modifiers,
+                            ScrollableLayerGuid, uint64_t>(
+              "layers::GeckoContentController::HandleTap", controller,
+              &GeckoContentController::HandleTap, aType, *geckoScreenPoint,
+              aModifiers, GetGuid(), touch ? touch->GetBlockId() : 0);
+
+      controller->PostDelayedTask(runnable.forget(), 0);
+      return nsEventStatus_eConsumeNoDefault;
+    }
+  }
+  return nsEventStatus_eIgnore;
+}
+
+void AsyncPanZoomController::OnTouchEndOrCancel() {
+  if (RefPtr<GeckoContentController> controller = GetGeckoContentController()) {
+    MOZ_ASSERT(GetCurrentTouchBlock());
+    controller->NotifyAPZStateChange(
+        GetGuid(), APZStateChange::eEndTouch,
+        GetCurrentTouchBlock()->SingleTapOccurred(),
+        Some(GetCurrentTouchBlock()->GetBlockId()));
+  }
+}
+
+nsEventStatus AsyncPanZoomController::OnSingleTapUp(
+    const TapGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a single-tap-up in state %s\n", this,
+                  ToString(mState).c_str());
+  // If mZoomConstraints.mAllowDoubleTapZoom is true we wait for a call to
+  // OnSingleTapConfirmed before sending event to content
+  MOZ_ASSERT(GetCurrentTouchBlock());
+  if (!(ZoomConstraintsAllowDoubleTapZoom() &&
+        GetCurrentTouchBlock()->TouchActionAllowsDoubleTapZoom())) {
+    return GenerateSingleTap(TapType::eSingleTap, aEvent.mPoint,
+                             aEvent.modifiers);
+  }
+  return nsEventStatus_eIgnore;
+}
+
+nsEventStatus AsyncPanZoomController::OnSingleTapConfirmed(
+    const TapGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a single-tap-confirmed in state %s\n", this,
+                  ToString(mState).c_str());
+  return GenerateSingleTap(TapType::eSingleTap, aEvent.mPoint,
+                           aEvent.modifiers);
+}
+
+nsEventStatus AsyncPanZoomController::OnDoubleTap(
+    const TapGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a double-tap in state %s\n", this,
+                  ToString(mState).c_str());
+  RefPtr<GeckoContentController> controller = GetGeckoContentController();
+  if (controller) {
+    if (ZoomConstraintsAllowDoubleTapZoom() &&
+        (!GetCurrentTouchBlock() ||
+         GetCurrentTouchBlock()->TouchActionAllowsDoubleTapZoom())) {
+      if (Maybe<LayoutDevicePoint> geckoScreenPoint =
+              ConvertToGecko(aEvent.mPoint)) {
+        controller->HandleTap(
+            TapType::eDoubleTap, *geckoScreenPoint, aEvent.modifiers, GetGuid(),
+            GetCurrentTouchBlock() ? GetCurrentTouchBlock()->GetBlockId() : 0);
+      }
+    }
+    return nsEventStatus_eConsumeNoDefault;
+  }
+  return nsEventStatus_eIgnore;
+}
+
+nsEventStatus AsyncPanZoomController::OnSecondTap(
+    const TapGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a second-tap in state %s\n", this,
+                  ToString(mState).c_str());
+  return GenerateSingleTap(TapType::eSecondTap, aEvent.mPoint,
+                           aEvent.modifiers);
+}
+
+nsEventStatus AsyncPanZoomController::OnCancelTap(
+    const TapGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a cancel-tap in state %s\n", this,
+                  ToString(mState).c_str());
+  // XXX: Implement this.
+  return nsEventStatus_eIgnore;
+}
+
+ScreenToParentLayerMatrix4x4 AsyncPanZoomController::GetTransformToThis()
+    const {
+  if (APZCTreeManager* treeManagerLocal = GetApzcTreeManager()) {
+    return treeManagerLocal->GetScreenToApzcTransform(this);
+  }
+  return ScreenToParentLayerMatrix4x4();
+}
+
+ScreenPoint AsyncPanZoomController::ToScreenCoordinates(
+    const ParentLayerPoint& aVector, const ParentLayerPoint& aAnchor) const {
+  return TransformVector(GetTransformToThis().Inverse(), aVector, aAnchor);
+}
+
+// TODO: figure out a good way to check the w-coordinate is positive and return
+// the result
+ParentLayerPoint AsyncPanZoomController::ToParentLayerCoordinates(
+    const ScreenPoint& aVector, const ScreenPoint& aAnchor) const {
+  return TransformVector(GetTransformToThis(), aVector, aAnchor);
+}
+
+ParentLayerPoint AsyncPanZoomController::ToParentLayerCoordinates(
+    const ScreenPoint& aVector, const ExternalPoint& aAnchor) const {
+  return ToParentLayerCoordinates(
+      aVector,
+      ViewAs<ScreenPixel>(aAnchor, PixelCastJustification::ExternalIsScreen));
+}
+
+ExternalPoint AsyncPanZoomController::ToExternalPoint(
+    const ExternalPoint& aScreenOffset, const ScreenPoint& aScreenPoint) {
+  return aScreenOffset +
+         ViewAs<ExternalPixel>(aScreenPoint,
+                               PixelCastJustification::ExternalIsScreen);
+}
+
+ScreenPoint AsyncPanZoomController::PanVector(const ExternalPoint& aPos) const {
+  return ScreenPoint(fabs(aPos.x - mStartTouch.x),
+                     fabs(aPos.y - mStartTouch.y));
+}
+
+bool AsyncPanZoomController::Contains(const ScreenIntPoint& aPoint) const {
+  ScreenToParentLayerMatrix4x4 transformToThis = GetTransformToThis();
+  Maybe<ParentLayerIntPoint> point = UntransformBy(transformToThis, aPoint);
+  if (!point) {
+    return false;
+  }
+
+  ParentLayerIntRect cb;
+  {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    GetFrameMetrics().GetCompositionBounds().ToIntRect(&cb);
+  }
+  return cb.Contains(*point);
+}
+
+bool AsyncPanZoomController::IsInOverscrollGutter(
+    const ScreenPoint& aHitTestPoint) const {
+  if (!IsPhysicallyOverscrolled()) {
+    return false;
+  }
+
+  Maybe<ParentLayerPoint> apzcPoint =
+      UntransformBy(GetTransformToThis(), aHitTestPoint);
+  if (!apzcPoint) return false;
+  return IsInOverscrollGutter(*apzcPoint);
+}
+
+bool AsyncPanZoomController::IsInOverscrollGutter(
+    const ParentLayerPoint& aHitTestPoint) const {
+  ParentLayerRect compositionBounds;
+  {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    compositionBounds = GetFrameMetrics().GetCompositionBounds();
+  }
+  if (!compositionBounds.Contains(aHitTestPoint)) {
+    // Point is outside of scrollable element's bounds altogether.
+    return false;
+  }
+  auto overscrollTransform = GetOverscrollTransform(eForHitTesting);
+  ParentLayerPoint overscrollUntransformed =
+      overscrollTransform.Inverse().TransformPoint(aHitTestPoint);
+
+  if (compositionBounds.Contains(overscrollUntransformed)) {
+    // Point is over scrollable content.
+    return false;
+  }
+
+  // Point is in gutter.
+  return true;
+}
+
+bool AsyncPanZoomController::IsOverscrolled() const {
+  return mOverscrollEffect->IsOverscrolled();
+}
+
+bool AsyncPanZoomController::IsPhysicallyOverscrolled() const {
+  // As an optimization, avoid calling Apply/UnapplyAsyncTestAttributes
+  // unless we're in a test environment where we need it.
+  if (StaticPrefs::apz_overscroll_test_async_scroll_offset_enabled()) {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    AutoApplyAsyncTestAttributes testAttributeApplier(this, lock);
+    return mX.IsOverscrolled() || mY.IsOverscrolled();
+  }
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return mX.IsOverscrolled() || mY.IsOverscrolled();
+}
+
+bool AsyncPanZoomController::IsInInvalidOverscroll() const {
+  return mX.IsInInvalidOverscroll() || mY.IsInInvalidOverscroll();
+}
+
+ParentLayerPoint AsyncPanZoomController::PanStart() const {
+  return ParentLayerPoint(mX.PanStart(), mY.PanStart());
+}
+
+const ParentLayerPoint AsyncPanZoomController::GetVelocityVector() const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return ParentLayerPoint(mX.GetVelocity(), mY.GetVelocity());
+}
+
+void AsyncPanZoomController::SetVelocityVector(
+    const ParentLayerPoint& aVelocityVector) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  mX.SetVelocity(aVelocityVector.x);
+  mY.SetVelocity(aVelocityVector.y);
+}
+
+void AsyncPanZoomController::HandlePanningWithTouchAction(double aAngle) {
+  // Handling of cross sliding will need to be added in this method after
+  // touch-action released enabled by default.
+  MOZ_ASSERT(GetCurrentTouchBlock());
+  RefPtr<const OverscrollHandoffChain> overscrollHandoffChain =
+      GetCurrentInputBlock()->GetOverscrollHandoffChain();
+  bool canScrollHorizontal =
+      !mX.IsAxisLocked() && overscrollHandoffChain->CanScrollInDirection(
+                                this, ScrollDirection::eHorizontal);
+  bool canScrollVertical =
+      !mY.IsAxisLocked() && overscrollHandoffChain->CanScrollInDirection(
+                                this, ScrollDirection::eVertical);
+  if (GetCurrentTouchBlock()->TouchActionAllowsPanningXY()) {
+    if (canScrollHorizontal && canScrollVertical) {
+      if (apz::IsCloseToHorizontal(aAngle,
+                                   StaticPrefs::apz_axis_lock_lock_angle())) {
+        mY.SetAxisLocked(true);
+        SetState(PANNING_LOCKED_X);
+      } else if (apz::IsCloseToVertical(
+                     aAngle, StaticPrefs::apz_axis_lock_lock_angle())) {
+        mX.SetAxisLocked(true);
+        SetState(PANNING_LOCKED_Y);
+      } else {
+        SetState(PANNING);
+      }
+    } else if (canScrollHorizontal || canScrollVertical) {
+      SetState(PANNING);
+    } else {
+      SetState(NOTHING);
+    }
+  } else if (GetCurrentTouchBlock()->TouchActionAllowsPanningX()) {
+    // Using bigger angle for panning to keep behavior consistent
+    // with IE.
+    if (apz::IsCloseToHorizontal(
+            aAngle, StaticPrefs::apz_axis_lock_direct_pan_angle())) {
+      mY.SetAxisLocked(true);
+      SetState(PANNING_LOCKED_X);
+      mPanDirRestricted = true;
+    } else {
+      // Don't treat these touches as pan/zoom movements since 'touch-action'
+      // value requires it.
+      SetState(NOTHING);
+    }
+  } else if (GetCurrentTouchBlock()->TouchActionAllowsPanningY()) {
+    if (apz::IsCloseToVertical(aAngle,
+                               StaticPrefs::apz_axis_lock_direct_pan_angle())) {
+      mX.SetAxisLocked(true);
+      SetState(PANNING_LOCKED_Y);
+      mPanDirRestricted = true;
+    } else {
+      SetState(NOTHING);
+    }
+  } else {
+    SetState(NOTHING);
+  }
+  if (!IsInPanningState()) {
+    // If we didn't enter a panning state because touch-action disallowed it,
+    // make sure to clear any leftover velocity from the pre-threshold
+    // touchmoves.
+    mX.SetVelocity(0);
+    mY.SetVelocity(0);
+  }
+}
+
+void AsyncPanZoomController::HandlePanning(double aAngle) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  MOZ_ASSERT(GetCurrentInputBlock());
+  RefPtr<const OverscrollHandoffChain> overscrollHandoffChain =
+      GetCurrentInputBlock()->GetOverscrollHandoffChain();
+  bool canScrollHorizontal =
+      !mX.IsAxisLocked() && overscrollHandoffChain->CanScrollInDirection(
+                                this, ScrollDirection::eHorizontal);
+  bool canScrollVertical =
+      !mY.IsAxisLocked() && overscrollHandoffChain->CanScrollInDirection(
+                                this, ScrollDirection::eVertical);
+
+  MOZ_ASSERT(UsingStatefulAxisLock());
+
+  if (!canScrollHorizontal || !canScrollVertical) {
+    SetState(PANNING);
+  } else if (apz::IsCloseToHorizontal(
+                 aAngle, StaticPrefs::apz_axis_lock_lock_angle())) {
+    mY.SetAxisLocked(true);
+    if (canScrollHorizontal) {
+      SetState(PANNING_LOCKED_X);
+    }
+  } else if (apz::IsCloseToVertical(aAngle,
+                                    StaticPrefs::apz_axis_lock_lock_angle())) {
+    mX.SetAxisLocked(true);
+    if (canScrollVertical) {
+      SetState(PANNING_LOCKED_Y);
+    }
+  } else {
+    SetState(PANNING);
+  }
+}
+
+void AsyncPanZoomController::HandlePanningUpdate(
+    const ScreenPoint& aPanDistance) {
+  // If we're axis-locked, check if the user is trying to break the lock
+  if (GetAxisLockMode() == STICKY && !mPanDirRestricted) {
+    ParentLayerPoint vector =
+        ToParentLayerCoordinates(aPanDistance, mStartTouch);
+
+    float angle = atan2f(vector.y, vector.x);  // range [-pi, pi]
+    angle = fabsf(angle);                      // range [0, pi]
+
+    float breakThreshold =
+        StaticPrefs::apz_axis_lock_breakout_threshold() * GetDPI();
+
+    if (fabs(aPanDistance.x) > breakThreshold ||
+        fabs(aPanDistance.y) > breakThreshold) {
+      switch (mState) {
+        case PANNING_LOCKED_X:
+          if (!apz::IsCloseToHorizontal(
+                  angle, StaticPrefs::apz_axis_lock_breakout_angle())) {
+            mY.SetAxisLocked(false);
+            // If we are within the lock angle from the Y axis, lock
+            // onto the Y axis.
+            if (apz::IsCloseToVertical(
+                    angle, StaticPrefs::apz_axis_lock_lock_angle())) {
+              mX.SetAxisLocked(true);
+              SetState(PANNING_LOCKED_Y);
+            } else {
+              SetState(PANNING);
+            }
+          }
+          break;
+
+        case PANNING_LOCKED_Y:
+          if (!apz::IsCloseToVertical(
+                  angle, StaticPrefs::apz_axis_lock_breakout_angle())) {
+            mX.SetAxisLocked(false);
+            // If we are within the lock angle from the X axis, lock
+            // onto the X axis.
+            if (apz::IsCloseToHorizontal(
+                    angle, StaticPrefs::apz_axis_lock_lock_angle())) {
+              mY.SetAxisLocked(true);
+              SetState(PANNING_LOCKED_X);
+            } else {
+              SetState(PANNING);
+            }
+          }
+          break;
+
+        case PANNING:
+          HandlePanning(angle);
+          break;
+
+        default:
+          break;
+      }
+    }
+  }
+}
+
+void AsyncPanZoomController::HandlePinchLocking(
+    const PinchGestureInput& aEvent) {
+  // Focus change and span distance calculated from an event buffer
+  // Used to handle pinch locking irrespective of touch screen sensitivity
+  // Note: both values fall back to the same value as
+  //       their un-buffered counterparts if there is only one (the latest)
+  //       event in the buffer. ie: when the touch screen is dispatching
+  //       events slower than the lifetime of the buffer
+  ParentLayerCoord bufferedSpanDistance;
+  ParentLayerPoint focusPoint, bufferedFocusChange;
+  {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+    focusPoint = mPinchEventBuffer.back().mLocalFocusPoint -
+                 Metrics().GetCompositionBounds().TopLeft();
+    ParentLayerPoint bufferedLastZoomFocus =
+        (mPinchEventBuffer.size() > 1)
+            ? mPinchEventBuffer.front().mLocalFocusPoint -
+                  Metrics().GetCompositionBounds().TopLeft()
+            : mLastZoomFocus;
+
+    bufferedFocusChange = bufferedLastZoomFocus - focusPoint;
+    bufferedSpanDistance = fabsf(mPinchEventBuffer.front().mPreviousSpan -
+                                 mPinchEventBuffer.back().mCurrentSpan);
+  }
+
+  // Convert to screen coordinates
+  ScreenCoord spanDistance =
+      ToScreenCoordinates(ParentLayerPoint(0, bufferedSpanDistance), focusPoint)
+          .Length();
+  ScreenPoint focusChange =
+      ToScreenCoordinates(bufferedFocusChange, focusPoint);
+
+  if (mPinchLocked) {
+    if (GetPinchLockMode() == PINCH_STICKY) {
+      ScreenCoord spanBreakoutThreshold =
+          StaticPrefs::apz_pinch_lock_span_breakout_threshold() * GetDPI();
+      mPinchLocked = !(spanDistance > spanBreakoutThreshold);
+    }
+  } else {
+    if (GetPinchLockMode() != PINCH_FREE) {
+      ScreenCoord spanLockThreshold =
+          StaticPrefs::apz_pinch_lock_span_lock_threshold() * GetDPI();
+      ScreenCoord scrollLockThreshold =
+          StaticPrefs::apz_pinch_lock_scroll_lock_threshold() * GetDPI();
+
+      if (spanDistance < spanLockThreshold &&
+          focusChange.Length() > scrollLockThreshold) {
+        mPinchLocked = true;
+
+        // We are transitioning to a two-finger pan that could trigger
+        // a fling at its end, so start tracking velocity.
+        StartTouch(aEvent.mLocalFocusPoint, aEvent.mTimeStamp);
+      }
+    }
+  }
+}
+
+nsEventStatus AsyncPanZoomController::StartPanning(
+    const ExternalPoint& aStartPoint, const TimeStamp& aEventTime) {
+  ParentLayerPoint vector =
+      ToParentLayerCoordinates(PanVector(aStartPoint), mStartTouch);
+  double angle = atan2(vector.y, vector.x);  // range [-pi, pi]
+  angle = fabs(angle);                       // range [0, pi]
+
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  HandlePanningWithTouchAction(angle);
+
+  if (IsInPanningState()) {
+    mTouchStartRestingTimeBeforePan = aEventTime - mTouchStartTime;
+    mMinimumVelocityDuringPan = Nothing();
+
+    if (RefPtr<GeckoContentController> controller =
+            GetGeckoContentController()) {
+      controller->NotifyAPZStateChange(GetGuid(),
+                                       APZStateChange::eStartPanning);
+    }
+    return nsEventStatus_eConsumeNoDefault;
+  }
+  // Don't consume an event that didn't trigger a panning.
+  return nsEventStatus_eIgnore;
+}
+
+void AsyncPanZoomController::UpdateWithTouchAtDevicePoint(
+    const MultiTouchInput& aEvent) {
+  const SingleTouchData& touchData = aEvent.mTouches[0];
+  // Take historical touch data into account in order to improve the accuracy
+  // of the velocity estimate. On many Android devices, the touch screen samples
+  // at a higher rate than vsync (e.g. 100Hz vs 60Hz), and the historical data
+  // lets us take advantage of those high-rate samples.
+  for (const auto& historicalData : touchData.mHistoricalData) {
+    ParentLayerPoint historicalPoint = historicalData.mLocalScreenPoint;
+    mX.UpdateWithTouchAtDevicePoint(historicalPoint.x,
+                                    historicalData.mTimeStamp);
+    mY.UpdateWithTouchAtDevicePoint(historicalPoint.y,
+                                    historicalData.mTimeStamp);
+  }
+  ParentLayerPoint point = touchData.mLocalScreenPoint;
+  mX.UpdateWithTouchAtDevicePoint(point.x, aEvent.mTimeStamp);
+  mY.UpdateWithTouchAtDevicePoint(point.y, aEvent.mTimeStamp);
+}
+
+Maybe<CompositionPayload> AsyncPanZoomController::NotifyScrollSampling() {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return mSampledState.front().TakeScrollPayload();
+}
+
+bool AsyncPanZoomController::AttemptScroll(
+    ParentLayerPoint& aStartPoint, ParentLayerPoint& aEndPoint,
+    OverscrollHandoffState& aOverscrollHandoffState) {
+  // "start - end" rather than "end - start" because e.g. moving your finger
+  // down (*positive* direction along y axis) causes the vertical scroll offset
+  // to *decrease* as the page follows your finger.
+  ParentLayerPoint displacement = aStartPoint - aEndPoint;
+
+  ParentLayerPoint overscroll;  // will be used outside monitor block
+
+  // If the direction of panning is reversed within the same input block,
+  // a later event in the block could potentially scroll an APZC earlier
+  // in the handoff chain, than an earlier event in the block (because
+  // the earlier APZC was scrolled to its extent in the original direction).
+  // We want to disallow this.
+  bool scrollThisApzc = false;
+  if (InputBlockState* block = GetCurrentInputBlock()) {
+    scrollThisApzc =
+        !block->GetScrolledApzc() || block->IsDownchainOfScrolledApzc(this);
+  }
+
+  ParentLayerPoint adjustedDisplacement;
+  if (scrollThisApzc) {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    bool respectDisregardedDirections =
+        ScrollSourceRespectsDisregardedDirections(
+            aOverscrollHandoffState.mScrollSource);
+    bool forcesVerticalOverscroll = respectDisregardedDirections &&
+                                    mScrollMetadata.GetDisregardedDirection() ==
+                                        Some(ScrollDirection::eVertical);
+    bool forcesHorizontalOverscroll =
+        respectDisregardedDirections &&
+        mScrollMetadata.GetDisregardedDirection() ==
+            Some(ScrollDirection::eHorizontal);
+
+    bool yChanged =
+        mY.AdjustDisplacement(displacement.y, adjustedDisplacement.y,
+                              overscroll.y, forcesVerticalOverscroll);
+    bool xChanged =
+        mX.AdjustDisplacement(displacement.x, adjustedDisplacement.x,
+                              overscroll.x, forcesHorizontalOverscroll);
+    if (xChanged || yChanged) {
+      ScheduleComposite();
+    }
+
+    if (!IsZero(adjustedDisplacement) &&
+        Metrics().GetZoom() != CSSToParentLayerScale(0)) {
+      ScrollBy(adjustedDisplacement / Metrics().GetZoom());
+      if (InputBlockState* block = GetCurrentInputBlock()) {
+        bool displacementIsUserVisible = true;
+
+        {  // Release the APZC lock before calling ToScreenCoordinates which
+          // acquires the APZ tree lock. Note that this just unlocks the mutex
+          // once, so if we're locking it multiple times on the callstack then
+          // this will be insufficient.
+          RecursiveMutexAutoUnlock unlock(mRecursiveMutex);
+
+          ScreenIntPoint screenDisplacement = RoundedToInt(
+              ToScreenCoordinates(adjustedDisplacement, aStartPoint));
+          // If the displacement we just applied rounds to zero in screen space,
+          // then it's probably not going to be visible to the user. In that
+          // case let's not mark this APZC as scrolled, so that even if the
+          // immediate handoff pref is disabled, we'll allow doing the handoff
+          // to the next APZC.
+          if (screenDisplacement == ScreenIntPoint()) {
+            displacementIsUserVisible = false;
+          }
+        }
+        if (displacementIsUserVisible) {
+          block->SetScrolledApzc(this);
+        }
+      }
+      // Note that in the case of instant scrolling, the last snap target ids
+      // will be set after AttemptScroll call so that we can clobber them
+      // unconditionally here.
+      mLastSnapTargetIds = ScrollSnapTargetIds{};
+      ScheduleCompositeAndMaybeRepaint();
+    }
+
+    // Adjust the start point to reflect the consumed portion of the scroll.
+    aStartPoint = aEndPoint + overscroll;
+  } else {
+    overscroll = displacement;
+  }
+
+  // Accumulate the amount of actual scrolling that occurred into the handoff
+  // state. Note that ToScreenCoordinates() needs to be called outside the
+  // mutex.
+  if (!IsZero(adjustedDisplacement)) {
+    aOverscrollHandoffState.mTotalMovement +=
+        ToScreenCoordinates(adjustedDisplacement, aEndPoint);
+  }
+
+  // If we consumed the entire displacement as a normal scroll, great.
+  if (IsZero(overscroll)) {
+    return true;
+  }
+
+  if (AllowScrollHandoffInCurrentBlock()) {
+    // If there is overscroll, first try to hand it off to an APZC later
+    // in the handoff chain to consume (either as a normal scroll or as
+    // overscroll).
+    // Note: "+ overscroll" rather than "- overscroll" because "overscroll"
+    // is what's left of "displacement", and "displacement" is "start - end".
+    ++aOverscrollHandoffState.mChainIndex;
+    bool consumed =
+        CallDispatchScroll(aStartPoint, aEndPoint, aOverscrollHandoffState);
+    if (consumed) {
+      return true;
+    }
+
+    overscroll = aStartPoint - aEndPoint;
+    MOZ_ASSERT(!IsZero(overscroll));
+  }
+
+  // If there is no APZC later in the handoff chain that accepted the
+  // overscroll, try to accept it ourselves. We only accept it if we
+  // are pannable.
+  if (ScrollSourceAllowsOverscroll(aOverscrollHandoffState.mScrollSource)) {
+    APZC_LOG("%p taking overscroll during panning\n", this);
+
+    ParentLayerPoint prevVisualOverscroll = GetOverscrollAmount();
+
+    OverscrollForPanning(overscroll, aOverscrollHandoffState.mPanDistance);
+
+    // Accumulate the amount of change to the overscroll that occurred into the
+    // handoff state. Note that the input amount, |overscroll|, is turned into
+    // some smaller visual overscroll amount (queried via GetOverscrollAmount())
+    // by applying resistance (Axis::ApplyResistance()), and it's the latter we
+    // want to count towards OverscrollHandoffState::mTotalMovement.
+    ParentLayerPoint visualOverscrollChange =
+        GetOverscrollAmount() - prevVisualOverscroll;
+    if (!IsZero(visualOverscrollChange)) {
+      aOverscrollHandoffState.mTotalMovement +=
+          ToScreenCoordinates(visualOverscrollChange, aEndPoint);
+    }
+  }
+
+  aStartPoint = aEndPoint + overscroll;
+
+  return IsZero(overscroll);
+}
+
+void AsyncPanZoomController::OverscrollForPanning(
+    ParentLayerPoint& aOverscroll, const ScreenPoint& aPanDistance) {
+  // Only allow entering overscroll along an axis if the pan distance along
+  // that axis is greater than the pan distance along the other axis by a
+  // configurable factor. If we are already overscrolled, don't check this.
+  if (!IsOverscrolled()) {
+    if (aPanDistance.x <
+        StaticPrefs::apz_overscroll_min_pan_distance_ratio() * aPanDistance.y) {
+      aOverscroll.x = 0;
+    }
+    if (aPanDistance.y <
+        StaticPrefs::apz_overscroll_min_pan_distance_ratio() * aPanDistance.x) {
+      aOverscroll.y = 0;
+    }
+  }
+
+  OverscrollBy(aOverscroll);
+}
+
+ScrollDirections AsyncPanZoomController::GetOverscrollableDirections() const {
+  ScrollDirections result;
+
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+  // If the target has the disregarded direction, it means it's single line
+  // text control, thus we don't want to overscroll in both directions.
+  if (mScrollMetadata.GetDisregardedDirection()) {
+    return result;
+  }
+
+  if (mX.CanScroll() && mX.OverscrollBehaviorAllowsOverscrollEffect()) {
+    result += ScrollDirection::eHorizontal;
+  }
+
+  if (mY.CanScroll() && mY.OverscrollBehaviorAllowsOverscrollEffect()) {
+    result += ScrollDirection::eVertical;
+  }
+
+  return result;
+}
+
+void AsyncPanZoomController::OverscrollBy(ParentLayerPoint& aOverscroll) {
+  if (!StaticPrefs::apz_overscroll_enabled()) {
+    return;
+  }
+
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  // Do not go into overscroll in a direction in which we have no room to
+  // scroll to begin with.
+  ScrollDirections overscrollableDirections = GetOverscrollableDirections();
+  if (IsZero(aOverscroll.x)) {
+    overscrollableDirections -= ScrollDirection::eHorizontal;
+  }
+  if (IsZero(aOverscroll.y)) {
+    overscrollableDirections -= ScrollDirection::eVertical;
+  }
+
+  mOverscrollEffect->ConsumeOverscroll(aOverscroll, overscrollableDirections);
+}
+
+RefPtr<const OverscrollHandoffChain>
+AsyncPanZoomController::BuildOverscrollHandoffChain() {
+  if (APZCTreeManager* treeManagerLocal = GetApzcTreeManager()) {
+    return treeManagerLocal->BuildOverscrollHandoffChain(this);
+  }
+
+  // This APZC IsDestroyed(). To avoid callers having to special-case this
+  // scenario, just build a 1-element chain containing ourselves.
+  OverscrollHandoffChain* result = new OverscrollHandoffChain;
+  result->Add(this);
+  return result;
+}
+
+ParentLayerPoint AsyncPanZoomController::AttemptFling(
+    const FlingHandoffState& aHandoffState) {
+  // The PLPPI computation acquires the tree lock, so it needs to be performed
+  // on the controller thread, and before the APZC lock is acquired.
+  APZThreadUtils::AssertOnControllerThread();
+  float PLPPI = ComputePLPPI(PanStart(), aHandoffState.mVelocity);
+
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+  if (!IsPannable()) {
+    return aHandoffState.mVelocity;
+  }
+
+  // We may have a pre-existing velocity for whatever reason (for example,
+  // a previously handed off fling). We don't want to clobber that.
+  APZC_LOG("%p accepting fling with velocity %s\n", this,
+           ToString(aHandoffState.mVelocity).c_str());
+  ParentLayerPoint residualVelocity = aHandoffState.mVelocity;
+  if (mX.CanScroll()) {
+    mX.SetVelocity(mX.GetVelocity() + aHandoffState.mVelocity.x);
+    residualVelocity.x = 0;
+  }
+  if (mY.CanScroll()) {
+    mY.SetVelocity(mY.GetVelocity() + aHandoffState.mVelocity.y);
+    residualVelocity.y = 0;
+  }
+
+  // If we're not scrollable in at least one of the directions in which we
+  // were handed velocity, don't start a fling animation.
+  // The |IsFinite()| condition should only fail when running some tests
+  // that generate events faster than the clock resolution.
+  ParentLayerPoint velocity = GetVelocityVector();
+  if (!velocity.IsFinite() ||
+      velocity.Length() <= StaticPrefs::apz_fling_min_velocity_threshold()) {
+    // Relieve overscroll now if needed, since we will not transition to a fling
+    // animation and then an overscroll animation, and relieve it then.
+    aHandoffState.mChain->SnapBackOverscrolledApzc(this);
+    return residualVelocity;
+  }
+
+  // If there's a scroll snap point near the predicted fling destination,
+  // scroll there using a smooth scroll animation. Otherwise, start a
+  // fling animation.
+  ScrollSnapToDestination();
+  if (mState != SMOOTHMSD_SCROLL) {
+    SetState(FLING);
+    AsyncPanZoomAnimation* fling =
+        GetPlatformSpecificState()->CreateFlingAnimation(*this, aHandoffState,
+                                                         PLPPI);
+    StartAnimation(fling);
+  }
+
+  return residualVelocity;
+}
+
+float AsyncPanZoomController::ComputePLPPI(ParentLayerPoint aPoint,
+                                           ParentLayerPoint aDirection) const {
+  // Avoid division-by-zero.
+  if (aDirection == ParentLayerPoint()) {
+    return GetDPI();
+  }
+
+  // Convert |aDirection| into a unit vector.
+  aDirection = aDirection / aDirection.Length();
+
+  // Place the vector at |aPoint| and convert to screen coordinates.
+  // The length of the resulting vector is the number of Screen coordinates
+  // that equal 1 ParentLayer coordinate in the given direction.
+  float screenPerParent = ToScreenCoordinates(aDirection, aPoint).Length();
+
+  // Finally, factor in the DPI scale.
+  return GetDPI() / screenPerParent;
+}
+
+Maybe<CSSPoint> AsyncPanZoomController::GetCurrentAnimationDestination(
+    const RecursiveMutexAutoLock& aProofOfLock) const {
+  if (mState == WHEEL_SCROLL) {
+    return Some(mAnimation->AsWheelScrollAnimation()->GetDestination());
+  }
+  if (mState == SMOOTH_SCROLL) {
+    return Some(mAnimation->AsSmoothScrollAnimation()->GetDestination());
+  }
+  if (mState == SMOOTHMSD_SCROLL) {
+    return Some(mAnimation->AsSmoothMsdScrollAnimation()->GetDestination());
+  }
+  if (mState == KEYBOARD_SCROLL) {
+    return Some(mAnimation->AsSmoothScrollAnimation()->GetDestination());
+  }
+
+  return Nothing();
+}
+
+ParentLayerPoint
+AsyncPanZoomController::AdjustHandoffVelocityForOverscrollBehavior(
+    ParentLayerPoint& aHandoffVelocity) const {
+  ParentLayerPoint residualVelocity;
+  ScrollDirections handoffDirections = GetAllowedHandoffDirections();
+  if (!handoffDirections.contains(ScrollDirection::eHorizontal)) {
+    residualVelocity.x = aHandoffVelocity.x;
+    aHandoffVelocity.x = 0;
+  }
+  if (!handoffDirections.contains(ScrollDirection::eVertical)) {
+    residualVelocity.y = aHandoffVelocity.y;
+    aHandoffVelocity.y = 0;
+  }
+  return residualVelocity;
+}
+
+bool AsyncPanZoomController::OverscrollBehaviorAllowsSwipe() const {
+  // Swipe navigation is a "non-local" overscroll behavior like handoff.
+  return GetAllowedHandoffDirections().contains(ScrollDirection::eHorizontal);
+}
+
+void AsyncPanZoomController::HandleFlingOverscroll(
+    const ParentLayerPoint& aVelocity, SideBits aOverscrollSideBits,
+    const RefPtr<const OverscrollHandoffChain>& aOverscrollHandoffChain,
+    const RefPtr<const AsyncPanZoomController>& aScrolledApzc) {
+  APZCTreeManager* treeManagerLocal = GetApzcTreeManager();
+  if (treeManagerLocal) {
+    const FlingHandoffState handoffState{
+        aVelocity, aOverscrollHandoffChain, Nothing(),
+        0,         true /* handoff */,      aScrolledApzc};
+    ParentLayerPoint residualVelocity =
+        treeManagerLocal->DispatchFling(this, handoffState);
+    FLING_LOG("APZC %p left with residual velocity %s\n", this,
+              ToString(residualVelocity).c_str());
+    if (!IsZero(residualVelocity) && IsPannable() &&
+        StaticPrefs::apz_overscroll_enabled()) {
+      // Obey overscroll-behavior.
+      RecursiveMutexAutoLock lock(mRecursiveMutex);
+      if (!mX.OverscrollBehaviorAllowsOverscrollEffect()) {
+        residualVelocity.x = 0;
+      }
+      if (!mY.OverscrollBehaviorAllowsOverscrollEffect()) {
+        residualVelocity.y = 0;
+      }
+
+      if (!IsZero(residualVelocity)) {
+        mOverscrollEffect->RelieveOverscroll(residualVelocity,
+                                             aOverscrollSideBits);
+      }
+    }
+  }
+}
+
+void AsyncPanZoomController::HandleSmoothScrollOverscroll(
+    const ParentLayerPoint& aVelocity, SideBits aOverscrollSideBits) {
+  // We must call BuildOverscrollHandoffChain from this deferred callback
+  // function in order to avoid a deadlock when acquiring the tree lock.
+  HandleFlingOverscroll(aVelocity, aOverscrollSideBits,
+                        BuildOverscrollHandoffChain(), nullptr);
+}
+
+void AsyncPanZoomController::SmoothScrollTo(const CSSPoint& aDestination,
+                                            const ScrollOrigin& aOrigin) {
+  // Convert velocity from ParentLayerPoints/ms to ParentLayerPoints/s and then
+  // to appunits/second.
+  nsPoint destination = CSSPoint::ToAppUnits(aDestination);
+  nsSize velocity;
+  if (Metrics().GetZoom() != CSSToParentLayerScale(0)) {
+    velocity = CSSSize::ToAppUnits(ParentLayerSize(mX.GetVelocity() * 1000.0f,
+                                                   mY.GetVelocity() * 1000.0f) /
+                                   Metrics().GetZoom());
+  }
+
+  if (mState == SMOOTH_SCROLL && mAnimation) {
+    RefPtr<SmoothScrollAnimation> animation(
+        mAnimation->AsSmoothScrollAnimation());
+    if (animation->GetScrollOrigin() == aOrigin) {
+      APZC_LOG("%p updating destination on existing animation\n", this);
+      animation->UpdateDestination(GetFrameTime().Time(), destination,
+                                   velocity);
+      return;
+    }
+  }
+
+  CancelAnimation();
+  SetState(SMOOTH_SCROLL);
+  nsPoint initialPosition =
+      CSSPoint::ToAppUnits(Metrics().GetVisualScrollOffset());
+  RefPtr<SmoothScrollAnimation> animation =
+      new SmoothScrollAnimation(*this, initialPosition, aOrigin);
+  animation->UpdateDestination(GetFrameTime().Time(), destination, velocity);
+  StartAnimation(animation.get());
+}
+
+void AsyncPanZoomController::SmoothMsdScrollTo(
+    CSSSnapTarget&& aDestination, ScrollTriggeredByScript aTriggeredByScript) {
+  if (mState == SMOOTHMSD_SCROLL && mAnimation) {
+    APZC_LOG("%p updating destination on existing animation\n", this);
+    RefPtr<SmoothMsdScrollAnimation> animation(
+        static_cast<SmoothMsdScrollAnimation*>(mAnimation.get()));
+    animation->SetDestination(aDestination.mPosition,
+                              std::move(aDestination.mTargetIds),
+                              aTriggeredByScript);
+  } else {
+    CancelAnimation();
+    SetState(SMOOTHMSD_SCROLL);
+    // Convert velocity from ParentLayerPoints/ms to ParentLayerPoints/s.
+    CSSPoint initialVelocity;
+    if (Metrics().GetZoom() != CSSToParentLayerScale(0)) {
+      initialVelocity = ParentLayerPoint(mX.GetVelocity() * 1000.0f,
+                                         mY.GetVelocity() * 1000.0f) /
+                        Metrics().GetZoom();
+    }
+
+    StartAnimation(new SmoothMsdScrollAnimation(
+        *this, Metrics().GetVisualScrollOffset(), initialVelocity,
+        aDestination.mPosition,
+        StaticPrefs::layout_css_scroll_behavior_spring_constant(),
+        StaticPrefs::layout_css_scroll_behavior_damping_ratio(),
+        std::move(aDestination.mTargetIds), aTriggeredByScript));
+  }
+}
+
+void AsyncPanZoomController::StartOverscrollAnimation(
+    const ParentLayerPoint& aVelocity, SideBits aOverscrollSideBits) {
+  MOZ_ASSERT(mState != OVERSCROLL_ANIMATION);
+
+  SetState(OVERSCROLL_ANIMATION);
+
+  ParentLayerPoint velocity = aVelocity;
+  AdjustDeltaForAllowedScrollDirections(velocity,
+                                        GetOverscrollableDirections());
+  StartAnimation(new OverscrollAnimation(*this, velocity, aOverscrollSideBits));
+}
+
+bool AsyncPanZoomController::CallDispatchScroll(
+    ParentLayerPoint& aStartPoint, ParentLayerPoint& aEndPoint,
+    OverscrollHandoffState& aOverscrollHandoffState) {
+  // Make a local copy of the tree manager pointer and check if it's not
+  // null before calling DispatchScroll(). This is necessary because
+  // Destroy(), which nulls out mTreeManager, could be called concurrently.
+  APZCTreeManager* treeManagerLocal = GetApzcTreeManager();
+  if (!treeManagerLocal) {
+    return false;
+  }
+
+  // Obey overscroll-behavior.
+  ParentLayerPoint endPoint = aEndPoint;
+  if (aOverscrollHandoffState.mChainIndex > 0) {
+    ScrollDirections handoffDirections = GetAllowedHandoffDirections();
+    if (!handoffDirections.contains(ScrollDirection::eHorizontal)) {
+      endPoint.x = aStartPoint.x;
+    }
+    if (!handoffDirections.contains(ScrollDirection::eVertical)) {
+      endPoint.y = aStartPoint.y;
+    }
+    if (aStartPoint == endPoint) {
+      // Handoff not allowed in either direction - don't even bother.
+      return false;
+    }
+  }
+
+  return treeManagerLocal->DispatchScroll(this, aStartPoint, endPoint,
+                                          aOverscrollHandoffState);
+}
+
+void AsyncPanZoomController::RecordScrollPayload(const TimeStamp& aTimeStamp) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  if (!mScrollPayload) {
+    mScrollPayload = Some(
+        CompositionPayload{CompositionPayloadType::eAPZScroll, aTimeStamp});
+  }
+}
+
+void AsyncPanZoomController::StartTouch(const ParentLayerPoint& aPoint,
+                                        TimeStamp aTimestamp) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  mX.StartTouch(aPoint.x, aTimestamp);
+  mY.StartTouch(aPoint.y, aTimestamp);
+}
+
+void AsyncPanZoomController::EndTouch(TimeStamp aTimestamp,
+                                      Axis::ClearAxisLock aClearAxisLock) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  mX.EndTouch(aTimestamp, aClearAxisLock);
+  mY.EndTouch(aTimestamp, aClearAxisLock);
+}
+
+void AsyncPanZoomController::TrackTouch(const MultiTouchInput& aEvent) {
+  ExternalPoint extPoint = GetFirstExternalTouchPoint(aEvent);
+  ScreenPoint panVector = PanVector(extPoint);
+  HandlePanningUpdate(panVector);
+
+  ParentLayerPoint prevTouchPoint(mX.GetPos(), mY.GetPos());
+  ParentLayerPoint touchPoint = GetFirstTouchPoint(aEvent);
+
+  UpdateWithTouchAtDevicePoint(aEvent);
+
+  auto velocity = GetVelocityVector().Length();
+  if (mMinimumVelocityDuringPan) {
+    mMinimumVelocityDuringPan =
+        Some(std::min(*mMinimumVelocityDuringPan, velocity));
+  } else {
+    mMinimumVelocityDuringPan = Some(velocity);
+  }
+
+  if (prevTouchPoint != touchPoint) {
+    MOZ_ASSERT(GetCurrentTouchBlock());
+    OverscrollHandoffState handoffState(
+        *GetCurrentTouchBlock()->GetOverscrollHandoffChain(), panVector,
+        ScrollSource::Touchscreen);
+    RecordScrollPayload(aEvent.mTimeStamp);
+    CallDispatchScroll(prevTouchPoint, touchPoint, handoffState);
+  }
+}
+
+ParentLayerPoint AsyncPanZoomController::GetFirstTouchPoint(
+    const MultiTouchInput& aEvent) {
+  return ((SingleTouchData&)aEvent.mTouches[0]).mLocalScreenPoint;
+}
+
+ExternalPoint AsyncPanZoomController::GetFirstExternalTouchPoint(
+    const MultiTouchInput& aEvent) {
+  return ToExternalPoint(aEvent.mScreenOffset,
+                         ((SingleTouchData&)aEvent.mTouches[0]).mScreenPoint);
+}
+
+ParentLayerPoint AsyncPanZoomController::GetOverscrollAmount() const {
+  if (StaticPrefs::apz_overscroll_test_async_scroll_offset_enabled()) {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    AutoApplyAsyncTestAttributes testAttributeApplier(this, lock);
+    return GetOverscrollAmountInternal();
+  }
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return GetOverscrollAmountInternal();
+}
+
+ParentLayerPoint AsyncPanZoomController::GetOverscrollAmountInternal() const {
+  return {mX.GetOverscroll(), mY.GetOverscroll()};
+}
+
+SideBits AsyncPanZoomController::GetOverscrollSideBits() const {
+  return apz::GetOverscrollSideBits({mX.GetOverscroll(), mY.GetOverscroll()});
+}
+
+void AsyncPanZoomController::RestoreOverscrollAmount(
+    const ParentLayerPoint& aOverscroll) {
+  mX.RestoreOverscroll(aOverscroll.x);
+  mY.RestoreOverscroll(aOverscroll.y);
+}
+
+void AsyncPanZoomController::StartAnimation(AsyncPanZoomAnimation* aAnimation) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  mAnimation = aAnimation;
+  mLastSampleTime = GetFrameTime();
+  ScheduleComposite();
+}
+
+void AsyncPanZoomController::CancelAnimation(CancelAnimationFlags aFlags) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  APZC_LOG_DETAIL("running CancelAnimation(0x%x) in state %s\n", this, aFlags,
+                  ToString(mState).c_str());
+
+  if ((aFlags & ExcludeWheel) && mState == WHEEL_SCROLL) {
+    return;
+  }
+
+  if (mAnimation) {
+    mAnimation->Cancel(aFlags);
+  }
+
+  SetState(NOTHING);
+  mLastSnapTargetIds = ScrollSnapTargetIds{};
+  mAnimation = nullptr;
+  // Since there is no animation in progress now the axes should
+  // have no velocity either. If we are dropping the velocity from a non-zero
+  // value we should trigger a repaint as the displayport margins are dependent
+  // on the velocity and the last repaint request might not have good margins
+  // any more.
+  bool repaint = !IsZero(GetVelocityVector());
+  mX.SetVelocity(0);
+  mY.SetVelocity(0);
+  mX.SetAxisLocked(false);
+  mY.SetAxisLocked(false);
+  // Setting the state to nothing and cancelling the animation can
+  // preempt normal mechanisms for relieving overscroll, so we need to clear
+  // overscroll here.
+  if (!(aFlags & ExcludeOverscroll) && IsOverscrolled()) {
+    ClearOverscroll();
+    repaint = true;
+  }
+  // Similar to relieving overscroll, we also need to snap to any snap points
+  // if appropriate.
+  if (aFlags & CancelAnimationFlags::ScrollSnap) {
+    ScrollSnap(ScrollSnapFlags::IntendedEndPosition);
+  }
+  if (repaint) {
+    RequestContentRepaint();
+    ScheduleComposite();
+  }
+}
+
+void AsyncPanZoomController::ClearOverscroll() {
+  mOverscrollEffect->ClearOverscroll();
+}
+
+void AsyncPanZoomController::ClearPhysicalOverscroll() {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  mX.ClearOverscroll();
+  mY.ClearOverscroll();
+}
+
+void AsyncPanZoomController::SetCompositorController(
+    CompositorController* aCompositorController) {
+  mCompositorController = aCompositorController;
+}
+
+void AsyncPanZoomController::SetVisualScrollOffset(const CSSPoint& aOffset) {
+  Metrics().SetVisualScrollOffset(aOffset);
+  Metrics().RecalculateLayoutViewportOffset();
+}
+
+void AsyncPanZoomController::ClampAndSetVisualScrollOffset(
+    const CSSPoint& aOffset) {
+  Metrics().ClampAndSetVisualScrollOffset(aOffset);
+  Metrics().RecalculateLayoutViewportOffset();
+}
+
+void AsyncPanZoomController::ScrollBy(const CSSPoint& aOffset) {
+  SetVisualScrollOffset(Metrics().GetVisualScrollOffset() + aOffset);
+}
+
+void AsyncPanZoomController::ScrollByAndClamp(const CSSPoint& aOffset) {
+  ClampAndSetVisualScrollOffset(Metrics().GetVisualScrollOffset() + aOffset);
+}
+
+void AsyncPanZoomController::ScaleWithFocus(float aScale,
+                                            const CSSPoint& aFocus) {
+  Metrics().ZoomBy(aScale);
+  // We want to adjust the scroll offset such that the CSS point represented by
+  // aFocus remains at the same position on the screen before and after the
+  // change in zoom. The below code accomplishes this; see
+  // https://bugzilla.mozilla.org/show_bug.cgi?id=923431#c6 for an in-depth
+  // explanation of how.
+  SetVisualScrollOffset((Metrics().GetVisualScrollOffset() + aFocus) -
+                        (aFocus / aScale));
+}
+
+/*static*/
+gfx::IntSize AsyncPanZoomController::GetDisplayportAlignmentMultiplier(
+    const ScreenSize& aBaseSize) {
+  gfx::IntSize multiplier(1, 1);
+  float baseWidth = aBaseSize.width;
+  while (baseWidth > 500) {
+    baseWidth /= 2;
+    multiplier.width *= 2;
+    if (multiplier.width >= 8) {
+      break;
+    }
+  }
+  float baseHeight = aBaseSize.height;
+  while (baseHeight > 500) {
+    baseHeight /= 2;
+    multiplier.height *= 2;
+    if (multiplier.height >= 8) {
+      break;
+    }
+  }
+  return multiplier;
+}
+
+/**
+ * Enlarges the displayport along both axes based on the velocity.
+ */
+static CSSSize CalculateDisplayPortSize(
+    const CSSSize& aCompositionSize, const CSSPoint& aVelocity,
+    AsyncPanZoomController::ZoomInProgress aZoomInProgress,
+    const CSSToScreenScale2D& aDpPerCSS) {
+  bool xIsStationarySpeed =
+      fabsf(aVelocity.x) < StaticPrefs::apz_min_skate_speed();
+  bool yIsStationarySpeed =
+      fabsf(aVelocity.y) < StaticPrefs::apz_min_skate_speed();
+  float xMultiplier = xIsStationarySpeed
+                          ? StaticPrefs::apz_x_stationary_size_multiplier()
+                          : StaticPrefs::apz_x_skate_size_multiplier();
+  float yMultiplier = yIsStationarySpeed
+                          ? StaticPrefs::apz_y_stationary_size_multiplier()
+                          : StaticPrefs::apz_y_skate_size_multiplier();
+
+  if (IsHighMemSystem() && !xIsStationarySpeed) {
+    xMultiplier += StaticPrefs::apz_x_skate_highmem_adjust();
+  }
+
+  if (IsHighMemSystem() && !yIsStationarySpeed) {
+    yMultiplier += StaticPrefs::apz_y_skate_highmem_adjust();
+  }
+
+  if (aZoomInProgress == AsyncPanZoomController::ZoomInProgress::Yes) {
+    // If a zoom is in progress, we will be making content visible on the
+    // x and y axes in equal proportion, because the zoom operation scales
+    // equally on the x and y axes. The default multipliers computed above are
+    // biased towards the y-axis since that's where most scrolling occurs, but
+    // in the case of zooming, we should really use equal multipliers on both
+    // axes. This does that while preserving the total displayport area
+    // quantity (aCompositionSize.Area() * xMultiplier * yMultiplier).
+    // Note that normally changing the shape of the displayport is expensive
+    // and should be avoided, but if a zoom is in progress the displayport
+    // is likely going to be fully repainted anyway due to changes in resolution
+    // so there should be no marginal cost to also changing the shape of it.
+    float areaMultiplier = xMultiplier * yMultiplier;
+    xMultiplier = sqrt(areaMultiplier);
+    yMultiplier = xMultiplier;
+  }
+
+  // Scale down the margin multipliers by the alignment multiplier because
+  // the alignment code will expand the displayport outward to the multiplied
+  // alignment. This is not necessary for correctness, but for performance;
+  // if we don't do this the displayport can end up much larger. The math here
+  // is actually just scaling the part of the multipler that is > 1, so that
+  // we never end up with xMultiplier or yMultiplier being less than 1 (that
+  // would result in a guaranteed checkerboarding situation). Note that the
+  // calculation doesn't cancel exactly the increased margin from applying
+  // the alignment multiplier, but this is simple and should provide
+  // reasonable behaviour in most cases.
+  gfx::IntSize alignmentMultipler =
+      AsyncPanZoomController::GetDisplayportAlignmentMultiplier(
+          aCompositionSize * aDpPerCSS);
+  if (xMultiplier > 1) {
+    xMultiplier = ((xMultiplier - 1) / alignmentMultipler.width) + 1;
+  }
+  if (yMultiplier > 1) {
+    yMultiplier = ((yMultiplier - 1) / alignmentMultipler.height) + 1;
+  }
+
+  return aCompositionSize * CSSSize(xMultiplier, yMultiplier);
+}
+
+/**
+ * Ensures that the displayport is at least as large as the visible area
+ * inflated by the danger zone. If this is not the case then the
+ * "AboutToCheckerboard" function in TiledContentClient.cpp will return true
+ * even in the stable state.
+ */
+static CSSSize ExpandDisplayPortToDangerZone(
+    const CSSSize& aDisplayPortSize, const FrameMetrics& aFrameMetrics) {
+  CSSSize dangerZone(0.0f, 0.0f);
+  if (aFrameMetrics.DisplayportPixelsPerCSSPixel().xScale != 0 &&
+      aFrameMetrics.DisplayportPixelsPerCSSPixel().yScale != 0) {
+    dangerZone = ScreenSize(StaticPrefs::apz_danger_zone_x(),
+                            StaticPrefs::apz_danger_zone_y()) /
+                 aFrameMetrics.DisplayportPixelsPerCSSPixel();
+  }
+  const CSSSize compositionSize =
+      aFrameMetrics.CalculateBoundedCompositedSizeInCssPixels();
+
+  const float xSize = std::max(aDisplayPortSize.width,
+                               compositionSize.width + (2 * dangerZone.width));
+
+  const float ySize =
+      std::max(aDisplayPortSize.height,
+               compositionSize.height + (2 * dangerZone.height));
+
+  return CSSSize(xSize, ySize);
+}
+
+/**
+ * Attempts to redistribute any area in the displayport that would get clipped
+ * by the scrollable rect, or be inaccessible due to disabled scrolling, to the
+ * other axis, while maintaining total displayport area.
+ */
+static void RedistributeDisplayPortExcess(CSSSize& aDisplayPortSize,
+                                          const CSSRect& aScrollableRect) {
+  // As aDisplayPortSize.height * aDisplayPortSize.width does not change,
+  // we are just scaling by the ratio and its inverse.
+  if (aDisplayPortSize.height > aScrollableRect.Height()) {
+    aDisplayPortSize.width *=
+        (aDisplayPortSize.height / aScrollableRect.Height());
+    aDisplayPortSize.height = aScrollableRect.Height();
+  } else if (aDisplayPortSize.width > aScrollableRect.Width()) {
+    aDisplayPortSize.height *=
+        (aDisplayPortSize.width / aScrollableRect.Width());
+    aDisplayPortSize.width = aScrollableRect.Width();
+  }
+}
+
+/* static */
+const ScreenMargin AsyncPanZoomController::CalculatePendingDisplayPort(
+    const FrameMetrics& aFrameMetrics, const ParentLayerPoint& aVelocity,
+    ZoomInProgress aZoomInProgress) {
+  if (aFrameMetrics.IsScrollInfoLayer()) {
+    // Don't compute margins. Since we can't asynchronously scroll this frame,
+    // we don't want to paint anything more than the composition bounds.
+    return ScreenMargin();
+  }
+
+  CSSSize compositionSize =
+      aFrameMetrics.CalculateBoundedCompositedSizeInCssPixels();
+  CSSPoint velocity;
+  if (aFrameMetrics.GetZoom() != CSSToParentLayerScale(0)) {
+    velocity = aVelocity / aFrameMetrics.GetZoom();  // avoid division by zero
+  }
+  CSSRect scrollableRect = aFrameMetrics.GetExpandedScrollableRect();
+
+  // Calculate the displayport size based on how fast we're moving along each
+  // axis.
+  CSSSize displayPortSize =
+      CalculateDisplayPortSize(compositionSize, velocity, aZoomInProgress,
+                               aFrameMetrics.DisplayportPixelsPerCSSPixel());
+
+  displayPortSize =
+      ExpandDisplayPortToDangerZone(displayPortSize, aFrameMetrics);
+
+  if (StaticPrefs::apz_enlarge_displayport_when_clipped()) {
+    RedistributeDisplayPortExcess(displayPortSize, scrollableRect);
+  }
+
+  // We calculate a "displayport" here which is relative to the scroll offset.
+  // Note that the scroll offset we have here in the APZ code may not be the
+  // same as the base rect that gets used on the layout side when the
+  // displayport margins are actually applied, so it is important to only
+  // consider the displayport as margins relative to a scroll offset rather than
+  // relative to something more unchanging like the scrollable rect origin.
+
+  // Center the displayport based on its expansion over the composition size.
+  CSSRect displayPort((compositionSize.width - displayPortSize.width) / 2.0f,
+                      (compositionSize.height - displayPortSize.height) / 2.0f,
+                      displayPortSize.width, displayPortSize.height);
+
+  // Offset the displayport, depending on how fast we're moving and the
+  // estimated time it takes to paint, to try to minimise checkerboarding.
+  float paintFactor = kDefaultEstimatedPaintDurationMs;
+  displayPort.MoveBy(velocity * paintFactor * StaticPrefs::apz_velocity_bias());
+
+  APZC_LOGV_FM(aFrameMetrics,
+               "Calculated displayport as %s from velocity %s zooming %d paint "
+               "time %f metrics",
+               ToString(displayPort).c_str(), ToString(aVelocity).c_str(),
+               (int)aZoomInProgress, paintFactor);
+
+  CSSMargin cssMargins;
+  cssMargins.left = -displayPort.X();
+  cssMargins.top = -displayPort.Y();
+  cssMargins.right =
+      displayPort.Width() - compositionSize.width - cssMargins.left;
+  cssMargins.bottom =
+      displayPort.Height() - compositionSize.height - cssMargins.top;
+
+  return cssMargins * aFrameMetrics.DisplayportPixelsPerCSSPixel();
+}
+
+void AsyncPanZoomController::ScheduleComposite() {
+  if (mCompositorController) {
+    mCompositorController->ScheduleRenderOnCompositorThread(
+        wr::RenderReasons::APZ);
+  }
+}
+
+void AsyncPanZoomController::ScheduleCompositeAndMaybeRepaint() {
+  ScheduleComposite();
+  RequestContentRepaint();
+}
+
+void AsyncPanZoomController::FlushRepaintForOverscrollHandoff() {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  RequestContentRepaint();
+}
+
+void AsyncPanZoomController::FlushRepaintForNewInputBlock() {
+  APZC_LOG("%p flushing repaint for new input block\n", this);
+
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  RequestContentRepaint();
+}
+
+bool AsyncPanZoomController::SnapBackIfOverscrolled() {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  if (SnapBackIfOverscrolledForMomentum(ParentLayerPoint(0, 0))) {
+    return true;
+  }
+  // If we don't kick off an overscroll animation, we still need to snap to any
+  // nearby snap points, assuming we haven't already done so when we started
+  // this fling
+  if (mState != FLING) {
+    ScrollSnap(ScrollSnapFlags::IntendedEndPosition);
+  }
+  return false;
+}
+
+bool AsyncPanZoomController::SnapBackIfOverscrolledForMomentum(
+    const ParentLayerPoint& aVelocity) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  // It's possible that we're already in the middle of an overscroll
+  // animation - if so, don't start a new one.
+  if (IsOverscrolled() && mState != OVERSCROLL_ANIMATION) {
+    APZC_LOG("%p is overscrolled, starting snap-back\n", this);
+    mOverscrollEffect->RelieveOverscroll(aVelocity, GetOverscrollSideBits());
+    return true;
+  }
+  return false;
+}
+
+bool AsyncPanZoomController::IsFlingingFast() const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  if (mState == FLING && GetVelocityVector().Length() >
+                             StaticPrefs::apz_fling_stop_on_tap_threshold()) {
+    APZC_LOG("%p is moving fast\n", this);
+    return true;
+  }
+  return false;
+}
+
+bool AsyncPanZoomController::IsPannable() const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return mX.CanScroll() || mY.CanScroll();
+}
+
+bool AsyncPanZoomController::IsScrollInfoLayer() const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return Metrics().IsScrollInfoLayer();
+}
+
+int32_t AsyncPanZoomController::GetLastTouchIdentifier() const {
+  RefPtr<GestureEventListener> listener = GetGestureEventListener();
+  return listener ? listener->GetLastTouchIdentifier() : -1;
+}
+
+void AsyncPanZoomController::RequestContentRepaint(
+    RepaintUpdateType aUpdateType) {
+  // Reinvoke this method on the repaint thread if it's not there already. It's
+  // important to do this before the call to CalculatePendingDisplayPort, so
+  // that CalculatePendingDisplayPort uses the most recent available version of
+  // Metrics(). just before the paint request is dispatched to content.
+  RefPtr<GeckoContentController> controller = GetGeckoContentController();
+  if (!controller) {
+    return;
+  }
+  if (!controller->IsRepaintThread()) {
+    // Even though we want to do the actual repaint request on the repaint
+    // thread, we want to update the expected gecko metrics synchronously.
+    // Otherwise we introduce a race condition where we might read from the
+    // expected gecko metrics on the controller thread before or after it gets
+    // updated on the repaint thread, when in fact we always want the updated
+    // version when reading.
+    {  // scope lock
+      RecursiveMutexAutoLock lock(mRecursiveMutex);
+      mExpectedGeckoMetrics.UpdateFrom(Metrics());
+    }
+
+    // use the local variable to resolve the function overload.
+    auto func =
+        static_cast<void (AsyncPanZoomController::*)(RepaintUpdateType)>(
+            &AsyncPanZoomController::RequestContentRepaint);
+    controller->DispatchToRepaintThread(NewRunnableMethod<RepaintUpdateType>(
+        "layers::AsyncPanZoomController::RequestContentRepaint", this, func,
+        aUpdateType));
+    return;
+  }
+
+  MOZ_ASSERT(controller->IsRepaintThread());
+
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  ParentLayerPoint velocity = GetVelocityVector();
+  ScreenMargin displayportMargins = CalculatePendingDisplayPort(
+      Metrics(), velocity,
+      (mState == PINCHING || mState == ANIMATING_ZOOM) ? ZoomInProgress::Yes
+                                                       : ZoomInProgress::No);
+  Metrics().SetPaintRequestTime(TimeStamp::Now());
+  RequestContentRepaint(velocity, displayportMargins, aUpdateType);
+}
+
+static CSSRect GetDisplayPortRect(const FrameMetrics& aFrameMetrics,
+                                  const ScreenMargin& aDisplayportMargins) {
+  // This computation is based on what happens in CalculatePendingDisplayPort.
+  // If that changes then this might need to change too.
+  // Note that the display port rect APZ computes is relative to the visual
+  // scroll offset. It's adjusted to be relative to the layout scroll offset
+  // when the main thread processes a repaint request (in
+  // APZCCallbackHelper::AdjustDisplayPortForScrollDelta()) and ultimately
+  // applied (in DisplayPortUtils::GetDisplayPort()) in this adjusted form.
+  CSSRect baseRect(aFrameMetrics.GetVisualScrollOffset(),
+                   aFrameMetrics.CalculateBoundedCompositedSizeInCssPixels());
+  baseRect.Inflate(aDisplayportMargins /
+                   aFrameMetrics.DisplayportPixelsPerCSSPixel());
+  return baseRect;
+}
+
+void AsyncPanZoomController::RequestContentRepaint(
+    const ParentLayerPoint& aVelocity, const ScreenMargin& aDisplayportMargins,
+    RepaintUpdateType aUpdateType) {
+  mRecursiveMutex.AssertCurrentThreadIn();
+
+  RefPtr<GeckoContentController> controller = GetGeckoContentController();
+  if (!controller) {
+    return;
+  }
+  MOZ_ASSERT(controller->IsRepaintThread());
+
+  APZScrollAnimationType animationType = APZScrollAnimationType::No;
+  if (mAnimation) {
+    animationType = mAnimation->WasTriggeredByScript()
+                        ? APZScrollAnimationType::TriggeredByScript
+                        : APZScrollAnimationType::TriggeredByUserInput;
+  }
+  RepaintRequest request(Metrics(), aDisplayportMargins, aUpdateType,
+                         animationType, mScrollGeneration, mLastSnapTargetIds,
+                         IsInScrollingGesture());
+
+  if (request.IsRootContent() && request.GetZoom() != mLastNotifiedZoom &&
+      mState != PINCHING && mState != ANIMATING_ZOOM) {
+    controller->NotifyScaleGestureComplete(
+        GetGuid(),
+        (request.GetZoom() / request.GetDevPixelsPerCSSPixel()).scale);
+    mLastNotifiedZoom = request.GetZoom();
+  }
+
+  // If we're trying to paint what we already think is painted, discard this
+  // request since it's a pointless paint.
+  if (request.GetDisplayPortMargins().WithinEpsilonOf(
+          mLastPaintRequestMetrics.GetDisplayPortMargins(), EPSILON) &&
+      request.GetVisualScrollOffset().WithinEpsilonOf(
+          mLastPaintRequestMetrics.GetVisualScrollOffset(), EPSILON) &&
+      request.GetPresShellResolution() ==
+          mLastPaintRequestMetrics.GetPresShellResolution() &&
+      request.GetZoom() == mLastPaintRequestMetrics.GetZoom() &&
+      request.GetLayoutViewport().WithinEpsilonOf(
+          mLastPaintRequestMetrics.GetLayoutViewport(), EPSILON) &&
+      request.GetScrollGeneration() ==
+          mLastPaintRequestMetrics.GetScrollGeneration() &&
+      request.GetScrollUpdateType() ==
+          mLastPaintRequestMetrics.GetScrollUpdateType() &&
+      request.GetScrollAnimationType() ==
+          mLastPaintRequestMetrics.GetScrollAnimationType() &&
+      request.GetLastSnapTargetIds() ==
+          mLastPaintRequestMetrics.GetLastSnapTargetIds()) {
+    return;
+  }
+
+  APZC_LOGV("%p requesting content repaint %s", this,
+            ToString(request).c_str());
+  {  // scope lock
+    MutexAutoLock lock(mCheckerboardEventLock);
+    if (mCheckerboardEvent && mCheckerboardEvent->IsRecordingTrace()) {
+      std::stringstream info;
+      info << " velocity " << aVelocity;
+      std::string str = info.str();
+      mCheckerboardEvent->UpdateRendertraceProperty(
+          CheckerboardEvent::RequestedDisplayPort,
+          GetDisplayPortRect(Metrics(), aDisplayportMargins), str);
+    }
+  }
+
+  controller->RequestContentRepaint(request);
+  mExpectedGeckoMetrics.UpdateFrom(Metrics());
+  mLastPaintRequestMetrics = request;
+
+  // We're holding the APZC lock here, so redispatch this so we can get
+  // the tree lock without the APZC lock.
+  controller->DispatchToRepaintThread(
+      NewRunnableMethod<AsyncPanZoomController*>(
+          "layers::APZCTreeManager::SendSubtreeTransformsToChromeMainThread",
+          GetApzcTreeManager(),
+          &APZCTreeManager::SendSubtreeTransformsToChromeMainThread, this));
+}
+
+bool AsyncPanZoomController::UpdateAnimation(
+    const RecursiveMutexAutoLock& aProofOfLock, const SampleTime& aSampleTime,
+    nsTArray<RefPtr<Runnable>>* aOutDeferredTasks) {
+  AssertOnSamplerThread();
+
+  // This function may get called multiple with the same sample time, if we
+  // composite multiple times at the same timestamp.
+  // However we only want to do one animation step per composition so we need
+  // to deduplicate these calls first.
+  if (mLastSampleTime == aSampleTime) {
+    return !!mAnimation;
+  }
+
+  // We're at a new timestamp, so advance to the next sample in the deque, if
+  // there is one. That one will be used for all the code that reads the
+  // eForCompositing transforms in this vsync interval.
+  AdvanceToNextSample();
+
+  // And then create a new sample, which will be used in the *next* vsync
+  // interval. We do the sample at this point and not later in order to try
+  // and enforce one frame delay between computing the async transform and
+  // compositing it to the screen. This one-frame delay gives code running on
+  // the main thread a chance to try and respond to the scroll position change,
+  // so that e.g. a main-thread animation can stay in sync with user-driven
+  // scrolling or a compositor animation.
+  bool needComposite = SampleCompositedAsyncTransform(aProofOfLock);
+
+  TimeDuration sampleTimeDelta = aSampleTime - mLastSampleTime;
+  mLastSampleTime = aSampleTime;
+
+  if (needComposite || mAnimation) {
+    // Bump the scroll generation before we call RequestContentRepaint below
+    // so that the RequestContentRepaint call will surely use the new
+    // generation.
+    if (APZCTreeManager* treeManagerLocal = GetApzcTreeManager()) {
+      mScrollGeneration = treeManagerLocal->NewAPZScrollGeneration();
+    }
+  }
+
+  if (mAnimation) {
+    bool continueAnimation = mAnimation->Sample(Metrics(), sampleTimeDelta);
+    bool wantsRepaints = mAnimation->WantsRepaints();
+    *aOutDeferredTasks = mAnimation->TakeDeferredTasks();
+    if (!continueAnimation) {
+      SetState(NOTHING);
+      if (mAnimation->AsSmoothMsdScrollAnimation()) {
+        {
+          RecursiveMutexAutoLock lock(mRecursiveMutex);
+          mLastSnapTargetIds =
+              mAnimation->AsSmoothMsdScrollAnimation()->TakeSnapTargetIds();
+        }
+      }
+      mAnimation = nullptr;
+    }
+    // Request a repaint at the end of the animation in case something such as a
+    // call to NotifyLayersUpdated was invoked during the animation and Gecko's
+    // current state is some intermediate point of the animation.
+    if (!continueAnimation || wantsRepaints) {
+      RequestContentRepaint();
+    }
+    needComposite = true;
+  }
+  return needComposite;
+}
+
+AsyncTransformComponentMatrix AsyncPanZoomController::GetOverscrollTransform(
+    AsyncTransformConsumer aMode) const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  AutoApplyAsyncTestAttributes testAttributeApplier(this, lock);
+
+  if (aMode == eForCompositing && mScrollMetadata.IsApzForceDisabled()) {
+    return AsyncTransformComponentMatrix();
+  }
+
+  if (!IsPhysicallyOverscrolled()) {
+    return AsyncTransformComponentMatrix();
+  }
+
+  // The overscroll effect is a simple translation by the overscroll offset.
+  ParentLayerPoint overscrollOffset(-mX.GetOverscroll(), -mY.GetOverscroll());
+  return AsyncTransformComponentMatrix().PostTranslate(overscrollOffset.x,
+                                                       overscrollOffset.y, 0);
+}
+
+bool AsyncPanZoomController::AdvanceAnimations(const SampleTime& aSampleTime) {
+  AssertOnSamplerThread();
+
+  // Don't send any state-change notifications until the end of the function,
+  // because we may go through some intermediate states while we finish
+  // animations and start new ones.
+  StateChangeNotificationBlocker blocker(this);
+
+  // The eventual return value of this function. The compositor needs to know
+  // whether or not to advance by a frame as soon as it can. For example, if a
+  // fling is happening, it has to keep compositing so that the animation is
+  // smooth. If an animation frame is requested, it is the compositor's
+  // responsibility to schedule a composite.
+  bool requestAnimationFrame = false;
+  nsTArray<RefPtr<Runnable>> deferredTasks;
+
+  {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    {  // scope lock
+      CSSRect visibleRect = GetVisibleRect(lock);
+      MutexAutoLock lock2(mCheckerboardEventLock);
+      // Update RendertraceProperty before UpdateAnimation() call, since
+      // the UpdateAnimation() updates effective ScrollOffset for next frame
+      // if APZFrameDelay is enabled.
+      if (mCheckerboardEvent) {
+        mCheckerboardEvent->UpdateRendertraceProperty(
+            CheckerboardEvent::UserVisible, visibleRect);
+      }
+    }
+
+    requestAnimationFrame = UpdateAnimation(lock, aSampleTime, &deferredTasks);
+  }
+  // Execute any deferred tasks queued up by mAnimation's Sample() (called by
+  // UpdateAnimation()). This needs to be done after the monitor is released
+  // since the tasks are allowed to call APZCTreeManager methods which can grab
+  // the tree lock.
+  for (uint32_t i = 0; i < deferredTasks.Length(); ++i) {
+    APZThreadUtils::RunOnControllerThread(std::move(deferredTasks[i]));
+  }
+
+  // If any of the deferred tasks starts a new animation, it will request a
+  // new composite directly, so we can just return requestAnimationFrame here.
+  return requestAnimationFrame;
+}
+
+ParentLayerPoint AsyncPanZoomController::GetCurrentAsyncScrollOffset(
+    AsyncTransformConsumer aMode) const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  AutoApplyAsyncTestAttributes testAttributeApplier(this, lock);
+
+  return GetEffectiveScrollOffset(aMode, lock) * GetEffectiveZoom(aMode, lock);
+}
+
+CSSRect AsyncPanZoomController::GetCurrentAsyncVisualViewport(
+    AsyncTransformConsumer aMode) const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  AutoApplyAsyncTestAttributes testAttributeApplier(this, lock);
+
+  return CSSRect(
+      GetEffectiveScrollOffset(aMode, lock),
+      FrameMetrics::CalculateCompositedSizeInCssPixels(
+          Metrics().GetCompositionBounds(), GetEffectiveZoom(aMode, lock)));
+}
+
+AsyncTransform AsyncPanZoomController::GetCurrentAsyncTransform(
+    AsyncTransformConsumer aMode, AsyncTransformComponents aComponents,
+    std::size_t aSampleIndex) const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  AutoApplyAsyncTestAttributes testAttributeApplier(this, lock);
+
+  CSSToParentLayerScale effectiveZoom;
+  if (aComponents.contains(AsyncTransformComponent::eVisual)) {
+    effectiveZoom = GetEffectiveZoom(aMode, lock, aSampleIndex);
+  } else {
+    effectiveZoom =
+        Metrics().LayersPixelsPerCSSPixel() * LayerToParentLayerScale(1.0f);
+  }
+
+  LayerToParentLayerScale compositedAsyncZoom =
+      effectiveZoom / Metrics().LayersPixelsPerCSSPixel();
+
+  ParentLayerPoint translation;
+  if (aComponents.contains(AsyncTransformComponent::eVisual)) {
+    // There is no "lastPaintVisualOffset" to subtract here; the visual offset
+    // is entirely async.
+
+    CSSPoint currentVisualOffset =
+        GetEffectiveScrollOffset(aMode, lock, aSampleIndex) -
+        GetEffectiveLayoutViewport(aMode, lock, aSampleIndex).TopLeft();
+
+    translation += currentVisualOffset * effectiveZoom;
+  }
+  if (aComponents.contains(AsyncTransformComponent::eLayout)) {
+    CSSPoint lastPaintLayoutOffset;
+    if (mLastContentPaintMetrics.IsScrollable()) {
+      lastPaintLayoutOffset = mLastContentPaintMetrics.GetLayoutScrollOffset();
+    }
+
+    CSSPoint currentLayoutOffset =
+        GetEffectiveLayoutViewport(aMode, lock, aSampleIndex).TopLeft();
+
+    translation +=
+        (currentLayoutOffset - lastPaintLayoutOffset) * effectiveZoom;
+  }
+
+  return AsyncTransform(compositedAsyncZoom, -translation);
+}
+
+AsyncTransformComponentMatrix
+AsyncPanZoomController::GetCurrentAsyncTransformWithOverscroll(
+    AsyncTransformConsumer aMode, AsyncTransformComponents aComponents,
+    std::size_t aSampleIndex) const {
+  AsyncTransformComponentMatrix asyncTransform =
+      GetCurrentAsyncTransform(aMode, aComponents, aSampleIndex);
+  // The overscroll transform is considered part of the layout component of
+  // the async transform, because it should not apply to fixed content.
+  if (aComponents.contains(AsyncTransformComponent::eLayout)) {
+    return asyncTransform * GetOverscrollTransform(aMode);
+  }
+  return asyncTransform;
+}
+
+LayoutDeviceToParentLayerScale AsyncPanZoomController::GetCurrentPinchZoomScale(
+    AsyncTransformConsumer aMode) const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  AutoApplyAsyncTestAttributes testAttributeApplier(this, lock);
+  CSSToParentLayerScale scale = GetEffectiveZoom(aMode, lock);
+  return scale / Metrics().GetDevPixelsPerCSSPixel();
+}
+
+AutoTArray<wr::SampledScrollOffset, 2>
+AsyncPanZoomController::GetSampledScrollOffsets() const {
+  AssertOnSamplerThread();
+
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+  const AsyncTransformComponents asyncTransformComponents =
+      GetZoomAnimationId()
+          ? AsyncTransformComponents{AsyncTransformComponent::eLayout}
+          : LayoutAndVisual;
+
+  // If layerTranslation includes only the layout component of the async
+  // transform then it has not been scaled by the async zoom, so we want to
+  // divide it by the resolution. If layerTranslation includes the visual
+  // component, then we should use the pinch zoom scale, which includes the
+  // async zoom. However, we only use LayoutAndVisual for non-zoomable APZCs,
+  // so it makes no difference.
+  LayoutDeviceToParentLayerScale resolution =
+      GetCumulativeResolution() * LayerToParentLayerScale(1.0f);
+
+  AutoTArray<wr::SampledScrollOffset, 2> sampledOffsets;
+
+  for (std::deque<SampledAPZCState>::size_type index = 0;
+       index < mSampledState.size(); index++) {
+    ParentLayerPoint layerTranslation =
+        GetCurrentAsyncTransform(AsyncPanZoomController::eForCompositing,
+                                 asyncTransformComponents, index)
+            .mTranslation;
+
+    // Include the overscroll transform here in scroll offsets transform
+    // to ensure that we do not overscroll fixed content.
+    layerTranslation =
+        GetOverscrollTransform(AsyncPanZoomController::eForCompositing)
+            .TransformPoint(layerTranslation);
+    // The positive translation means the painted content is supposed to
+    // move down (or to the right), and that corresponds to a reduction in
+    // the scroll offset. Since we are effectively giving WR the async
+    // scroll delta here, we want to negate the translation.
+    LayoutDevicePoint asyncScrollDelta = -layerTranslation / resolution;
+    sampledOffsets.AppendElement(wr::SampledScrollOffset{
+        wr::ToLayoutVector2D(asyncScrollDelta),
+        wr::ToWrAPZScrollGeneration(mSampledState[index].Generation())});
+  }
+
+  return sampledOffsets;
+}
+
+bool AsyncPanZoomController::SuppressAsyncScrollOffset() const {
+  return mScrollMetadata.IsApzForceDisabled() ||
+         (Metrics().IsMinimalDisplayPort() &&
+          StaticPrefs::apz_prefer_jank_minimal_displayports());
+}
+
+CSSRect AsyncPanZoomController::GetEffectiveLayoutViewport(
+    AsyncTransformConsumer aMode, const RecursiveMutexAutoLock& aProofOfLock,
+    std::size_t aSampleIndex) const {
+  if (aMode == eForCompositing && SuppressAsyncScrollOffset()) {
+    return mLastContentPaintMetrics.GetLayoutViewport();
+  }
+  if (aMode == eForCompositing) {
+    return mSampledState[aSampleIndex].GetLayoutViewport();
+  }
+  return Metrics().GetLayoutViewport();
+}
+
+CSSPoint AsyncPanZoomController::GetEffectiveScrollOffset(
+    AsyncTransformConsumer aMode, const RecursiveMutexAutoLock& aProofOfLock,
+    std::size_t aSampleIndex) const {
+  if (aMode == eForCompositing && SuppressAsyncScrollOffset()) {
+    return mLastContentPaintMetrics.GetVisualScrollOffset();
+  }
+  if (aMode == eForCompositing) {
+    return mSampledState[aSampleIndex].GetVisualScrollOffset();
+  }
+  return Metrics().GetVisualScrollOffset();
+}
+
+CSSToParentLayerScale AsyncPanZoomController::GetEffectiveZoom(
+    AsyncTransformConsumer aMode, const RecursiveMutexAutoLock& aProofOfLock,
+    std::size_t aSampleIndex) const {
+  if (aMode == eForCompositing && SuppressAsyncScrollOffset()) {
+    return mLastContentPaintMetrics.GetZoom();
+  }
+  if (aMode == eForCompositing) {
+    return mSampledState[aSampleIndex].GetZoom();
+  }
+  return Metrics().GetZoom();
+}
+
+void AsyncPanZoomController::AdvanceToNextSample() {
+  AssertOnSamplerThread();
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  // Always keep at least one state in mSampledState.
+  if (mSampledState.size() > 1) {
+    mSampledState.pop_front();
+  }
+}
+
+bool AsyncPanZoomController::SampleCompositedAsyncTransform(
+    const RecursiveMutexAutoLock& aProofOfLock) {
+  MOZ_ASSERT(mSampledState.size() <= 2);
+  bool sampleChanged = (mSampledState.back() != SampledAPZCState(Metrics()));
+  mSampledState.emplace_back(Metrics(), std::move(mScrollPayload),
+                             mScrollGeneration);
+  return sampleChanged;
+}
+
+void AsyncPanZoomController::ResampleCompositedAsyncTransform(
+    const RecursiveMutexAutoLock& aProofOfLock) {
+  // This only gets called during testing situations, so the fact that this
+  // drops the scroll payload from mSampledState.front() is not really a
+  // problem.
+  if (APZCTreeManager* treeManagerLocal = GetApzcTreeManager()) {
+    mScrollGeneration = treeManagerLocal->NewAPZScrollGeneration();
+  }
+  mSampledState.front() = SampledAPZCState(Metrics(), {}, mScrollGeneration);
+}
+
+void AsyncPanZoomController::ApplyAsyncTestAttributes(
+    const RecursiveMutexAutoLock& aProofOfLock) {
+  if (mTestAttributeAppliers == 0) {
+    if (mTestAsyncScrollOffset != CSSPoint() ||
+        mTestAsyncZoom != LayerToParentLayerScale()) {
+      // TODO Currently we update Metrics() and resample, which will cause
+      // the very latest user input to get immediately captured in the sample,
+      // and may defeat our attempt at "frame delay" (i.e. delaying the user
+      // input from affecting composition by one frame).
+      // Instead, maybe we should just apply the mTest* stuff directly to
+      // mSampledState.front(). We can even save/restore that SampledAPZCState
+      // instance in the AutoApplyAsyncTestAttributes instead of Metrics().
+      Metrics().ZoomBy(mTestAsyncZoom.scale);
+      CSSPoint asyncScrollPosition = Metrics().GetVisualScrollOffset();
+      CSSPoint requestedPoint =
+          asyncScrollPosition + this->mTestAsyncScrollOffset;
+      CSSPoint clampedPoint =
+          Metrics().CalculateScrollRange().ClampPoint(requestedPoint);
+      CSSPoint difference = mTestAsyncScrollOffset - clampedPoint;
+
+      ScrollByAndClamp(mTestAsyncScrollOffset);
+
+      if (StaticPrefs::apz_overscroll_test_async_scroll_offset_enabled()) {
+        ParentLayerPoint overscroll = difference * Metrics().GetZoom();
+        OverscrollBy(overscroll);
+      }
+      ResampleCompositedAsyncTransform(aProofOfLock);
+    }
+  }
+  ++mTestAttributeAppliers;
+}
+
+void AsyncPanZoomController::UnapplyAsyncTestAttributes(
+    const RecursiveMutexAutoLock& aProofOfLock,
+    const FrameMetrics& aPrevFrameMetrics,
+    const ParentLayerPoint& aPrevOverscroll) {
+  MOZ_ASSERT(mTestAttributeAppliers >= 1);
+  --mTestAttributeAppliers;
+  if (mTestAttributeAppliers == 0) {
+    if (mTestAsyncScrollOffset != CSSPoint() ||
+        mTestAsyncZoom != LayerToParentLayerScale()) {
+      Metrics() = aPrevFrameMetrics;
+      RestoreOverscrollAmount(aPrevOverscroll);
+      ResampleCompositedAsyncTransform(aProofOfLock);
+    }
+  }
+}
+
+Matrix4x4 AsyncPanZoomController::GetTransformToLastDispatchedPaint(
+    const AsyncTransformComponents& aComponents) const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  CSSPoint componentOffset;
+
+  // The computation of the componentOffset should roughly be the negation
+  // of the translation in GetCurrentAsyncTransform() with the expected
+  // gecko metrics substituted for the effective scroll offsets.
+  if (aComponents.contains(AsyncTransformComponent::eVisual)) {
+    componentOffset += mExpectedGeckoMetrics.GetLayoutScrollOffset() -
+                       mExpectedGeckoMetrics.GetVisualScrollOffset();
+  }
+
+  if (aComponents.contains(AsyncTransformComponent::eLayout)) {
+    CSSPoint lastPaintLayoutOffset;
+
+    if (mLastContentPaintMetrics.IsScrollable()) {
+      lastPaintLayoutOffset = mLastContentPaintMetrics.GetLayoutScrollOffset();
+    }
+
+    componentOffset +=
+        lastPaintLayoutOffset - mExpectedGeckoMetrics.GetLayoutScrollOffset();
+  }
+
+  LayerPoint scrollChange = componentOffset *
+                            mLastContentPaintMetrics.GetDevPixelsPerCSSPixel() *
+                            mLastContentPaintMetrics.GetCumulativeResolution();
+
+  // We're interested in the async zoom change. Factor out the content scale
+  // that may change when dragging the window to a monitor with a different
+  // content scale.
+  LayoutDeviceToParentLayerScale lastContentZoom =
+      mLastContentPaintMetrics.GetZoom() /
+      mLastContentPaintMetrics.GetDevPixelsPerCSSPixel();
+  LayoutDeviceToParentLayerScale lastDispatchedZoom =
+      mExpectedGeckoMetrics.GetZoom() /
+      mExpectedGeckoMetrics.GetDevPixelsPerCSSPixel();
+  float zoomChange = 1.0;
+  if (aComponents.contains(AsyncTransformComponent::eVisual) &&
+      lastDispatchedZoom != LayoutDeviceToParentLayerScale(0)) {
+    zoomChange = lastContentZoom.scale / lastDispatchedZoom.scale;
+  }
+  return Matrix4x4::Translation(scrollChange.x, scrollChange.y, 0)
+      .PostScale(zoomChange, zoomChange, 1);
+}
+
+CSSRect AsyncPanZoomController::GetVisibleRect(
+    const RecursiveMutexAutoLock& aProofOfLock) const {
+  AutoApplyAsyncTestAttributes testAttributeApplier(this, aProofOfLock);
+  CSSPoint currentScrollOffset = GetEffectiveScrollOffset(
+      AsyncPanZoomController::eForCompositing, aProofOfLock);
+  CSSRect visible = CSSRect(currentScrollOffset,
+                            Metrics().CalculateCompositedSizeInCssPixels());
+  return visible;
+}
+
+static CSSRect GetPaintedRect(const FrameMetrics& aFrameMetrics) {
+  CSSRect displayPort = aFrameMetrics.GetDisplayPort();
+  if (displayPort.IsEmpty()) {
+    // Fallback to use the viewport if the diplayport hasn't been set.
+    // This situation often happens non-scrollable iframe's root scroller in
+    // Fission.
+    return aFrameMetrics.GetVisualViewport();
+  }
+
+  return displayPort + aFrameMetrics.GetLayoutScrollOffset();
+}
+
+uint32_t AsyncPanZoomController::GetCheckerboardMagnitude(
+    const ParentLayerRect& aClippedCompositionBounds) const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+  CSSRect painted = GetPaintedRect(mLastContentPaintMetrics);
+  painted.Inflate(CSSMargin::FromAppUnits(
+      nsMargin(1, 1, 1, 1)));  // fuzz for rounding error
+
+  CSSRect visible = GetVisibleRect(lock);  // relative to scrolled frame origin
+  if (visible.IsEmpty() || painted.Contains(visible)) {
+    // early-exit if we're definitely not checkerboarding
+    return 0;
+  }
+
+  // aClippedCompositionBounds and Metrics().GetCompositionBounds() are both
+  // relative to the layer tree origin.
+  // The "*RelativeToItself*" variables are relative to the comp bounds origin
+  ParentLayerRect visiblePartOfCompBoundsRelativeToItself =
+      aClippedCompositionBounds - Metrics().GetCompositionBounds().TopLeft();
+  CSSRect visiblePartOfCompBoundsRelativeToItselfInCssSpace;
+  if (Metrics().GetZoom() != CSSToParentLayerScale(0)) {
+    visiblePartOfCompBoundsRelativeToItselfInCssSpace =
+        (visiblePartOfCompBoundsRelativeToItself / Metrics().GetZoom());
+  }
+
+  // This one is relative to the scrolled frame origin, same as `visible`
+  CSSRect visiblePartOfCompBoundsInCssSpace =
+      visiblePartOfCompBoundsRelativeToItselfInCssSpace + visible.TopLeft();
+
+  visible = visible.Intersect(visiblePartOfCompBoundsInCssSpace);
+
+  CSSIntRegion checkerboard;
+  // Round so as to minimize checkerboarding; if we're only showing fractional
+  // pixels of checkerboarding it's not really worth counting
+  checkerboard.Sub(RoundedIn(visible), RoundedOut(painted));
+  uint32_t area = checkerboard.Area();
+  if (area) {
+    APZC_LOG_FM(Metrics(),
+                "%p is currently checkerboarding (painted %s visible %s)", this,
+                ToString(painted).c_str(), ToString(visible).c_str());
+  }
+  return area;
+}
+
+void AsyncPanZoomController::ReportCheckerboard(
+    const SampleTime& aSampleTime,
+    const ParentLayerRect& aClippedCompositionBounds) {
+  if (mLastCheckerboardReport == aSampleTime) {
+    // This function will get called multiple times for each APZC on a single
+    // composite (once for each layer it is attached to). Only report the
+    // checkerboard once per composite though.
+    return;
+  }
+  mLastCheckerboardReport = aSampleTime;
+
+  bool recordTrace = StaticPrefs::apz_record_checkerboarding();
+  bool forTelemetry = Telemetry::CanRecordBase();
+  uint32_t magnitude = GetCheckerboardMagnitude(aClippedCompositionBounds);
+
+  // IsInTransformingState() acquires the APZC lock and thus needs to
+  // be called before acquiring mCheckerboardEventLock.
+  bool inTransformingState = IsInTransformingState();
+
+  MutexAutoLock lock(mCheckerboardEventLock);
+  if (!mCheckerboardEvent && (recordTrace || forTelemetry)) {
+    mCheckerboardEvent = MakeUnique<CheckerboardEvent>(recordTrace);
+  }
+  mPotentialCheckerboardTracker.InTransform(inTransformingState,
+                                            recordTrace || forTelemetry);
+  if (magnitude) {
+    mPotentialCheckerboardTracker.CheckerboardSeen();
+  }
+  UpdateCheckerboardEvent(lock, magnitude);
+}
+
+void AsyncPanZoomController::UpdateCheckerboardEvent(
+    const MutexAutoLock& aProofOfLock, uint32_t aMagnitude) {
+  if (mCheckerboardEvent && mCheckerboardEvent->RecordFrameInfo(aMagnitude)) {
+    // This checkerboard event is done. Report some metrics to telemetry.
+    mozilla::Telemetry::Accumulate(mozilla::Telemetry::CHECKERBOARD_SEVERITY,
+                                   mCheckerboardEvent->GetSeverity());
+    mozilla::Telemetry::Accumulate(mozilla::Telemetry::CHECKERBOARD_PEAK,
+                                   mCheckerboardEvent->GetPeak());
+    mozilla::Telemetry::Accumulate(
+        mozilla::Telemetry::CHECKERBOARD_DURATION,
+        (uint32_t)mCheckerboardEvent->GetDuration().ToMilliseconds());
+
+    // mCheckerboardEvent only gets created if we are supposed to record
+    // telemetry so we always pass true for aRecordTelemetry.
+    mPotentialCheckerboardTracker.CheckerboardDone(
+        /* aRecordTelemetry = */ true);
+
+    if (StaticPrefs::apz_record_checkerboarding()) {
+      // if the pref is enabled, also send it to the storage class. it may be
+      // chosen for public display on about:checkerboard, the hall of fame for
+      // checkerboard events.
+      uint32_t severity = mCheckerboardEvent->GetSeverity();
+      std::string log = mCheckerboardEvent->GetLog();
+      CheckerboardEventStorage::Report(severity, log);
+    }
+    mCheckerboardEvent = nullptr;
+  }
+}
+
+void AsyncPanZoomController::FlushActiveCheckerboardReport() {
+  MutexAutoLock lock(mCheckerboardEventLock);
+  // Pretend like we got a frame with 0 pixels checkerboarded. This will
+  // terminate the checkerboard event and flush it out
+  UpdateCheckerboardEvent(lock, 0);
+}
+
+void AsyncPanZoomController::NotifyLayersUpdated(
+    const ScrollMetadata& aScrollMetadata, bool aIsFirstPaint,
+    bool aThisLayerTreeUpdated) {
+  AssertOnUpdaterThread();
+
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  bool isDefault = mScrollMetadata.IsDefault();
+
+  const FrameMetrics& aLayerMetrics = aScrollMetadata.GetMetrics();
+
+  if ((aScrollMetadata == mLastContentPaintMetadata) && !isDefault) {
+    // No new information here, skip it.
+    APZC_LOGV("%p NotifyLayersUpdated short-circuit\n", this);
+    return;
+  }
+
+  // If the Metrics scroll offset is different from the last scroll offset
+  // that the main-thread sent us, then we know that the user has been doing
+  // something that triggers a scroll. This check is the APZ equivalent of the
+  // check on the main-thread at
+  // https://hg.mozilla.org/mozilla-central/file/97a52326b06a/layout/generic/nsGfxScrollFrame.cpp#l4050
+  // There is code below (the use site of userScrolled) that prevents a
+  // restored- scroll-position update from overwriting a user scroll, again
+  // equivalent to how the main thread code does the same thing.
+  // XXX Suspicious comparison between layout and visual scroll offsets.
+  // This may not do the right thing when we're zoomed in.
+  CSSPoint lastScrollOffset = mLastContentPaintMetrics.GetLayoutScrollOffset();
+  bool userScrolled = !FuzzyEqualsAdditive(Metrics().GetVisualScrollOffset().x,
+                                           lastScrollOffset.x) ||
+                      !FuzzyEqualsAdditive(Metrics().GetVisualScrollOffset().y,
+                                           lastScrollOffset.y);
+
+  if (aScrollMetadata.DidContentGetPainted()) {
+    mLastContentPaintMetadata = aScrollMetadata;
+  }
+
+  mScrollMetadata.SetScrollParentId(aScrollMetadata.GetScrollParentId());
+  APZC_LOGV_FM(aLayerMetrics,
+               "%p got a NotifyLayersUpdated with aIsFirstPaint=%d, "
+               "aThisLayerTreeUpdated=%d",
+               this, aIsFirstPaint, aThisLayerTreeUpdated);
+
+  {  // scope lock
+    MutexAutoLock lock(mCheckerboardEventLock);
+    if (mCheckerboardEvent && mCheckerboardEvent->IsRecordingTrace()) {
+      std::string str;
+      if (aThisLayerTreeUpdated) {
+        if (!aLayerMetrics.GetPaintRequestTime().IsNull()) {
+          // Note that we might get the paint request time as non-null, but with
+          // aThisLayerTreeUpdated false. That can happen if we get a layer
+          // transaction from a different process right after we get the layer
+          // transaction with aThisLayerTreeUpdated == true. In this case we
+          // want to ignore the paint request time because it was already dumped
+          // in the previous layer transaction.
+          TimeDuration paintTime =
+              TimeStamp::Now() - aLayerMetrics.GetPaintRequestTime();
+          std::stringstream info;
+          info << " painttime " << paintTime.ToMilliseconds();
+          str = info.str();
+        } else {
+          // This might be indicative of a wasted paint particularly if it
+          // happens during a checkerboard event.
+          str = " (this layertree updated)";
+        }
+      }
+      mCheckerboardEvent->UpdateRendertraceProperty(
+          CheckerboardEvent::Page, aLayerMetrics.GetScrollableRect());
+      mCheckerboardEvent->UpdateRendertraceProperty(
+          CheckerboardEvent::PaintedDisplayPort, GetPaintedRect(aLayerMetrics),
+          str);
+    }
+  }
+
+  // The main thread may send us a visual scroll offset update. This is
+  // different from a layout viewport offset update in that the layout viewport
+  // offset is limited to the layout scroll range, while the visual viewport
+  // offset is not.
+  // However, there are some conditions in which the layout update will clobber
+  // the visual update, and we want to ignore the visual update in those cases.
+  // This variable tracks that.
+  bool ignoreVisualUpdate = false;
+
+  // TODO if we're in a drag and scrollOffsetUpdated is set then we want to
+  // ignore it
+
+  bool needContentRepaint = false;
+  RepaintUpdateType contentRepaintType = RepaintUpdateType::eNone;
+  bool viewportSizeUpdated = false;
+  bool needToReclampScroll = false;
+
+  if ((aIsFirstPaint && aThisLayerTreeUpdated) || isDefault ||
+      Metrics().IsRootContent() != aLayerMetrics.IsRootContent()) {
+    if (Metrics().IsRootContent() && !aLayerMetrics.IsRootContent()) {
+      // We only support zooming on root content APZCs
+      SetZoomAnimationId(Nothing());
+    }
+
+    // Initialize our internal state to something sane when the content
+    // that was just painted is something we knew nothing about previously
+    CancelAnimation();
+
+    // Keep our existing scroll generation, if there are scroll updates. In this
+    // case we'll update our scroll generation when processing the scroll update
+    // array below. If there are no scroll updates, take the generation from the
+    // incoming metrics. Bug 1662019 will simplify this later.
+    ScrollGeneration oldScrollGeneration = Metrics().GetScrollGeneration();
+    mScrollMetadata = aScrollMetadata;
+    if (!aScrollMetadata.GetScrollUpdates().IsEmpty()) {
+      Metrics().SetScrollGeneration(oldScrollGeneration);
+    }
+
+    mExpectedGeckoMetrics.UpdateFrom(aLayerMetrics);
+
+    for (auto& sampledState : mSampledState) {
+      sampledState.UpdateScrollProperties(Metrics());
+      sampledState.UpdateZoomProperties(Metrics());
+    }
+
+    if (aLayerMetrics.HasNonZeroDisplayPortMargins()) {
+      // A non-zero display port margin here indicates a displayport has
+      // been set by a previous APZC for the content at this guid. The
+      // scrollable rect may have changed since then, making the margins
+      // wrong, so we need to calculate a new display port.
+      // It is important that we request a repaint here only when we need to
+      // otherwise we will end up setting a display port on every frame that
+      // gets a view id.
+      APZC_LOG("%p detected non-empty margins which probably need updating\n",
+               this);
+      needContentRepaint = true;
+    }
+  } else {
+    // If we're not taking the aLayerMetrics wholesale we still need to pull
+    // in some things into our local Metrics() because these things are
+    // determined by Gecko and our copy in Metrics() may be stale.
+
+    if (Metrics().GetLayoutViewport().Size() !=
+        aLayerMetrics.GetLayoutViewport().Size()) {
+      CSSRect layoutViewport = Metrics().GetLayoutViewport();
+      // The offset will be updated if necessary via
+      // RecalculateLayoutViewportOffset().
+      layoutViewport.SizeTo(aLayerMetrics.GetLayoutViewport().Size());
+      Metrics().SetLayoutViewport(layoutViewport);
+
+      needContentRepaint = true;
+      viewportSizeUpdated = true;
+    }
+
+    // TODO: Rely entirely on |aScrollMetadata.IsResolutionUpdated()| to
+    //       determine which branch to take, and drop the other conditions.
+    CSSToParentLayerScale oldZoom = Metrics().GetZoom();
+    if (FuzzyEqualsAdditive(
+            Metrics().GetCompositionBoundsWidthIgnoringScrollbars(),
+            aLayerMetrics.GetCompositionBoundsWidthIgnoringScrollbars()) &&
+        Metrics().GetDevPixelsPerCSSPixel() ==
+            aLayerMetrics.GetDevPixelsPerCSSPixel() &&
+        !viewportSizeUpdated && !aScrollMetadata.IsResolutionUpdated()) {
+      // Any change to the pres shell resolution was requested by APZ and is
+      // already included in our zoom; however, other components of the
+      // cumulative resolution (a parent document's pres-shell resolution, or
+      // the css-driven resolution) may have changed, and we need to update
+      // our zoom to reflect that. Note that we can't just take
+      // aLayerMetrics.mZoom because the APZ may have additional async zoom
+      // since the repaint request.
+      float totalResolutionChange = 1.0;
+
+      if (Metrics().GetCumulativeResolution() != LayoutDeviceToLayerScale(0)) {
+        totalResolutionChange = aLayerMetrics.GetCumulativeResolution().scale /
+                                Metrics().GetCumulativeResolution().scale;
+      }
+
+      float presShellResolutionChange = aLayerMetrics.GetPresShellResolution() /
+                                        Metrics().GetPresShellResolution();
+      if (presShellResolutionChange != 1.0f) {
+        needContentRepaint = true;
+      }
+      Metrics().ZoomBy(totalResolutionChange / presShellResolutionChange);
+      for (auto& sampledState : mSampledState) {
+        sampledState.ZoomBy(totalResolutionChange / presShellResolutionChange);
+      }
+    } else {
+      // Take the new zoom as either device scale or composition width or
+      // viewport size got changed (e.g. due to orientation change, or content
+      // changing the meta-viewport tag), or the main thread originated a
+      // resolution change for another reason (e.g. Ctrl+0 was pressed to
+      // reset the zoom).
+      Metrics().SetZoom(aLayerMetrics.GetZoom());
+      for (auto& sampledState : mSampledState) {
+        sampledState.UpdateZoomProperties(aLayerMetrics);
+      }
+      Metrics().SetDevPixelsPerCSSPixel(
+          aLayerMetrics.GetDevPixelsPerCSSPixel());
+    }
+
+    if (Metrics().GetZoom() != oldZoom) {
+      // If the zoom changed, the scroll range in CSS pixels may have changed
+      // even if the composition bounds didn't.
+      needToReclampScroll = true;
+    }
+
+    mExpectedGeckoMetrics.UpdateZoomFrom(aLayerMetrics);
+
+    if (!Metrics().GetScrollableRect().IsEqualEdges(
+            aLayerMetrics.GetScrollableRect())) {
+      Metrics().SetScrollableRect(aLayerMetrics.GetScrollableRect());
+      needContentRepaint = true;
+      needToReclampScroll = true;
+    }
+    if (!Metrics().GetCompositionBounds().IsEqualEdges(
+            aLayerMetrics.GetCompositionBounds())) {
+      Metrics().SetCompositionBounds(aLayerMetrics.GetCompositionBounds());
+      needToReclampScroll = true;
+    }
+    Metrics().SetCompositionBoundsWidthIgnoringScrollbars(
+        aLayerMetrics.GetCompositionBoundsWidthIgnoringScrollbars());
+
+    if (Metrics().IsRootContent() &&
+        Metrics().GetCompositionSizeWithoutDynamicToolbar() !=
+            aLayerMetrics.GetCompositionSizeWithoutDynamicToolbar()) {
+      Metrics().SetCompositionSizeWithoutDynamicToolbar(
+          aLayerMetrics.GetCompositionSizeWithoutDynamicToolbar());
+      needToReclampScroll = true;
+    }
+    Metrics().SetBoundingCompositionSize(
+        aLayerMetrics.GetBoundingCompositionSize());
+    Metrics().SetPresShellResolution(aLayerMetrics.GetPresShellResolution());
+    Metrics().SetCumulativeResolution(aLayerMetrics.GetCumulativeResolution());
+    Metrics().SetTransformToAncestorScale(
+        aLayerMetrics.GetTransformToAncestorScale());
+    mScrollMetadata.SetHasScrollgrab(aScrollMetadata.GetHasScrollgrab());
+    mScrollMetadata.SetLineScrollAmount(aScrollMetadata.GetLineScrollAmount());
+    mScrollMetadata.SetPageScrollAmount(aScrollMetadata.GetPageScrollAmount());
+    mScrollMetadata.SetSnapInfo(ScrollSnapInfo(aScrollMetadata.GetSnapInfo()));
+    mScrollMetadata.SetIsLayersIdRoot(aScrollMetadata.IsLayersIdRoot());
+    mScrollMetadata.SetIsAutoDirRootContentRTL(
+        aScrollMetadata.IsAutoDirRootContentRTL());
+    Metrics().SetIsScrollInfoLayer(aLayerMetrics.IsScrollInfoLayer());
+    Metrics().SetHasNonZeroDisplayPortMargins(
+        aLayerMetrics.HasNonZeroDisplayPortMargins());
+    Metrics().SetMinimalDisplayPort(aLayerMetrics.IsMinimalDisplayPort());
+    mScrollMetadata.SetForceDisableApz(aScrollMetadata.IsApzForceDisabled());
+    mScrollMetadata.SetIsRDMTouchSimulationActive(
+        aScrollMetadata.GetIsRDMTouchSimulationActive());
+    mScrollMetadata.SetForceMousewheelAutodir(
+        aScrollMetadata.ForceMousewheelAutodir());
+    mScrollMetadata.SetForceMousewheelAutodirHonourRoot(
+        aScrollMetadata.ForceMousewheelAutodirHonourRoot());
+    mScrollMetadata.SetIsPaginatedPresentation(
+        aScrollMetadata.IsPaginatedPresentation());
+    mScrollMetadata.SetDisregardedDirection(
+        aScrollMetadata.GetDisregardedDirection());
+    mScrollMetadata.SetOverscrollBehavior(
+        aScrollMetadata.GetOverscrollBehavior());
+  }
+
+  bool scrollOffsetUpdated = false;
+  bool smoothScrollRequested = false;
+  bool didCancelAnimation = false;
+  Maybe<CSSPoint> cumulativeRelativeDelta;
+  for (const auto& scrollUpdate : aScrollMetadata.GetScrollUpdates()) {
+    APZC_LOG("%p processing scroll update %s\n", this,
+             ToString(scrollUpdate).c_str());
+    if (!(Metrics().GetScrollGeneration() < scrollUpdate.GetGeneration())) {
+      // This is stale, let's ignore it
+      APZC_LOG("%p scrollupdate generation stale, dropping\n", this);
+      continue;
+    }
+    Metrics().SetScrollGeneration(scrollUpdate.GetGeneration());
+
+    MOZ_ASSERT(scrollUpdate.GetOrigin() != ScrollOrigin::Apz);
+    if (userScrolled &&
+        !nsLayoutUtils::CanScrollOriginClobberApz(scrollUpdate.GetOrigin())) {
+      APZC_LOG("%p scrollupdate cannot clobber APZ userScrolled\n", this);
+      continue;
+    }
+    // XXX: if we get here, |scrollUpdate| is clobbering APZ, so we may want
+    // to reset |userScrolled| back to false so that subsequent scrollUpdates
+    // in this loop don't get dropped by the check above. Need to add a test
+    // that exercises this scenario, as we don't currently have one.
+
+    if (scrollUpdate.GetMode() == ScrollMode::Smooth ||
+        scrollUpdate.GetMode() == ScrollMode::SmoothMsd) {
+      smoothScrollRequested = true;
+
+      // Requests to animate the visual scroll position override requests to
+      // simply update the visual scroll offset to a particular point. Since
+      // we have an animation request, we set ignoreVisualUpdate to true to
+      // indicate we don't need to apply the visual scroll update in
+      // aLayerMetrics.
+      ignoreVisualUpdate = true;
+
+      // For relative updates we want to add the relative offset to any existing
+      // destination, or the current visual offset if there is no existing
+      // destination.
+      CSSPoint base = GetCurrentAnimationDestination(lock).valueOr(
+          Metrics().GetVisualScrollOffset());
+
+      CSSPoint destination;
+      if (scrollUpdate.GetType() == ScrollUpdateType::Relative) {
+        CSSPoint delta =
+            scrollUpdate.GetDestination() - scrollUpdate.GetSource();
+        APZC_LOG("%p relative smooth scrolling from %s by %s\n", this,
+                 ToString(base).c_str(), ToString(delta).c_str());
+        destination = Metrics().CalculateScrollRange().ClampPoint(base + delta);
+      } else if (scrollUpdate.GetType() == ScrollUpdateType::PureRelative) {
+        CSSPoint delta = scrollUpdate.GetDelta();
+        APZC_LOG("%p pure-relative smooth scrolling from %s by %s\n", this,
+                 ToString(base).c_str(), ToString(delta).c_str());
+        destination = Metrics().CalculateScrollRange().ClampPoint(base + delta);
+      } else {
+        APZC_LOG("%p smooth scrolling to %s\n", this,
+                 ToString(scrollUpdate.GetDestination()).c_str());
+        destination = scrollUpdate.GetDestination();
+      }
+
+      if (scrollUpdate.GetMode() == ScrollMode::SmoothMsd) {
+        SmoothMsdScrollTo(
+            CSSSnapTarget{destination, scrollUpdate.GetSnapTargetIds()},
+            scrollUpdate.GetScrollTriggeredByScript());
+      } else {
+        MOZ_ASSERT(scrollUpdate.GetMode() == ScrollMode::Smooth);
+        MOZ_ASSERT(!scrollUpdate.WasTriggeredByScript());
+        SmoothScrollTo(destination, scrollUpdate.GetOrigin());
+      }
+      continue;
+    }
+
+    MOZ_ASSERT(scrollUpdate.GetMode() == ScrollMode::Instant ||
+               scrollUpdate.GetMode() == ScrollMode::Normal);
+
+    // If the layout update is of a higher priority than the visual update, then
+    // we don't want to apply the visual update.
+    // If the layout update is of a clobbering type (or a smooth scroll request,
+    // which is handled above) then it takes precedence over an eRestore visual
+    // update. But we also allow the possibility for the main thread to ask us
+    // to scroll both the layout and visual viewports to distinct (but
+    // compatible) locations (via e.g. both updates being of a non-clobbering/
+    // eRestore type).
+    if (nsLayoutUtils::CanScrollOriginClobberApz(scrollUpdate.GetOrigin()) &&
+        aLayerMetrics.GetVisualScrollUpdateType() !=
+            FrameMetrics::eMainThread) {
+      ignoreVisualUpdate = true;
+    }
+
+    Maybe<CSSPoint> relativeDelta;
+    if (scrollUpdate.GetType() == ScrollUpdateType::Relative) {
+      APZC_LOG(
+          "%p relative updating scroll offset from %s by %s\n", this,
+          ToString(Metrics().GetVisualScrollOffset()).c_str(),
+          ToString(scrollUpdate.GetDestination() - scrollUpdate.GetSource())
+              .c_str());
+
+      scrollOffsetUpdated = true;
+
+      // It's possible that the main thread has ignored an APZ scroll offset
+      // update for the pending relative scroll that we have just received.
+      // When this happens, we need to send a new scroll offset update with
+      // the combined scroll offset or else the main thread may have an
+      // incorrect scroll offset for a period of time.
+      if (Metrics().HasPendingScroll(aLayerMetrics)) {
+        needContentRepaint = true;
+        contentRepaintType = RepaintUpdateType::eUserAction;
+      }
+
+      relativeDelta =
+          Some(Metrics().ApplyRelativeScrollUpdateFrom(scrollUpdate));
+      Metrics().RecalculateLayoutViewportOffset();
+    } else if (scrollUpdate.GetType() == ScrollUpdateType::PureRelative) {
+      APZC_LOG("%p pure-relative updating scroll offset from %s by %s\n", this,
+               ToString(Metrics().GetVisualScrollOffset()).c_str(),
+               ToString(scrollUpdate.GetDelta()).c_str());
+
+      scrollOffsetUpdated = true;
+
+      // Always need a repaint request with a repaint type for pure relative
+      // scrolls because apz is doing the scroll at the main thread's request.
+      // The main thread has not updated it's scroll offset yet, it is depending
+      // on apz to tell it where to scroll.
+      needContentRepaint = true;
+      contentRepaintType = RepaintUpdateType::eVisualUpdate;
+
+      // We have to ignore a visual scroll offset update otherwise it will
+      // clobber the relative scrolling we are about to do. We perform
+      // visualScrollOffset = visualScrollOffset + delta. Then the
+      // visualScrollOffsetUpdated block below will do visualScrollOffset =
+      // aLayerMetrics.GetVisualDestination(). We need visual scroll offset
+      // updates to be incorporated into this scroll update loop to properly fix
+      // this.
+      ignoreVisualUpdate = true;
+
+      relativeDelta =
+          Some(Metrics().ApplyPureRelativeScrollUpdateFrom(scrollUpdate));
+      Metrics().RecalculateLayoutViewportOffset();
+    } else {
+      APZC_LOG("%p updating scroll offset from %s to %s\n", this,
+               ToString(Metrics().GetVisualScrollOffset()).c_str(),
+               ToString(scrollUpdate.GetDestination()).c_str());
+      bool offsetChanged = Metrics().ApplyScrollUpdateFrom(scrollUpdate);
+      Metrics().RecalculateLayoutViewportOffset();
+
+      if (offsetChanged || scrollUpdate.GetMode() != ScrollMode::Instant ||
+          scrollUpdate.GetType() != ScrollUpdateType::Absolute ||
+          scrollUpdate.GetOrigin() != ScrollOrigin::None) {
+        // We get a NewScrollFrame update for newly created scroll frames. Only
+        // if this was not a NewScrollFrame update or the offset changed do we
+        // request repaint. This is important so that we don't request repaint
+        // for every new content and set a full display port on it.
+        scrollOffsetUpdated = true;
+      }
+    }
+
+    if (relativeDelta) {
+      cumulativeRelativeDelta =
+          !cumulativeRelativeDelta
+              ? relativeDelta
+              : Some(*cumulativeRelativeDelta + *relativeDelta);
+    } else {
+      // If the scroll update is not relative, clobber the cumulative delta,
+      // i.e. later updates win.
+      cumulativeRelativeDelta.reset();
+    }
+
+    // If an animation is underway, tell it about the scroll offset update.
+    // Some animations can handle some scroll offset updates and continue
+    // running. Those that can't will return false, and we cancel them.
+    if (ShouldCancelAnimationForScrollUpdate(relativeDelta)) {
+      // Cancel the animation (which might also trigger a repaint request)
+      // after we update the scroll offset above. Otherwise we can be left
+      // in a state where things are out of sync.
+      CancelAnimation();
+      didCancelAnimation = true;
+    }
+  }
+
+  if (scrollOffsetUpdated) {
+    for (auto& sampledState : mSampledState) {
+      if (!didCancelAnimation && cumulativeRelativeDelta.isSome()) {
+        sampledState.UpdateScrollPropertiesWithRelativeDelta(
+            Metrics(), *cumulativeRelativeDelta);
+      } else {
+        sampledState.UpdateScrollProperties(Metrics());
+      }
+    }
+
+    // Because of the scroll generation update, any inflight paint requests
+    // are going to be ignored by layout, and so mExpectedGeckoMetrics becomes
+    // incorrect for the purposes of calculating the LD transform. To correct
+    // this we need to update mExpectedGeckoMetrics to be the last thing we
+    // know was painted by Gecko.
+    mExpectedGeckoMetrics.UpdateFrom(aLayerMetrics);
+
+    // Since the scroll offset has changed, we need to recompute the
+    // displayport margins and send them to layout. Otherwise there might be
+    // scenarios where for example we scroll from the top of a page (where the
+    // top displayport margin is zero) to the bottom of a page, which will
+    // result in a displayport that doesn't extend upwards at all.
+    // Note that even if the CancelAnimation call above requested a repaint
+    // this is fine because we already have repaint request deduplication.
+    needContentRepaint = true;
+    // Since the main-thread scroll offset changed we should trigger a
+    // recomposite to make sure it becomes user-visible.
+    ScheduleComposite();
+  } else if (needToReclampScroll) {
+    // Even if we didn't accept a new scroll offset from content, the
+    // scrollable rect or composition bounds may have changed in a way that
+    // makes our local scroll offset out of bounds, so re-clamp it.
+    ClampAndSetVisualScrollOffset(Metrics().GetVisualScrollOffset());
+    for (auto& sampledState : mSampledState) {
+      sampledState.ClampVisualScrollOffset(Metrics());
+    }
+  }
+
+  // If our scroll range changed (for example, because the page dynamically
+  // loaded new content, thereby increasing the size of the scrollable rect),
+  // and we're overscrolled, being overscrolled may no longer be a valid
+  // state (for example, we may no longer be at the edge of our scroll range),
+  // so clear overscroll and discontinue any overscroll animation.
+  // Ideas for improvements here:
+  //    - Instead of collapsing the overscroll gutter, try to "fill it"
+  //      with newly loaded content. This would basically entail checking
+  //      if (GetVisualScrollOffset() + GetOverscrollAmount()) is a valid
+  //      visual scroll offset in our new scroll range, and if so, scrolling
+  //      there.
+  if (needToReclampScroll) {
+    if (IsInInvalidOverscroll()) {
+      if (mState == OVERSCROLL_ANIMATION) {
+        CancelAnimation();
+      } else if (IsOverscrolled()) {
+        ClearOverscroll();
+      }
+    }
+  }
+
+  if (smoothScrollRequested && !scrollOffsetUpdated) {
+    mExpectedGeckoMetrics.UpdateFrom(aLayerMetrics);
+    // Need to acknowledge the request.
+    needContentRepaint = true;
+  }
+
+  // If `isDefault` is true, this APZC is a "new" one (this is the first time
+  // it's getting a NotifyLayersUpdated call). In this case we want to apply the
+  // visual scroll offset from the main thread to our scroll offset.
+  // The main thread may also ask us to scroll the visual viewport to a
+  // particular location. However, in all cases, we want to ignore the visual
+  // offset update if ignoreVisualUpdate is true, because we're clobbering
+  // the visual update with a layout update.
+  bool visualScrollOffsetUpdated =
+      !ignoreVisualUpdate &&
+      (isDefault ||
+       aLayerMetrics.GetVisualScrollUpdateType() != FrameMetrics::eNone);
+
+  if (visualScrollOffsetUpdated) {
+    APZC_LOG("%p updating visual scroll offset from %s to %s (updateType %d)\n",
+             this, ToString(Metrics().GetVisualScrollOffset()).c_str(),
+             ToString(aLayerMetrics.GetVisualDestination()).c_str(),
+             (int)aLayerMetrics.GetVisualScrollUpdateType());
+    bool offsetChanged = Metrics().ClampAndSetVisualScrollOffset(
+        aLayerMetrics.GetVisualDestination());
+
+    // If this is the first time we got metrics for this content (isDefault) and
+    // the update type was none and the offset didn't change then we don't have
+    // to do anything. This is important because we don't want to request
+    // repaint on the initial NotifyLayersUpdated for every content and thus set
+    // a full display port.
+    if (aLayerMetrics.GetVisualScrollUpdateType() == FrameMetrics::eNone &&
+        !offsetChanged) {
+      visualScrollOffsetUpdated = false;
+    }
+  }
+  if (visualScrollOffsetUpdated) {
+    // The rest of this branch largely follows the code in the
+    // |if (scrollOffsetUpdated)| branch above. Eventually it should get
+    // merged into that branch.
+    Metrics().RecalculateLayoutViewportOffset();
+    for (auto& sampledState : mSampledState) {
+      sampledState.UpdateScrollProperties(Metrics());
+    }
+    mExpectedGeckoMetrics.UpdateFrom(aLayerMetrics);
+    if (ShouldCancelAnimationForScrollUpdate(Nothing())) {
+      CancelAnimation();
+    }
+    // The main thread did not actually paint a displayport at the target
+    // visual offset, so we need to ask it to repaint. We need to set the
+    // contentRepaintType to something other than eNone, otherwise the main
+    // thread will short-circuit the repaint request.
+    // Don't do this for eRestore visual updates as a repaint coming from APZ
+    // breaks the scroll offset restoration mechanism.
+    needContentRepaint = true;
+    if (aLayerMetrics.GetVisualScrollUpdateType() ==
+        FrameMetrics::eMainThread) {
+      contentRepaintType = RepaintUpdateType::eVisualUpdate;
+    }
+    ScheduleComposite();
+  }
+
+  if (viewportSizeUpdated) {
+    // While we want to accept the main thread's layout viewport _size_,
+    // its position may be out of date in light of async scrolling, to
+    // adjust it if necessary to make sure it continues to enclose the
+    // visual viewport.
+    // Note: it's important to do this _after_ we've accepted any
+    // updated composition bounds.
+    Metrics().RecalculateLayoutViewportOffset();
+  }
+
+  if (needContentRepaint) {
+    // This repaint request could be driven by a user action if we accept a
+    // relative scroll offset update
+    RequestContentRepaint(contentRepaintType);
+  }
+}
+
+FrameMetrics& AsyncPanZoomController::Metrics() {
+  mRecursiveMutex.AssertCurrentThreadIn();
+  return mScrollMetadata.GetMetrics();
+}
+
+const FrameMetrics& AsyncPanZoomController::Metrics() const {
+  mRecursiveMutex.AssertCurrentThreadIn();
+  return mScrollMetadata.GetMetrics();
+}
+
+GeckoViewMetrics AsyncPanZoomController::GetGeckoViewMetrics() const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return GeckoViewMetrics{GetEffectiveScrollOffset(eForCompositing, lock),
+                          GetEffectiveZoom(eForCompositing, lock)};
+}
+
+bool AsyncPanZoomController::UpdateRootFrameMetricsIfChanged(
+    GeckoViewMetrics& aMetrics) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+  if (!Metrics().IsRootContent()) {
+    return false;
+  }
+
+  GeckoViewMetrics newMetrics = GetGeckoViewMetrics();
+  bool hasChanged = RoundedToInt(aMetrics.mVisualScrollOffset) !=
+                        RoundedToInt(newMetrics.mVisualScrollOffset) ||
+                    aMetrics.mZoom != newMetrics.mZoom;
+
+  if (hasChanged) {
+    aMetrics = newMetrics;
+  }
+
+  return hasChanged;
+}
+
+const FrameMetrics& AsyncPanZoomController::GetFrameMetrics() const {
+  return Metrics();
+}
+
+const ScrollMetadata& AsyncPanZoomController::GetScrollMetadata() const {
+  mRecursiveMutex.AssertCurrentThreadIn();
+  return mScrollMetadata;
+}
+
+void AsyncPanZoomController::AssertOnSamplerThread() const {
+  if (APZCTreeManager* treeManagerLocal = GetApzcTreeManager()) {
+    treeManagerLocal->AssertOnSamplerThread();
+  }
+}
+
+void AsyncPanZoomController::AssertOnUpdaterThread() const {
+  if (APZCTreeManager* treeManagerLocal = GetApzcTreeManager()) {
+    treeManagerLocal->AssertOnUpdaterThread();
+  }
+}
+
+APZCTreeManager* AsyncPanZoomController::GetApzcTreeManager() const {
+  mRecursiveMutex.AssertNotCurrentThreadIn();
+  return mTreeManager;
+}
+
+void AsyncPanZoomController::ZoomToRect(const ZoomTarget& aZoomTarget,
+                                        const uint32_t aFlags) {
+  CSSRect rect = aZoomTarget.targetRect;
+  if (!rect.IsFinite()) {
+    NS_WARNING("ZoomToRect got called with a non-finite rect; ignoring...");
+    return;
+  }
+
+  if (rect.IsEmpty() && (aFlags & DISABLE_ZOOM_OUT)) {
+    // Double-tap-to-zooming uses an empty rect to mean "zoom out".
+    // If zooming out is disabled, an empty rect is nonsensical
+    // and will produce undesirable scrolling.
+    NS_WARNING(
+        "ZoomToRect got called with an empty rect and zoom out disabled; "
+        "ignoring...");
+    return;
+  }
+
+  SetState(ANIMATING_ZOOM);
+
+  {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+    MOZ_ASSERT(Metrics().IsRootContent());
+
+    const float defaultZoomInAmount =
+        StaticPrefs::apz_doubletapzoom_defaultzoomin();
+
+    ParentLayerRect compositionBounds = Metrics().GetCompositionBounds();
+    CSSRect cssPageRect = Metrics().GetScrollableRect();
+    CSSPoint scrollOffset = Metrics().GetVisualScrollOffset();
+    CSSSize sizeBeforeZoom = Metrics().CalculateCompositedSizeInCssPixels();
+    CSSToParentLayerScale currentZoom = Metrics().GetZoom();
+    CSSToParentLayerScale targetZoom;
+
+    // The minimum zoom to prevent over-zoom-out.
+    // If the zoom factor is lower than this (i.e. we are zoomed more into the
+    // page), then the CSS content rect, in layers pixels, will be smaller than
+    // the composition bounds. If this happens, we can't fill the target
+    // composited area with this frame.
+    CSSToParentLayerScale localMinZoom(
+        std::max(compositionBounds.Width() / cssPageRect.Width(),
+                 compositionBounds.Height() / cssPageRect.Height()));
+
+    localMinZoom.scale =
+        clamped(localMinZoom.scale, mZoomConstraints.mMinZoom.scale,
+                mZoomConstraints.mMaxZoom.scale);
+
+    localMinZoom = std::max(mZoomConstraints.mMinZoom, localMinZoom);
+    CSSToParentLayerScale localMaxZoom =
+        std::max(localMinZoom, mZoomConstraints.mMaxZoom);
+
+    if (!rect.IsEmpty()) {
+      // Intersect the zoom-to-rect to the CSS rect to make sure it fits.
+      rect = rect.Intersect(cssPageRect);
+      targetZoom = CSSToParentLayerScale(
+          std::min(compositionBounds.Width() / rect.Width(),
+                   compositionBounds.Height() / rect.Height()));
+      if (aFlags & DISABLE_ZOOM_OUT) {
+        targetZoom = std::max(targetZoom, currentZoom);
+      }
+    }
+
+    // 1. If the rect is empty, the content-side logic for handling a double-tap
+    //    requested that we zoom out.
+    // 2. currentZoom is equal to mZoomConstraints.mMaxZoom and user still
+    // double-tapping it
+    // Treat these cases as a request to zoom out as much as possible
+    // unless cantZoomOutBehavior == ZoomIn and currentZoom
+    // is equal to localMinZoom and user still double-tapping it, then try to
+    // zoom in a small amount to provide feedback to the user.
+    bool zoomOut = false;
+    // True if we are already zoomed out and we are asked to either stay there
+    // or zoom out more and cantZoomOutBehavior == ZoomIn.
+    bool zoomInDefaultAmount = false;
+    if (aFlags & DISABLE_ZOOM_OUT) {
+      zoomOut = false;
+    } else {
+      if (rect.IsEmpty()) {
+        if (currentZoom == localMinZoom &&
+            aZoomTarget.cantZoomOutBehavior == CantZoomOutBehavior::ZoomIn &&
+            (defaultZoomInAmount != 1.f)) {
+          zoomInDefaultAmount = true;
+        } else {
+          zoomOut = true;
+        }
+      } else if (currentZoom == localMaxZoom && targetZoom >= localMaxZoom) {
+        zoomOut = true;
+      }
+    }
+
+    // already at min zoom and asked to zoom out further
+    if (!zoomOut && currentZoom == localMinZoom && targetZoom <= localMinZoom &&
+        aZoomTarget.cantZoomOutBehavior == CantZoomOutBehavior::ZoomIn &&
+        (defaultZoomInAmount != 1.f)) {
+      zoomInDefaultAmount = true;
+    }
+    MOZ_ASSERT(!(zoomInDefaultAmount && zoomOut));
+
+    if (zoomInDefaultAmount) {
+      targetZoom =
+          CSSToParentLayerScale(currentZoom.scale * defaultZoomInAmount);
+    }
+
+    if (zoomOut) {
+      targetZoom = localMinZoom;
+    }
+
+    if (aFlags & PAN_INTO_VIEW_ONLY) {
+      targetZoom = currentZoom;
+    } else if (aFlags & ONLY_ZOOM_TO_DEFAULT_SCALE) {
+      CSSToParentLayerScale zoomAtDefaultScale =
+          Metrics().GetDevPixelsPerCSSPixel() *
+          LayoutDeviceToParentLayerScale(1.0);
+      if (targetZoom.scale > zoomAtDefaultScale.scale) {
+        // Only change the zoom if we are less than the default zoom
+        if (currentZoom.scale < zoomAtDefaultScale.scale) {
+          targetZoom = zoomAtDefaultScale;
+        } else {
+          targetZoom = currentZoom;
+        }
+      }
+    }
+
+    targetZoom.scale =
+        clamped(targetZoom.scale, localMinZoom.scale, localMaxZoom.scale);
+
+    FrameMetrics endZoomToMetrics = Metrics();
+    endZoomToMetrics.SetZoom(CSSToParentLayerScale(targetZoom));
+    CSSSize sizeAfterZoom =
+        endZoomToMetrics.CalculateCompositedSizeInCssPixels();
+
+    if (zoomInDefaultAmount || zoomOut) {
+      // For the zoom out case we should always center what was visible
+      // otherwise it feels like we are scrolling as well as zooming out. For
+      // the non-zoomOut case, if we've been provided a pointer location, zoom
+      // around that, otherwise just zoom in to the center of what's currently
+      // visible.
+      if (!zoomOut && aZoomTarget.documentRelativePointerPosition.isSome()) {
+        rect = CSSRect(aZoomTarget.documentRelativePointerPosition->x -
+                           sizeAfterZoom.width / 2,
+                       aZoomTarget.documentRelativePointerPosition->y -
+                           sizeAfterZoom.height / 2,
+                       sizeAfterZoom.Width(), sizeAfterZoom.Height());
+      } else {
+        rect = CSSRect(
+            scrollOffset.x + (sizeBeforeZoom.width - sizeAfterZoom.width) / 2,
+            scrollOffset.y + (sizeBeforeZoom.height - sizeAfterZoom.height) / 2,
+            sizeAfterZoom.Width(), sizeAfterZoom.Height());
+      }
+
+      rect = rect.Intersect(cssPageRect);
+    }
+
+    // Check if we can fit the full elementBoundingRect.
+    if (!aZoomTarget.targetRect.IsEmpty() && !zoomOut &&
+        aZoomTarget.elementBoundingRect.isSome()) {
+      MOZ_ASSERT(aZoomTarget.elementBoundingRect->Contains(rect));
+      CSSRect elementBoundingRect =
+          aZoomTarget.elementBoundingRect->Intersect(cssPageRect);
+      if (elementBoundingRect.width <= sizeAfterZoom.width &&
+          elementBoundingRect.height <= sizeAfterZoom.height) {
+        rect = elementBoundingRect;
+      }
+    }
+
+    // Vertically center the zoomed element in the screen.
+    if (!zoomOut && (sizeAfterZoom.height > rect.Height())) {
+      rect.MoveByY(-(sizeAfterZoom.height - rect.Height()) * 0.5f);
+      if (rect.Y() < 0.0f) {
+        rect.MoveToY(0.0f);
+      }
+    }
+
+    // Horizontally center the zoomed element in the screen.
+    if (!zoomOut && (sizeAfterZoom.width > rect.Width())) {
+      rect.MoveByX(-(sizeAfterZoom.width - rect.Width()) * 0.5f);
+      if (rect.X() < 0.0f) {
+        rect.MoveToX(0.0f);
+      }
+    }
+
+    bool intersectRectAgain = false;
+    // If we can't zoom out enough to show the full rect then shift the rect we
+    // are able to show to center what was visible.
+    // Note that this calculation works no matter the relation of sizeBeforeZoom
+    // to sizeAfterZoom, ie whether we are increasing or decreasing zoom.
+    if (!zoomOut && (sizeAfterZoom.height < rect.Height())) {
+      rect.y =
+          scrollOffset.y + (sizeBeforeZoom.height - sizeAfterZoom.height) / 2;
+      rect.height = sizeAfterZoom.Height();
+
+      intersectRectAgain = true;
+    }
+
+    if (!zoomOut && (sizeAfterZoom.width < rect.Width())) {
+      rect.x =
+          scrollOffset.x + (sizeBeforeZoom.width - sizeAfterZoom.width) / 2;
+      rect.width = sizeAfterZoom.Width();
+
+      intersectRectAgain = true;
+    }
+    if (intersectRectAgain) {
+      rect = rect.Intersect(cssPageRect);
+    }
+
+    // If any of these conditions are met, the page will be overscrolled after
+    // zoomed. Attempting to scroll outside of the valid scroll range will cause
+    // problems.
+    if (rect.Y() + sizeAfterZoom.height > cssPageRect.YMost()) {
+      rect.MoveToY(std::max(cssPageRect.Y(),
+                            cssPageRect.YMost() - sizeAfterZoom.height));
+    }
+    if (rect.Y() < cssPageRect.Y()) {
+      rect.MoveToY(cssPageRect.Y());
+    }
+    if (rect.X() + sizeAfterZoom.width > cssPageRect.XMost()) {
+      rect.MoveToX(
+          std::max(cssPageRect.X(), cssPageRect.XMost() - sizeAfterZoom.width));
+    }
+    if (rect.X() < cssPageRect.X()) {
+      rect.MoveToY(cssPageRect.X());
+    }
+
+    endZoomToMetrics.SetVisualScrollOffset(rect.TopLeft());
+    endZoomToMetrics.RecalculateLayoutViewportOffset();
+
+    StartAnimation(new ZoomAnimation(
+        *this, Metrics().GetVisualScrollOffset(), Metrics().GetZoom(),
+        endZoomToMetrics.GetVisualScrollOffset(), endZoomToMetrics.GetZoom()));
+
+    RequestContentRepaint(RepaintUpdateType::eUserAction);
+  }
+}
+
+InputBlockState* AsyncPanZoomController::GetCurrentInputBlock() const {
+  return GetInputQueue()->GetCurrentBlock();
+}
+
+TouchBlockState* AsyncPanZoomController::GetCurrentTouchBlock() const {
+  return GetInputQueue()->GetCurrentTouchBlock();
+}
+
+PanGestureBlockState* AsyncPanZoomController::GetCurrentPanGestureBlock()
+    const {
+  return GetInputQueue()->GetCurrentPanGestureBlock();
+}
+
+PinchGestureBlockState* AsyncPanZoomController::GetCurrentPinchGestureBlock()
+    const {
+  return GetInputQueue()->GetCurrentPinchGestureBlock();
+}
+
+void AsyncPanZoomController::ResetTouchInputState() {
+  MultiTouchInput cancel(MultiTouchInput::MULTITOUCH_CANCEL, 0,
+                         TimeStamp::Now(), 0);
+  RefPtr<GestureEventListener> listener = GetGestureEventListener();
+  if (listener) {
+    listener->HandleInputEvent(cancel);
+  }
+  CancelAnimationAndGestureState();
+  // Clear overscroll along the entire handoff chain, in case an APZC
+  // later in the chain is overscrolled.
+  if (TouchBlockState* block = GetCurrentTouchBlock()) {
+    block->GetOverscrollHandoffChain()->ClearOverscroll();
+  }
+}
+
+void AsyncPanZoomController::ResetPanGestureInputState() {
+  // No point sending a PANGESTURE_INTERRUPTED as all it does is
+  // call CancelAnimation(), which we also do here.
+  CancelAnimationAndGestureState();
+  // Clear overscroll along the entire handoff chain, in case an APZC
+  // later in the chain is overscrolled.
+  if (PanGestureBlockState* block = GetCurrentPanGestureBlock()) {
+    block->GetOverscrollHandoffChain()->ClearOverscroll();
+  }
+}
+
+void AsyncPanZoomController::CancelAnimationAndGestureState() {
+  mX.CancelGesture();
+  mY.CancelGesture();
+  CancelAnimation(CancelAnimationFlags::ScrollSnap);
+}
+
+bool AsyncPanZoomController::HasReadyTouchBlock() const {
+  return GetInputQueue()->HasReadyTouchBlock();
+}
+
+bool AsyncPanZoomController::CanHandleScrollOffsetUpdate(PanZoomState aState) {
+  return aState == PAN_MOMENTUM || aState == TOUCHING || IsPanningState(aState);
+}
+
+bool AsyncPanZoomController::ShouldCancelAnimationForScrollUpdate(
+    const Maybe<CSSPoint>& aRelativeDelta) {
+  // Never call CancelAnimation() for a no-op relative update.
+  if (aRelativeDelta == Some(CSSPoint())) {
+    return false;
+  }
+
+  if (mAnimation) {
+    return !mAnimation->HandleScrollOffsetUpdate(aRelativeDelta);
+  }
+
+  return !CanHandleScrollOffsetUpdate(mState);
+}
+
+AsyncPanZoomController::PanZoomState
+AsyncPanZoomController::SetStateNoContentControllerDispatch(
+    PanZoomState aNewState) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  APZC_LOG_DETAIL("changing from state %s to %s\n", this,
+                  ToString(mState).c_str(), ToString(aNewState).c_str());
+  PanZoomState oldState = mState;
+  mState = aNewState;
+  return oldState;
+}
+
+void AsyncPanZoomController::SetState(PanZoomState aNewState) {
+  // When a state transition to a transforming state is occuring and a delayed
+  // transform end notification exists, send the TransformEnd notification
+  // before the TransformBegin notification is sent for the input state change.
+  if (IsTransformingState(aNewState) && IsDelayedTransformEndSet()) {
+    MOZ_ASSERT(!IsTransformingState(mState));
+    SetDelayedTransformEnd(false);
+    DispatchStateChangeNotification(PANNING, NOTHING);
+  }
+
+  PanZoomState oldState = SetStateNoContentControllerDispatch(aNewState);
+
+  DispatchStateChangeNotification(oldState, aNewState);
+}
+
+auto AsyncPanZoomController::GetState() const -> PanZoomState {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return mState;
+}
+
+void AsyncPanZoomController::DispatchStateChangeNotification(
+    PanZoomState aOldState, PanZoomState aNewState) {
+  {  // scope the lock
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    if (mNotificationBlockers > 0) {
+      return;
+    }
+  }
+
+  if (RefPtr<GeckoContentController> controller = GetGeckoContentController()) {
+    if (!IsTransformingState(aOldState) && IsTransformingState(aNewState)) {
+      controller->NotifyAPZStateChange(GetGuid(),
+                                       APZStateChange::eTransformBegin);
+    } else if (IsTransformingState(aOldState) &&
+               !IsTransformingState(aNewState)) {
+      controller->NotifyAPZStateChange(GetGuid(),
+                                       APZStateChange::eTransformEnd);
+    }
+  }
+}
+
+bool AsyncPanZoomController::IsInTransformingState() const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return IsTransformingState(mState);
+}
+
+bool AsyncPanZoomController::IsTransformingState(PanZoomState aState) {
+  return !(aState == NOTHING || aState == TOUCHING);
+}
+
+bool AsyncPanZoomController::IsPanningState(PanZoomState aState) {
+  return (aState == PANNING || aState == PANNING_LOCKED_X ||
+          aState == PANNING_LOCKED_Y);
+}
+
+bool AsyncPanZoomController::IsInPanningState() const {
+  return IsPanningState(mState);
+}
+
+bool AsyncPanZoomController::IsInScrollingGesture() const {
+  return IsPanningState(mState) || mState == SCROLLBAR_DRAG ||
+         mState == TOUCHING || mState == PINCHING;
+}
+
+bool AsyncPanZoomController::IsDelayedTransformEndSet() {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return mDelayedTransformEnd;
+}
+
+void AsyncPanZoomController::SetDelayedTransformEnd(bool aDelayedTransformEnd) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  mDelayedTransformEnd = aDelayedTransformEnd;
+}
+
+void AsyncPanZoomController::UpdateZoomConstraints(
+    const ZoomConstraints& aConstraints) {
+  if ((MOZ_LOG_TEST(sApzCtlLog, LogLevel::Debug) &&
+       (aConstraints != mZoomConstraints)) ||
+      MOZ_LOG_TEST(sApzCtlLog, LogLevel::Verbose)) {
+    APZC_LOG("%p updating zoom constraints to %d %d %f %f\n", this,
+             aConstraints.mAllowZoom, aConstraints.mAllowDoubleTapZoom,
+             aConstraints.mMinZoom.scale, aConstraints.mMaxZoom.scale);
+  }
+
+  if (std::isnan(aConstraints.mMinZoom.scale) ||
+      std::isnan(aConstraints.mMaxZoom.scale)) {
+    NS_WARNING("APZC received zoom constraints with NaN values; dropping...");
+    return;
+  }
+
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  CSSToParentLayerScale min = Metrics().GetDevPixelsPerCSSPixel() *
+                              ViewportMinScale() / ParentLayerToScreenScale(1);
+  CSSToParentLayerScale max = Metrics().GetDevPixelsPerCSSPixel() *
+                              ViewportMaxScale() / ParentLayerToScreenScale(1);
+
+  // inf float values and other bad cases should be sanitized by the code below.
+  mZoomConstraints.mAllowZoom = aConstraints.mAllowZoom;
+  mZoomConstraints.mAllowDoubleTapZoom = aConstraints.mAllowDoubleTapZoom;
+  mZoomConstraints.mMinZoom =
+      (min > aConstraints.mMinZoom ? min : aConstraints.mMinZoom);
+  mZoomConstraints.mMaxZoom =
+      (max > aConstraints.mMaxZoom ? aConstraints.mMaxZoom : max);
+  if (mZoomConstraints.mMaxZoom < mZoomConstraints.mMinZoom) {
+    mZoomConstraints.mMaxZoom = mZoomConstraints.mMinZoom;
+  }
+}
+
+bool AsyncPanZoomController::ZoomConstraintsAllowZoom() const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return mZoomConstraints.mAllowZoom;
+}
+
+bool AsyncPanZoomController::ZoomConstraintsAllowDoubleTapZoom() const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return mZoomConstraints.mAllowDoubleTapZoom;
+}
+
+void AsyncPanZoomController::PostDelayedTask(already_AddRefed<Runnable> aTask,
+                                             int aDelayMs) {
+  APZThreadUtils::AssertOnControllerThread();
+  RefPtr<Runnable> task = aTask;
+  RefPtr<GeckoContentController> controller = GetGeckoContentController();
+  if (controller) {
+    controller->PostDelayedTask(task.forget(), aDelayMs);
+  }
+  // If there is no controller, that means this APZC has been destroyed, and
+  // we probably don't need to run the task. It will get destroyed when the
+  // RefPtr goes out of scope.
+}
+
+bool AsyncPanZoomController::Matches(const ScrollableLayerGuid& aGuid) {
+  return aGuid == GetGuid();
+}
+
+bool AsyncPanZoomController::HasTreeManager(
+    const APZCTreeManager* aTreeManager) const {
+  return GetApzcTreeManager() == aTreeManager;
+}
+
+void AsyncPanZoomController::GetGuid(ScrollableLayerGuid* aGuidOut) const {
+  if (aGuidOut) {
+    *aGuidOut = GetGuid();
+  }
+}
+
+ScrollableLayerGuid AsyncPanZoomController::GetGuid() const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return ScrollableLayerGuid(mLayersId, Metrics().GetPresShellId(),
+                             Metrics().GetScrollId());
+}
+
+void AsyncPanZoomController::SetTestAsyncScrollOffset(const CSSPoint& aPoint) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  mTestAsyncScrollOffset = aPoint;
+  ScheduleComposite();
+}
+
+void AsyncPanZoomController::SetTestAsyncZoom(
+    const LayerToParentLayerScale& aZoom) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  mTestAsyncZoom = aZoom;
+  ScheduleComposite();
+}
+
+Maybe<CSSSnapTarget> AsyncPanZoomController::FindSnapPointNear(
+    const CSSPoint& aDestination, ScrollUnit aUnit,
+    ScrollSnapFlags aSnapFlags) {
+  mRecursiveMutex.AssertCurrentThreadIn();
+  APZC_LOG("%p scroll snapping near %s\n", this,
+           ToString(aDestination).c_str());
+  CSSRect scrollRange = Metrics().CalculateScrollRange();
+  if (auto snapTarget = ScrollSnapUtils::GetSnapPointForDestination(
+          mScrollMetadata.GetSnapInfo(), aUnit, aSnapFlags,
+          CSSRect::ToAppUnits(scrollRange),
+          CSSPoint::ToAppUnits(Metrics().GetVisualScrollOffset()),
+          CSSPoint::ToAppUnits(aDestination))) {
+    CSSPoint cssSnapPoint = CSSPoint::FromAppUnits(snapTarget->mPosition);
+    // GetSnapPointForDestination() can produce a destination that's outside
+    // of the scroll frame's scroll range. Clamp it here (this matches the
+    // behaviour of the main-thread code path, which clamps it in
+    // nsGfxScrollFrame::ScrollTo()).
+    return Some(CSSSnapTarget{scrollRange.ClampPoint(cssSnapPoint),
+                              snapTarget->mTargetIds});
+  }
+  return Nothing();
+}
+
+Maybe<std::pair<MultiTouchInput, MultiTouchInput>>
+AsyncPanZoomController::MaybeSplitTouchMoveEvent(
+    const MultiTouchInput& aOriginalEvent, ScreenCoord aPanThreshold,
+    float aVectorLength, ExternalPoint& aExtPoint) {
+  if (aVectorLength <= aPanThreshold) {
+    return Nothing();
+  }
+
+  auto splitEvent = std::make_pair(aOriginalEvent, aOriginalEvent);
+
+  SingleTouchData& firstTouchData = splitEvent.first.mTouches[0];
+  SingleTouchData& secondTouchData = splitEvent.second.mTouches[0];
+
+  firstTouchData.mHistoricalData.Clear();
+  secondTouchData.mHistoricalData.Clear();
+
+  ExternalPoint destination = aExtPoint;
+  ExternalPoint thresholdPosition;
+
+  const float ratio = aPanThreshold / aVectorLength;
+  thresholdPosition.x = mStartTouch.x + ratio * (destination.x - mStartTouch.x);
+  thresholdPosition.y = mStartTouch.y + ratio * (destination.y - mStartTouch.y);
+
+  TouchSample start{mLastTouch};
+  // To compute the timestamp of the first event (which is at the threshold),
+  // use linear interpolation with the starting point |start| being the last
+  // event that's before the threshold, and the end point |end| being the first
+  // event after the threshold.
+
+  // The initial choice for |start| is the last touch event before
+  // |aOriginalEvent|, and the initial choice for |end| is |aOriginalEvent|.
+
+  // However, the historical data points stored in |aOriginalEvent| may contain
+  // intermediate positions that can serve as tighter bounds for the
+  // interpolation.
+  TouchSample end{destination, aOriginalEvent.mTimeStamp};
+
+  for (const auto& historicalData :
+       aOriginalEvent.mTouches[0].mHistoricalData) {
+    ExternalPoint histExtPoint = ToExternalPoint(aOriginalEvent.mScreenOffset,
+                                                 historicalData.mScreenPoint);
+
+    if (PanVector(histExtPoint).Length() <
+        PanVector(thresholdPosition).Length()) {
+      start = {histExtPoint, historicalData.mTimeStamp};
+    } else {
+      break;
+    }
+  }
+
+  for (const SingleTouchData::HistoricalTouchData& histData :
+       Reversed(aOriginalEvent.mTouches[0].mHistoricalData)) {
+    ExternalPoint histExtPoint =
+        ToExternalPoint(aOriginalEvent.mScreenOffset, histData.mScreenPoint);
+
+    if (PanVector(histExtPoint).Length() >
+        PanVector(thresholdPosition).Length()) {
+      end = {histExtPoint, histData.mTimeStamp};
+    } else {
+      break;
+    }
+  }
+
+  const float totalLength =
+      ScreenPoint(fabs(end.mPosition.x - start.mPosition.x),
+                  fabs(end.mPosition.y - start.mPosition.y))
+          .Length();
+  const float thresholdLength =
+      ScreenPoint(fabs(thresholdPosition.x - start.mPosition.x),
+                  fabs(thresholdPosition.y - start.mPosition.y))
+          .Length();
+  const float splitRatio = thresholdLength / totalLength;
+
+  splitEvent.first.mTimeStamp =
+      start.mTimeStamp +
+      (end.mTimeStamp - start.mTimeStamp).MultDouble(splitRatio);
+
+  for (const auto& historicalData :
+       aOriginalEvent.mTouches[0].mHistoricalData) {
+    if (historicalData.mTimeStamp > splitEvent.first.mTimeStamp) {
+      secondTouchData.mHistoricalData.AppendElement(historicalData);
+    } else {
+      firstTouchData.mHistoricalData.AppendElement(historicalData);
+    }
+  }
+
+  firstTouchData.mScreenPoint = RoundedToInt(
+      ViewAs<ScreenPixel>(thresholdPosition - splitEvent.first.mScreenOffset,
+                          PixelCastJustification::ExternalIsScreen));
+
+  // Recompute firstTouchData.mLocalScreenPoint.
+  splitEvent.first.TransformToLocal(GetTransformToThis());
+
+  // Pass |thresholdPosition| back out to the caller via |aExtPoint|
+  aExtPoint = thresholdPosition;
+
+  return Some(splitEvent);
+}
+
+void AsyncPanZoomController::ScrollSnapNear(const CSSPoint& aDestination,
+                                            ScrollSnapFlags aSnapFlags) {
+  if (Maybe<CSSSnapTarget> snapTarget = FindSnapPointNear(
+          aDestination, ScrollUnit::DEVICE_PIXELS, aSnapFlags)) {
+    if (snapTarget->mPosition != Metrics().GetVisualScrollOffset()) {
+      APZC_LOG("%p smooth scrolling to snap point %s\n", this,
+               ToString(snapTarget->mPosition).c_str());
+      SmoothMsdScrollTo(std::move(*snapTarget), ScrollTriggeredByScript::No);
+    }
+  }
+}
+
+void AsyncPanZoomController::ScrollSnap(ScrollSnapFlags aSnapFlags) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  ScrollSnapNear(Metrics().GetVisualScrollOffset(), aSnapFlags);
+}
+
+void AsyncPanZoomController::ScrollSnapToDestination() {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+  float friction = StaticPrefs::apz_fling_friction();
+  ParentLayerPoint velocity(mX.GetVelocity(), mY.GetVelocity());
+  ParentLayerPoint predictedDelta;
+  // "-velocity / log(1.0 - friction)" is the integral of the deceleration
+  // curve modeled for flings in the "Axis" class.
+  if (velocity.x != 0.0f && friction != 0.0f) {
+    predictedDelta.x = -velocity.x / log(1.0 - friction);
+  }
+  if (velocity.y != 0.0f && friction != 0.0f) {
+    predictedDelta.y = -velocity.y / log(1.0 - friction);
+  }
+
+  // If the fling will overscroll, don't scroll snap, because then the user
+  // user would not see any overscroll animation.
+  bool flingWillOverscroll =
+      IsOverscrolled() && ((velocity.x.value * mX.GetOverscroll() >= 0) ||
+                           (velocity.y.value * mY.GetOverscroll() >= 0));
+  if (flingWillOverscroll) {
+    return;
+  }
+
+  CSSPoint startPosition = Metrics().GetVisualScrollOffset();
+  ScrollSnapFlags snapFlags = ScrollSnapFlags::IntendedEndPosition;
+  if (predictedDelta != ParentLayerPoint()) {
+    snapFlags |= ScrollSnapFlags::IntendedDirection;
+  }
+  if (Maybe<CSSSnapTarget> snapTarget = MaybeAdjustDeltaForScrollSnapping(
+          ScrollUnit::DEVICE_PIXELS, snapFlags, predictedDelta,
+          startPosition)) {
+    APZC_LOG(
+        "%p fling snapping.  friction: %f velocity: %f, %f "
+        "predictedDelta: %f, %f position: %f, %f "
+        "snapDestination: %f, %f\n",
+        this, friction, velocity.x.value, velocity.y.value,
+        predictedDelta.x.value, predictedDelta.y.value,
+        Metrics().GetVisualScrollOffset().x.value,
+        Metrics().GetVisualScrollOffset().y.value, startPosition.x.value,
+        startPosition.y.value);
+
+    // Ensure that any queued transform-end due to a pan-end is not
+    // sent. Instead rely on the transform-end sent due to the
+    // scroll snap animation.
+    SetDelayedTransformEnd(false);
+
+    SmoothMsdScrollTo(std::move(*snapTarget), ScrollTriggeredByScript::No);
+  }
+}
+
+Maybe<CSSSnapTarget> AsyncPanZoomController::MaybeAdjustDeltaForScrollSnapping(
+    ScrollUnit aUnit, ScrollSnapFlags aSnapFlags, ParentLayerPoint& aDelta,
+    CSSPoint& aStartPosition) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  CSSToParentLayerScale zoom = Metrics().GetZoom();
+  if (zoom == CSSToParentLayerScale(0)) {
+    return Nothing();
+  }
+  CSSPoint destination = Metrics().CalculateScrollRange().ClampPoint(
+      aStartPosition + (aDelta / zoom));
+
+  if (Maybe<CSSSnapTarget> snapTarget =
+          FindSnapPointNear(destination, aUnit, aSnapFlags)) {
+    aDelta = (snapTarget->mPosition - aStartPosition) * zoom;
+    aStartPosition = snapTarget->mPosition;
+    return snapTarget;
+  }
+  return Nothing();
+}
+
+Maybe<CSSSnapTarget>
+AsyncPanZoomController::MaybeAdjustDeltaForScrollSnappingOnWheelInput(
+    const ScrollWheelInput& aEvent, ParentLayerPoint& aDelta,
+    CSSPoint& aStartPosition) {
+  // Don't scroll snap for pixel scrolls. This matches the main thread
+  // behaviour in EventStateManager::DoScrollText().
+  if (aEvent.mDeltaType == ScrollWheelInput::SCROLLDELTA_PIXEL) {
+    return Nothing();
+  }
+
+  // Note that this MaybeAdjustDeltaForScrollSnappingOnWheelInput also gets
+  // called for pan gestures at least on older Mac and Windows. In such cases
+  // `aEvent.mDeltaType` is `SCROLLDELTA_PIXEL` which should be filtered out by
+  // the above `if` block, so we assume all incoming `aEvent` are purely wheel
+  // events, thus we basically use `IntendedDirection` here.
+  // If we want to change the behavior, i.e. we want to do scroll snap for
+  // such cases as well, we need to use `IntendedEndPoint`.
+  ScrollSnapFlags snapFlags = ScrollSnapFlags::IntendedDirection;
+  if (aEvent.mDeltaType == ScrollWheelInput::SCROLLDELTA_PAGE) {
+    // On Windows there are a couple of cases where scroll events happen with
+    // SCROLLDELTA_PAGE, in such case we consider it's a page scroll.
+    snapFlags |= ScrollSnapFlags::IntendedEndPosition;
+  }
+  return MaybeAdjustDeltaForScrollSnapping(
+      ScrollWheelInput::ScrollUnitForDeltaType(aEvent.mDeltaType),
+      ScrollSnapFlags::IntendedDirection, aDelta, aStartPosition);
+}
+
+Maybe<CSSSnapTarget>
+AsyncPanZoomController::MaybeAdjustDestinationForScrollSnapping(
+    const KeyboardInput& aEvent, CSSPoint& aDestination,
+    ScrollSnapFlags aSnapFlags) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  ScrollUnit unit = KeyboardScrollAction::GetScrollUnit(aEvent.mAction.mType);
+
+  if (Maybe<CSSSnapTarget> snapPoint =
+          FindSnapPointNear(aDestination, unit, aSnapFlags)) {
+    aDestination = snapPoint->mPosition;
+    return snapPoint;
+  }
+  return Nothing();
+}
+
+void AsyncPanZoomController::SetZoomAnimationId(
+    const Maybe<uint64_t>& aZoomAnimationId) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  mZoomAnimationId = aZoomAnimationId;
+}
+
+Maybe<uint64_t> AsyncPanZoomController::GetZoomAnimationId() const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return mZoomAnimationId;
+}
+
+std::ostream& operator<<(std::ostream& aOut,
+                         const AsyncPanZoomController::PanZoomState& aState) {
+  switch (aState) {
+    case AsyncPanZoomController::PanZoomState::NOTHING:
+      aOut << "NOTHING";
+      break;
+    case AsyncPanZoomController::PanZoomState::FLING:
+      aOut << "FLING";
+      break;
+    case AsyncPanZoomController::PanZoomState::TOUCHING:
+      aOut << "TOUCHING";
+      break;
+    case AsyncPanZoomController::PanZoomState::PANNING:
+      aOut << "PANNING";
+      break;
+    case AsyncPanZoomController::PanZoomState::PANNING_LOCKED_X:
+      aOut << "PANNING_LOCKED_X";
+      break;
+    case AsyncPanZoomController::PanZoomState::PANNING_LOCKED_Y:
+      aOut << "PANNING_LOCKED_Y";
+      break;
+    case AsyncPanZoomController::PanZoomState::PAN_MOMENTUM:
+      aOut << "PAN_MOMENTUM";
+      break;
+    case AsyncPanZoomController::PanZoomState::PINCHING:
+      aOut << "PINCHING";
+      break;
+    case AsyncPanZoomController::PanZoomState::ANIMATING_ZOOM:
+      aOut << "ANIMATING_ZOOM";
+      break;
+    case AsyncPanZoomController::PanZoomState::OVERSCROLL_ANIMATION:
+      aOut << "OVERSCROLL_ANIMATION";
+      break;
+    case AsyncPanZoomController::PanZoomState::SMOOTH_SCROLL:
+      aOut << "SMOOTH_SCROLL";
+      break;
+    case AsyncPanZoomController::PanZoomState::SMOOTHMSD_SCROLL:
+      aOut << "SMOOTHMSD_SCROLL";
+      break;
+    case AsyncPanZoomController::PanZoomState::WHEEL_SCROLL:
+      aOut << "WHEEL_SCROLL";
+      break;
+    case AsyncPanZoomController::PanZoomState::KEYBOARD_SCROLL:
+      aOut << "KEYBOARD_SCROLL";
+      break;
+    case AsyncPanZoomController::PanZoomState::AUTOSCROLL:
+      aOut << "AUTOSCROLL";
+      break;
+    case AsyncPanZoomController::PanZoomState::SCROLLBAR_DRAG:
+      aOut << "SCROLLBAR_DRAG";
+      break;
+    default:
+      aOut << "UNKNOWN_STATE";
+      break;
+  }
+  return aOut;
+}
+
+bool operator==(const PointerEventsConsumableFlags& aLhs,
+                const PointerEventsConsumableFlags& aRhs) {
+  return (aLhs.mHasRoom == aRhs.mHasRoom) &&
+         (aLhs.mAllowedByTouchAction == aRhs.mAllowedByTouchAction);
+}
+
+std::ostream& operator<<(std::ostream& aOut,
+                         const PointerEventsConsumableFlags& aFlags) {
+  aOut << std::boolalpha << "{ hasRoom: " << aFlags.mHasRoom
+       << ", allowedByTouchAction: " << aFlags.mAllowedByTouchAction << "}";
+  return aOut;
+}
+
+}  // namespace layers
+}  // namespace mozilla