1 files changed, 415 insertions, 0 deletions
diff --git a/layout/generic/StickyScrollContainer.cpp b/layout/generic/StickyScrollContainer.cpp
new file mode 100644
index 0000000000..2e9d32ab5f
--- /dev/null
+++ b/layout/generic/StickyScrollContainer.cpp
@@ -0,0 +1,415 @@
+/* -*- 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/. */
+
+/**
+ * compute sticky positioning, both during reflow and when the scrolling
+ * container scrolls
+ */
+
+#include "StickyScrollContainer.h"
+
+#include "mozilla/OverflowChangedTracker.h"
+#include "nsIFrame.h"
+#include "nsIFrameInlines.h"
+#include "nsIScrollableFrame.h"
+#include "nsLayoutUtils.h"
+
+namespace mozilla {
+
+NS_DECLARE_FRAME_PROPERTY_DELETABLE(StickyScrollContainerProperty,
+                                    StickyScrollContainer)
+
+StickyScrollContainer::StickyScrollContainer(nsIScrollableFrame* aScrollFrame)
+    : mScrollFrame(aScrollFrame) {
+  mScrollFrame->AddScrollPositionListener(this);
+}
+
+StickyScrollContainer::~StickyScrollContainer() {
+  mScrollFrame->RemoveScrollPositionListener(this);
+}
+
+// static
+StickyScrollContainer* StickyScrollContainer::GetStickyScrollContainerForFrame(
+    nsIFrame* aFrame) {
+  nsIScrollableFrame* scrollFrame = nsLayoutUtils::GetNearestScrollableFrame(
+      aFrame->GetParent(), nsLayoutUtils::SCROLLABLE_SAME_DOC |
+                               nsLayoutUtils::SCROLLABLE_STOP_AT_PAGE |
+                               nsLayoutUtils::SCROLLABLE_INCLUDE_HIDDEN);
+  if (!scrollFrame) {
+    // We might not find any, for instance in the case of
+    // <html style="position: fixed">
+    return nullptr;
+  }
+  nsIFrame* frame = do_QueryFrame(scrollFrame);
+  StickyScrollContainer* s =
+      frame->GetProperty(StickyScrollContainerProperty());
+  if (!s) {
+    s = new StickyScrollContainer(scrollFrame);
+    frame->SetProperty(StickyScrollContainerProperty(), s);
+  }
+  return s;
+}
+
+// static
+void StickyScrollContainer::NotifyReparentedFrameAcrossScrollFrameBoundary(
+    nsIFrame* aFrame, nsIFrame* aOldParent) {
+  nsIScrollableFrame* oldScrollFrame = nsLayoutUtils::GetNearestScrollableFrame(
+      aOldParent, nsLayoutUtils::SCROLLABLE_SAME_DOC |
+                      nsLayoutUtils::SCROLLABLE_INCLUDE_HIDDEN);
+  if (!oldScrollFrame) {
+    // XXX maybe aFrame has sticky descendants that can be sticky now, but
+    // we aren't going to handle that.
+    return;
+  }
+
+  StickyScrollContainer* oldSSC =
+      static_cast<nsIFrame*>(do_QueryFrame(oldScrollFrame))
+          ->GetProperty(StickyScrollContainerProperty());
+  if (!oldSSC) {
+    // aOldParent had no sticky descendants, so aFrame doesn't have any sticky
+    // descendants, and we're done here.
+    return;
+  }
+
+  auto i = oldSSC->mFrames.Length();
+  while (i-- > 0) {
+    nsIFrame* f = oldSSC->mFrames[i];
+    StickyScrollContainer* newSSC = GetStickyScrollContainerForFrame(f);
+    if (newSSC != oldSSC) {
+      oldSSC->RemoveFrame(f);
+      if (newSSC) {
+        newSSC->AddFrame(f);
+      }
+    }
+  }
+}
+
+// static
+StickyScrollContainer*
+StickyScrollContainer::GetStickyScrollContainerForScrollFrame(
+    nsIFrame* aFrame) {
+  return aFrame->GetProperty(StickyScrollContainerProperty());
+}
+
+static nscoord ComputeStickySideOffset(
+    Side aSide, const StyleRect<LengthPercentageOrAuto>& aOffset,
+    nscoord aPercentBasis) {
+  auto& side = aOffset.Get(aSide);
+  if (side.IsAuto()) {
+    return NS_AUTOOFFSET;
+  }
+  return nsLayoutUtils::ComputeCBDependentValue(aPercentBasis, side);
+}
+
+// static
+void StickyScrollContainer::ComputeStickyOffsets(nsIFrame* aFrame) {
+  nsIScrollableFrame* scrollableFrame =
+      nsLayoutUtils::GetNearestScrollableFrame(
+          aFrame->GetParent(), nsLayoutUtils::SCROLLABLE_SAME_DOC |
+                                   nsLayoutUtils::SCROLLABLE_INCLUDE_HIDDEN);
+
+  if (!scrollableFrame) {
+    // Bail.
+    return;
+  }
+
+  nsSize scrollContainerSize = scrollableFrame->GetScrolledFrame()
+                                   ->GetContentRectRelativeToSelf()
+                                   .Size();
+
+  nsMargin computedOffsets;
+  const nsStylePosition* position = aFrame->StylePosition();
+
+  computedOffsets.left = ComputeStickySideOffset(eSideLeft, position->mOffset,
+                                                 scrollContainerSize.width);
+  computedOffsets.right = ComputeStickySideOffset(eSideRight, position->mOffset,
+                                                  scrollContainerSize.width);
+  computedOffsets.top = ComputeStickySideOffset(eSideTop, position->mOffset,
+                                                scrollContainerSize.height);
+  computedOffsets.bottom = ComputeStickySideOffset(
+      eSideBottom, position->mOffset, scrollContainerSize.height);
+
+  // Store the offset
+  nsMargin* offsets = aFrame->GetProperty(nsIFrame::ComputedOffsetProperty());
+  if (offsets) {
+    *offsets = computedOffsets;
+  } else {
+    aFrame->SetProperty(nsIFrame::ComputedOffsetProperty(),
+                        new nsMargin(computedOffsets));
+  }
+}
+
+static constexpr nscoord gUnboundedNegative = nscoord_MIN / 2;
+static constexpr nscoord gUnboundedExtent = nscoord_MAX;
+static constexpr nscoord gUnboundedPositive =
+    gUnboundedNegative + gUnboundedExtent;
+
+void StickyScrollContainer::ComputeStickyLimits(nsIFrame* aFrame,
+                                                nsRect* aStick,
+                                                nsRect* aContain) const {
+  NS_ASSERTION(nsLayoutUtils::IsFirstContinuationOrIBSplitSibling(aFrame),
+               "Can't sticky position individual continuations");
+
+  aStick->SetRect(gUnboundedNegative, gUnboundedNegative, gUnboundedExtent,
+                  gUnboundedExtent);
+  aContain->SetRect(gUnboundedNegative, gUnboundedNegative, gUnboundedExtent,
+                    gUnboundedExtent);
+
+  const nsMargin* computedOffsets =
+      aFrame->GetProperty(nsIFrame::ComputedOffsetProperty());
+  if (!computedOffsets) {
+    // We haven't reflowed the scroll frame yet, so offsets haven't been
+    // computed. Bail.
+    return;
+  }
+
+  nsIFrame* scrolledFrame = mScrollFrame->GetScrolledFrame();
+  nsIFrame* cbFrame = aFrame->GetContainingBlock();
+  NS_ASSERTION(cbFrame == scrolledFrame ||
+                   nsLayoutUtils::IsProperAncestorFrame(scrolledFrame, cbFrame),
+               "Scroll frame should be an ancestor of the containing block");
+
+  nsRect rect =
+      nsLayoutUtils::GetAllInFlowRectsUnion(aFrame, aFrame->GetParent());
+
+  // FIXME(bug 1421660): Table row groups aren't supposed to be containing
+  // blocks, but we treat them as such (maybe it's the right thing to do!).
+  // Anyway, not having this basically disables position: sticky on table cells,
+  // which would be really unfortunate, and doesn't match what other browsers
+  // do.
+  if (cbFrame != scrolledFrame && cbFrame->IsTableRowGroupFrame()) {
+    cbFrame = cbFrame->GetContainingBlock();
+  }
+
+  // Containing block limits for the position of aFrame relative to its parent.
+  // The margin box of the sticky element stays within the content box of the
+  // contaning-block element.
+  if (cbFrame == scrolledFrame) {
+    // cbFrame is the scrolledFrame, and it won't have continuations. Unlike the
+    // else clause, we consider scrollable overflow rect because and the union
+    // of its in-flow rects doesn't include the scrollable overflow area.
+    *aContain = cbFrame->ScrollableOverflowRectRelativeToSelf();
+    nsLayoutUtils::TransformRect(cbFrame, aFrame->GetParent(), *aContain);
+  } else {
+    *aContain = nsLayoutUtils::GetAllInFlowRectsUnion(
+        cbFrame, aFrame->GetParent(), nsLayoutUtils::RECTS_USE_CONTENT_BOX);
+  }
+
+  nsRect marginRect = nsLayoutUtils::GetAllInFlowRectsUnion(
+      aFrame, aFrame->GetParent(), nsLayoutUtils::RECTS_USE_MARGIN_BOX);
+
+  // Deflate aContain by the difference between the union of aFrame's
+  // continuations' margin boxes and the union of their border boxes, so that
+  // by keeping aFrame within aContain, we keep the union of the margin boxes
+  // within the containing block's content box.
+  aContain->Deflate(marginRect - rect);
+
+  // Deflate aContain by the border-box size, to form a constraint on the
+  // upper-left corner of aFrame and continuations.
+  aContain->Deflate(nsMargin(0, rect.width, rect.height, 0));
+
+  nsMargin sfPadding = scrolledFrame->GetUsedPadding();
+  nsPoint sfOffset = aFrame->GetParent()->GetOffsetTo(scrolledFrame);
+
+  // Top
+  if (computedOffsets->top != NS_AUTOOFFSET) {
+    aStick->SetTopEdge(mScrollPosition.y + sfPadding.top +
+                       computedOffsets->top - sfOffset.y);
+  }
+
+  nsSize sfSize = scrolledFrame->GetContentRectRelativeToSelf().Size();
+
+  // Bottom
+  if (computedOffsets->bottom != NS_AUTOOFFSET &&
+      (computedOffsets->top == NS_AUTOOFFSET ||
+       rect.height <= sfSize.height - computedOffsets->TopBottom())) {
+    aStick->SetBottomEdge(mScrollPosition.y + sfPadding.top + sfSize.height -
+                          computedOffsets->bottom - rect.height - sfOffset.y);
+  }
+
+  StyleDirection direction = cbFrame->StyleVisibility()->mDirection;
+
+  // Left
+  if (computedOffsets->left != NS_AUTOOFFSET &&
+      (computedOffsets->right == NS_AUTOOFFSET ||
+       direction == StyleDirection::Ltr ||
+       rect.width <= sfSize.width - computedOffsets->LeftRight())) {
+    aStick->SetLeftEdge(mScrollPosition.x + sfPadding.left +
+                        computedOffsets->left - sfOffset.x);
+  }
+
+  // Right
+  if (computedOffsets->right != NS_AUTOOFFSET &&
+      (computedOffsets->left == NS_AUTOOFFSET ||
+       direction == StyleDirection::Rtl ||
+       rect.width <= sfSize.width - computedOffsets->LeftRight())) {
+    aStick->SetRightEdge(mScrollPosition.x + sfPadding.left + sfSize.width -
+                         computedOffsets->right - rect.width - sfOffset.x);
+  }
+
+  // These limits are for the bounding box of aFrame's continuations. Convert
+  // to limits for aFrame itself.
+  nsPoint frameOffset = aFrame->GetPosition() - rect.TopLeft();
+  aStick->MoveBy(frameOffset);
+  aContain->MoveBy(frameOffset);
+}
+
+nsPoint StickyScrollContainer::ComputePosition(nsIFrame* aFrame) const {
+  nsRect stick;
+  nsRect contain;
+  ComputeStickyLimits(aFrame, &stick, &contain);
+
+  nsPoint position = aFrame->GetNormalPosition();
+
+  // For each sticky direction (top, bottom, left, right), move the frame along
+  // the appropriate axis, based on the scroll position, but limit this to keep
+  // the element's margin box within the containing block.
+  position.y = std::max(position.y, std::min(stick.y, contain.YMost()));
+  position.y = std::min(position.y, std::max(stick.YMost(), contain.y));
+  position.x = std::max(position.x, std::min(stick.x, contain.XMost()));
+  position.x = std::min(position.x, std::max(stick.XMost(), contain.x));
+
+  return position;
+}
+
+bool StickyScrollContainer::IsStuckInYDirection(nsIFrame* aFrame) const {
+  nsPoint position = ComputePosition(aFrame);
+  return position.y != aFrame->GetNormalPosition().y;
+}
+
+void StickyScrollContainer::GetScrollRanges(nsIFrame* aFrame,
+                                            nsRectAbsolute* aOuter,
+                                            nsRectAbsolute* aInner) const {
+  // We need to use the first in flow; continuation frames should not move
+  // relative to each other and should get identical scroll ranges.
+  // Also, ComputeStickyLimits requires this.
+  nsIFrame* firstCont =
+      nsLayoutUtils::FirstContinuationOrIBSplitSibling(aFrame);
+
+  nsRect stickRect;
+  nsRect containRect;
+  ComputeStickyLimits(firstCont, &stickRect, &containRect);
+
+  nsRectAbsolute stick = nsRectAbsolute::FromRect(stickRect);
+  nsRectAbsolute contain = nsRectAbsolute::FromRect(containRect);
+
+  aOuter->SetBox(gUnboundedNegative, gUnboundedNegative, gUnboundedPositive,
+                 gUnboundedPositive);
+  aInner->SetBox(gUnboundedNegative, gUnboundedNegative, gUnboundedPositive,
+                 gUnboundedPositive);
+
+  const nsPoint normalPosition = firstCont->GetNormalPosition();
+
+  // Bottom and top
+  if (stick.YMost() != gUnboundedPositive) {
+    aOuter->SetTopEdge(contain.Y() - stick.YMost());
+    aInner->SetTopEdge(normalPosition.y - stick.YMost());
+  }
+
+  if (stick.Y() != gUnboundedNegative) {
+    aInner->SetBottomEdge(normalPosition.y - stick.Y());
+    aOuter->SetBottomEdge(contain.YMost() - stick.Y());
+  }
+
+  // Right and left
+  if (stick.XMost() != gUnboundedPositive) {
+    aOuter->SetLeftEdge(contain.X() - stick.XMost());
+    aInner->SetLeftEdge(normalPosition.x - stick.XMost());
+  }
+
+  if (stick.X() != gUnboundedNegative) {
+    aInner->SetRightEdge(normalPosition.x - stick.X());
+    aOuter->SetRightEdge(contain.XMost() - stick.X());
+  }
+
+  // Make sure |inner| does not extend outside of |outer|. (The consumers of
+  // the Layers API, to which this information is propagated, expect this
+  // invariant to hold.) The calculated value of |inner| can sometimes extend
+  // outside of |outer|, for example due to margin collapsing, since
+  // GetNormalPosition() returns the actual position after margin collapsing,
+  // while |contain| is calculated based on the frame's GetUsedMargin() which
+  // is pre-collapsing.
+  // Note that this doesn't necessarily solve all problems stemming from
+  // comparing pre- and post-collapsing margins (TODO: find a proper solution).
+  *aInner = aInner->Intersect(*aOuter);
+  if (aInner->IsEmpty()) {
+    // This might happen if aInner didn't intersect aOuter at all initially,
+    // in which case aInner is empty and outside aOuter. Make sure it doesn't
+    // extend outside aOuter.
+    *aInner = aInner->MoveInsideAndClamp(*aOuter);
+  }
+}
+
+void StickyScrollContainer::PositionContinuations(nsIFrame* aFrame) {
+  NS_ASSERTION(nsLayoutUtils::IsFirstContinuationOrIBSplitSibling(aFrame),
+               "Should be starting from the first continuation");
+  bool hadProperty;
+  nsPoint translation =
+      ComputePosition(aFrame) - aFrame->GetNormalPosition(&hadProperty);
+  if (NS_WARN_IF(!hadProperty)) {
+    // If the frame was never relatively positioned, don't move its position
+    // dynamically. There are a variety of frames for which `position` doesn't
+    // really apply like frames inside svg which would get here and be sticky
+    // only in one direction.
+    return;
+  }
+
+  // Move all continuation frames by the same amount.
+  for (nsIFrame* cont = aFrame; cont;
+       cont = nsLayoutUtils::GetNextContinuationOrIBSplitSibling(cont)) {
+    cont->SetPosition(cont->GetNormalPosition() + translation);
+  }
+}
+
+void StickyScrollContainer::UpdatePositions(nsPoint aScrollPosition,
+                                            nsIFrame* aSubtreeRoot) {
+#ifdef DEBUG
+  {
+    nsIFrame* scrollFrameAsFrame = do_QueryFrame(mScrollFrame);
+    NS_ASSERTION(!aSubtreeRoot || aSubtreeRoot == scrollFrameAsFrame,
+                 "If reflowing, should be reflowing the scroll frame");
+  }
+#endif
+  mScrollPosition = aScrollPosition;
+
+  OverflowChangedTracker oct;
+  oct.SetSubtreeRoot(aSubtreeRoot);
+  for (nsTArray<nsIFrame*>::size_type i = 0; i < mFrames.Length(); i++) {
+    nsIFrame* f = mFrames[i];
+    if (!nsLayoutUtils::IsFirstContinuationOrIBSplitSibling(f)) {
+      // This frame was added in nsIFrame::Init before we knew it wasn't
+      // the first ib-split-sibling.
+      mFrames.RemoveElementAt(i);
+      --i;
+      continue;
+    }
+
+    if (aSubtreeRoot) {
+      // Reflowing the scroll frame, so recompute offsets.
+      ComputeStickyOffsets(f);
+    }
+    // mFrames will only contain first continuations, because we filter in
+    // nsIFrame::Init.
+    PositionContinuations(f);
+
+    f = f->GetParent();
+    if (f != aSubtreeRoot) {
+      for (nsIFrame* cont = f; cont;
+           cont = nsLayoutUtils::GetNextContinuationOrIBSplitSibling(cont)) {
+        oct.AddFrame(cont, OverflowChangedTracker::CHILDREN_CHANGED);
+      }
+    }
+  }
+  oct.Flush();
+}
+
+void StickyScrollContainer::ScrollPositionWillChange(nscoord aX, nscoord aY) {}
+
+void StickyScrollContainer::ScrollPositionDidChange(nscoord aX, nscoord aY) {
+  UpdatePositions(nsPoint(aX, aY), nullptr);
+}
+
+}  // namespace mozilla