Fully (?) fix overflow: auto with delayed scroll updates

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@gmail.com>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 594 matching lines @@
 
 void PaintLayerScrollableArea::scrollToPosition(const DoublePoint& scrollPosition, ScrollOffsetClamping clamp, ScrollBehavior scrollBehavior, ScrollType scrollType)
 {
     cancelProgrammaticScrollAnimation();
 
     DoublePoint newScrollPosition = clamp == ScrollOffsetClamped ? clampScrollPosition(scrollPosition) : scrollPosition;
     if (newScrollPosition != scrollPositionDouble())
         ScrollableArea::setScrollPosition(newScrollPosition, scrollType, scrollBehavior);
 }
 
-void PaintLayerScrollableArea::updateAfterLayout()
+bool PaintLayerScrollableArea::updateAfterLayout(SubtreeLayoutScope* delayedLayoutScope)
 {
     ASSERT(box().hasOverflowClip());
 
+    bool didMarkForLayout = false;
+
     if (needsScrollbarReconstruction()) {
         m_scrollbarManager.setCanDetachScrollbars(false);
         setHasHorizontalScrollbar(false);
         setHasVerticalScrollbar(false);
     }
 
     m_scrollbarManager.setCanDetachScrollbars(true);
 
     DoubleSize originalScrollOffset = adjustedScrollOffset();
     computeScrollDimensions();
@@ skipping 47 matching lines @@
         layer()->updateSelfPaintingLayer();
 
         // Force an update since we know the scrollbars have changed things.
         if (box().document().hasAnnotatedRegions())
             box().document().setAnnotatedRegionsDirty(true);
 
         // Our proprietary overflow: overlay value doesn't trigger a layout.
         if ((horizontalScrollBarChanged && box().style()->overflowX() != OOVERLAY) || (verticalScrollBarChanged && box().style()->overflowY() != OOVERLAY)) {
             if (!m_inOverflowRelayout) {
                 m_inOverflowRelayout = true;
-                SubtreeLayoutScope layoutScope(box());
-                layoutScope.setNeedsLayout(&box(), LayoutInvalidationReason::ScrollbarChanged);
-                if (box().isLayoutBlock()) {
-                    LayoutBlock& block = toLayoutBlock(box());
-                    block.scrollbarsChanged(horizontalScrollBarChanged, verticalScrollBarChanged);
-                    block.layoutBlock(true);
+                if (delayedLayoutScope) {
+                    delayedLayoutScope->setNeedsLayout(&box(), LayoutInvalidationReason::ScrollbarChanged);
                 } else {
-                    box().layout();
+                    SubtreeLayoutScope layoutScope(box());
+                    layoutScope.setNeedsLayout(&box(), LayoutInvalidationReason::ScrollbarChanged);
+                    if (box().isLayoutBlock()) {
+                        LayoutBlock& block = toLayoutBlock(box());
+                        block.scrollbarsChanged(horizontalScrollBarChanged, verticalScrollBarChanged);
+                        block.layoutBlock(true);
+                    } else {
+                        box().layout();
+                    }
                 }
                 LayoutObject* parent = box().parent();
                 if (parent && parent->isFlexibleBox())
                     toLayoutFlexibleBox(parent)->clearCachedMainSizeForChild(box());
+                didMarkForLayout = true;

[szager1, 2016/02/25 21:08:08]

Should this go inside the "if (delayedLayoutScope)" conditional?  If the second
layout happens immediately, then it's not necessary to re-flex later.

I don't think that could ever happen with this version of the code (i.e., if
flexing is happening, then delayedLayoutScope will always be non-null), but it
seems like something that could get broken in the future.

                 m_inOverflowRelayout = false;
             }
         }
     }
 
     {
         // Hits in compositing/overflow/automatically-opt-into-composited-scrolling-after-style-change.html.
         DisableCompositingQueryAsserts disabler;
 
         // Set up the range (and page step/line step).
@@ skipping 12 matching lines @@
             setHasHorizontalScrollbar(false);
         if (!scrollSize(VerticalScrollbar))
             setHasVerticalScrollbar(false);
     }
 
     bool hasOverflow = hasScrollableHorizontalOverflow() || hasScrollableVerticalOverflow();
     updateScrollableAreaSet(hasOverflow);
 
     DisableCompositingQueryAsserts disabler;
     positionOverflowControls();
+
+    return didMarkForLayout;
 }
 
 ScrollBehavior PaintLayerScrollableArea::scrollBehaviorStyle() const
 {
     return box().style()->scrollBehavior();
 }
 
 bool PaintLayerScrollableArea::hasHorizontalOverflow() const
 {
     return pixelSnappedScrollWidth() > box().pixelSnappedClientWidth();
@@ skipping 809 matching lines @@
 }
 
 DEFINE_TRACE(PaintLayerScrollableArea::ScrollbarManager)
 {
     visitor->trace(m_scrollableArea);
     visitor->trace(m_hBar);
     visitor->trace(m_vBar);
 }
 
 } // namespace blink