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

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

Index: third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
index ba2878b3f7468b7f66b5582495755e38850c1913..bf90d39f4dcad2efc4c4314620129e3e230d6556 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -612,10 +612,12 @@ void PaintLayerScrollableArea::scrollToPosition(const DoublePoint& scrollPositio
         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);
@@ -683,18 +685,23 @@ void PaintLayerScrollableArea::updateAfterLayout()
         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;
             }
         }
@@ -727,6 +734,8 @@ void PaintLayerScrollableArea::updateAfterLayout()
 
     DisableCompositingQueryAsserts disabler;
     positionOverflowControls();
+
+    return didMarkForLayout;
 }
 
 ScrollBehavior PaintLayerScrollableArea::scrollBehaviorStyle() const