Restore PaintLayerScrollableArea::ScrollbarManager::canDetach behavior.

third_party/WebKit/Source/core/layout/LayoutFlexibleBox.cpp

 /*
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 761 matching lines @@
             true :
             child.computePercentageLogicalHeight(length) != -1;
     }
     // TODO(cbiesinger): Eventually we should support other types of sizes here. Requires updating
     // computeMainSizeFromAspectRatioUsing.
     return length.isFixed();
 }
 
 bool LayoutFlexibleBox::childFlexBaseSizeRequiresLayout(const LayoutBox& child) const
 {
-    return !mainAxisLengthIsDefinite(child, flexBasisForChild(child)) && (
-        hasOrthogonalFlow(child) || crossAxisOverflowForChild(child) == OverflowAuto);
+    return (!mainAxisLengthIsDefinite(child, flexBasisForChild(child))
+        && (hasOrthogonalFlow(child)
+            || (crossAxisOverflowForChild(child) == OverflowAuto
+                && !PaintLayerScrollableArea::FreezeScrollbarsScope::scrollbarsAreFrozen())));

[cbiesinger, 2016/06/06 23:23:42]

We might want to split this up into separate statements; the nested boolean
operators are kinda hard to read at this point...

I'm also a little concerned about the side effect here:
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/layout/La...

We now take the other branch in that if. I *think* that's ok, so lgtm.

 }
 
 void LayoutFlexibleBox::cacheChildMainSize(const LayoutBox& child)
 {
     ASSERT(!child.needsLayout());
     LayoutUnit mainSize;
     if (hasOrthogonalFlow(child)) {
         mainSize = child.logicalHeight();
     } else {
         // The max preferred logical width includes the intrinsic scrollbar logical width, which is only set for
@@ skipping 1067 matching lines @@
             ASSERT(child);
             LayoutUnit lineCrossAxisExtent = lineContexts[lineNumber].crossAxisExtent;
             LayoutUnit originalOffset = lineContexts[lineNumber].crossAxisOffset - crossAxisStartEdge;
             LayoutUnit newOffset = contentExtent - originalOffset - lineCrossAxisExtent;
             adjustAlignmentForChild(*child, newOffset - originalOffset);
         }
     }
 }
 
 } // namespace blink