Don't clamp scroll positions until the end of all layouts

third_party/WebKit/Source/core/layout/LayoutBlock.h

Index: third_party/WebKit/Source/core/layout/LayoutBlock.h
diff --git a/third_party/WebKit/Source/core/layout/LayoutBlock.h b/third_party/WebKit/Source/core/layout/LayoutBlock.h
index c621c78145195ca30c53d77ea6996935f1e84276..0a150fea6327184447f5c5450fff0c478887e4eb 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBlock.h
+++ b/third_party/WebKit/Source/core/layout/LayoutBlock.h
@@ -341,8 +341,17 @@ protected:
     // finishDelayUpdateScrollInfo() will do nothing until finishDelayUpdateScrollInfo()
     // is called the same number of times.
     // finishDelayUpdateScrollInfo returns true when it marked something for layout.
+    // It will also return a map of saved scroll positions that the caller should restore
+    // on the given scrollable areas after performing the layout.
+    // This can be necessary because Flexbox's multi-pass layout can lose the scroll position.
+    // TODO(cbiesinger): This is a temporary hack. The right solution is to delay the scroll
+    // clamping that currently happens in PaintLayerScrollableArea::updateAfterLayout to only
+    // happen after all layout is done, i.e. during updateLayerPositionsAfterLayout. However,
+    // that currently fails a layout test. To fix this bug in time for M50, we use this temporary
+    // hack. The real fix is tracked in crbug.com/600036
+    typedef WTF::HashMap<PaintLayerScrollableArea*, DoublePoint> ScrollPositionMap;
     static void startDelayUpdateScrollInfo();
-    static bool finishDelayUpdateScrollInfo(SubtreeLayoutScope*);
+    static bool finishDelayUpdateScrollInfo(SubtreeLayoutScope*, ScrollPositionMap*);
 
     void updateScrollInfoAfterLayout();