Disable overlay scrollbars in Blink when hidden by the compositor.

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 d24d79670ec318bbe425e2f953f40ce35ecc177f..edcba00f7e7e0af53a2d28f112856e8e16eb141b 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -652,6 +652,22 @@ void PaintLayerScrollableArea::setScrollOffsetUnconditionally(
   scrollOffsetChanged(offset, scrollType);
 }
 
+void PaintLayerScrollableArea::didChangeScrollbarsHidden() {
+  updateScrollbarsEnabledState();
+}
+
+void PaintLayerScrollableArea::updateScrollbarsEnabledState() {
+  // overflow:scroll should just enable/disable.
+  if (box().style()->overflowX() == OverflowScroll && horizontalScrollbar()) {
+    horizontalScrollbar()->setEnabled(hasHorizontalOverflow() &&
+                                      !scrollbarsHidden());
+  }
+  if (box().style()->overflowY() == OverflowScroll && verticalScrollbar()) {
+    verticalScrollbar()->setEnabled(hasVerticalOverflow() &&
+                                    !scrollbarsHidden());
+  }
+}
+
 void PaintLayerScrollableArea::updateAfterLayout() {
   ASSERT(box().hasOverflowClip());
 
@@ -738,11 +754,7 @@ void PaintLayerScrollableArea::updateAfterLayout() {
     // compositing/overflow/automatically-opt-into-composited-scrolling-after-style-change.html.
     DisableCompositingQueryAsserts disabler;
 
-    // overflow:scroll should just enable/disable.
-    if (box().style()->overflowX() == OverflowScroll && horizontalScrollbar())
-      horizontalScrollbar()->setEnabled(hasHorizontalOverflow());
-    if (box().style()->overflowY() == OverflowScroll && verticalScrollbar())
-      verticalScrollbar()->setEnabled(hasVerticalOverflow());
+    updateScrollbarsEnabledState();
 
     // Set up the range (and page step/line step).
     if (Scrollbar* horizontalScrollbar = this->horizontalScrollbar()) {