Implement overlay scrollbar fade out for non-composited scrollers.

third_party/WebKit/Source/core/paint/PaintLayer.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@web.de>
@@ skipping 787 matching lines @@
 }
 
 void PaintLayer::updateLayerPosition() {
   LayoutPoint localPoint;
 
   if (layoutObject()->isInline() && layoutObject()->isLayoutInline()) {
     LayoutInline* inlineFlow = toLayoutInline(layoutObject());
     IntRect lineBox = enclosingIntRect(inlineFlow->linesBoundingBox());
     m_size = lineBox.size();
   } else if (LayoutBox* box = layoutBox()) {
-    m_size = pixelSnappedIntSize(box->size(), box->location());
+    IntSize newSize = pixelSnappedIntSize(box->size(), box->location());
+    if (newSize != m_size && m_scrollableArea)
+      m_scrollableArea->visibleContentResized();

[dtapuska, 2016/11/02 16:53:21]

I fear you have a re-entry problem here. You are notifying an observer in the
middle of what should be an atomic operation.

Can we call visibleContentResized() after we've assigned m_size ? Or perhaps
even better at the bottom of this method?

[bokan, 2016/11/02 22:41:57]

Good catch, I'll add it to the bottom.

+    m_size = newSize;
     localPoint.moveBy(box->topLeftLocation());
   }
 
   if (!layoutObject()->isOutOfFlowPositioned() &&
       !layoutObject()->isColumnSpanAll() && layoutObject()->parent()) {
     // We must adjust our position by walking up the layout tree looking for the
     // nearest enclosing object with a layer.
     LayoutObject* curr = layoutObject()->parent();
     while (curr && !curr->hasLayer()) {
       if (curr->isBox() && !curr->isTableRow()) {
@@ skipping 2447 matching lines @@
 }
 
 void showLayerTree(const blink::LayoutObject* layoutObject) {
   if (!layoutObject) {
     fprintf(stderr, "Cannot showLayerTree. Root is (nil)\n");
     return;
   }
   showLayerTree(layoutObject->enclosingLayer());
 }
 #endif