Explicitly invalidate LayoutView when overflow is recalculated.

third_party/WebKit/Source/core/frame/FrameView.cpp

 /*
  * Copyright (C) 1998, 1999 Torben Weis <weis@kde.org>
  *                     1999 Lars Knoll <knoll@kde.org>
  *                     1999 Antti Koivisto <koivisto@kde.org>
  *                     2000 Dirk Mueller <mueller@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *           (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
@@ skipping 797 matching lines @@
     scrollingCoordinator->notifyOverflowUpdated();
 
   IntRect documentRect = layoutViewItem.documentRect();
   if (scrollOrigin() == -documentRect.location() &&
       contentsSize() == documentRect.size())
     return;
 
   if (needsLayout())
     return;
 
+  layoutViewItem.setMayNeedPaintInvalidation();

[pdr., 2017/02/10 22:25:33]

Yeah I think this patch looks like the bug. We should only need to do this when
the visual viewport supplies the scrollbars though, so WDYT of something like:
if (visualViewportSuppliesScrollbars()) {
  layoutViewItem.setMayNeedPaintInvalidation();
} else {
  // do the regular computeScrollbarExistence code here.
}

If you can launch a debug build with "--enable-viewport
--enable-blink-features=PaintUnderInvalidationChecking" and run the demo, you
can confirm this is under-invalidation (missing call to
setMayNeedPaintInvalidation). This is significantly less useful _after_ you've
spent a day locating the bug but maybe it can be of use in the future :)

Idea for testing: you can add a test under paint/invalidation and ensure the
right invalidation rect is output.

[bokan, 2017/02/21 22:14:14]

Yah, I agree we can avoid the scrollbar updates below if
visualViewportSuppliesScrollbars. However, given that this needs to get merged
to M57, I'm not keen to change any more behavior than necessary.  How about I
land a version like this:

if (visualViewportSuppliesScrollbars())
  layoutViewItem.setMayNeedPaintInvalidation()

And put up a followup patch that conditionally does the scrollbar stuff below?
 
Neat, thanks for the tip!
 
Thanks, will do.

+
   // TODO(pdr): This should be refactored to just block scrollbar updates as
   // we are not in a scrollbar update here and m_inUpdateScrollbars has other
   // side effects. This scope is only for preventing a synchronous layout from
   // scroll origin changes which would not be allowed during style recalc.
   InUpdateScrollbarsScope inUpdateScrollbarsScope(this);
 
   bool shouldHaveHorizontalScrollbar = false;
   bool shouldHaveVerticalScrollbar = false;
   computeScrollbarExistence(shouldHaveHorizontalScrollbar,
                             shouldHaveVerticalScrollbar, documentRect.size());
@@ skipping 4271 matching lines @@
     std::unique_ptr<CompositorAnimationTimeline> timeline) {
   m_animationTimeline = std::move(timeline);
 }
 
 void FrameView::setAnimationHost(
     std::unique_ptr<CompositorAnimationHost> host) {
   m_animationHost = std::move(host);
 }
 
 }  // namespace blink