Use ScrollAnchor in FrameView and PaintLayerScrollableArea.

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

 /*
  * Copyright (C) 2013 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 269 matching lines @@
     // If we are self-collapsing with self-collapsing descendants this will get set to save us burrowing through our
     // descendants every time in |isSelfCollapsingBlock|. We reset it here so that |isSelfCollapsingBlock| attempts to burrow
     // at least once and so that it always gives a reliable result reflecting the latest layout.
     m_hasOnlySelfCollapsingChildren = false;
 
     if (!relayoutChildren && simplifiedLayout())
         return;
 
     LayoutAnalyzer::BlockScope analyzer(*this);
     SubtreeLayoutScope layoutScope(*this);
+    if (scrollAnchoringEnabled() && hasOverflowClip())
+        scrollableArea()->scrollAnchor().save();

[ojan, 2016/02/02 06:51:01]

Can you instead do this in LayoutBlock::layout()? That way it will work for all
of the other LayoutBlock subclasses as well.

[skobes, 2016/02/02 22:39:10]

I thought about this but was scared off by the comment in LayoutBlock::layout:

    // Table cells call layoutBlock directly, so don't add any logic here.  Put
code into
    // layoutBlock().

Do you think it would be better to instrument LayoutBlock::layout and
LayoutTableCell::layout separately?

[ojan, 2016/02/03 06:17:52]

Yes, that way the code will work for flex/grid/etc as well. At some point we
should make test cases for all the different display types, but I don't think
that needs to block this patch landing. Are you keeping track somewhere for
test-cases we should write? Each case we find that didn't do the right thing, we
should make a test case for so that at the end we have a nice robust test suite
to use to build a spec and to share with other browser vendors.

 
     // Multiple passes might be required for column based layout.
     // The number of passes could be as high as the number of columns.
     bool done = false;
     LayoutUnit pageLogicalHeight = 0;
     while (!done)
         done = layoutBlockFlow(relayoutChildren, pageLogicalHeight, layoutScope);
 
     LayoutView* layoutView = view();
     if (layoutView->layoutState()->pageLogicalHeight())
         setPageLogicalOffset(layoutView->layoutState()->pageLogicalOffset(*this, logicalTop()));
 
     updateLayerTransformAfterLayout();
 
     // Update our scroll information if we're overflow:auto/scroll/hidden now that we know if
     // we overflow or not.
     updateScrollInfoAfterLayout();
+    if (scrollAnchoringEnabled() && hasOverflowClip())
+        scrollableArea()->scrollAnchor().restore();
 
     if (m_paintInvalidationLogicalTop != m_paintInvalidationLogicalBottom) {
         bool hasVisibleContent = style()->visibility() == VISIBLE;
         if (!hasVisibleContent) {
             PaintLayer* layer = enclosingLayer();
             layer->updateDescendantDependentFlags();
             hasVisibleContent = layer->hasVisibleContent();
         }
         if (hasVisibleContent)
             setShouldInvalidateOverflowForPaint();
@@ skipping 2807 matching lines @@
     FrameView* frameView = document().view();
     LayoutUnit top = (style()->position() == FixedPosition) ? 0 : frameView->scrollOffset().height();
     int visibleHeight = frameView->visibleContentRect(IncludeScrollbars).height();
     if (size().height() < visibleHeight)
         top += (visibleHeight - size().height()) / 2;
     setY(top);
     dialog->setCentered(top);
 }
 
 } // namespace blink