Use ScrollAnchor in FrameView and PaintLayerScrollableArea.

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

Index: third_party/WebKit/Source/core/layout/LayoutBlockFlow.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutBlockFlow.cpp b/third_party/WebKit/Source/core/layout/LayoutBlockFlow.cpp
index a53a7603bd935af6e556c0e1855197b420c4c62f..c3936cbacd784f5846da1c674e8217643aa71230 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBlockFlow.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBlockFlow.cpp
@@ -287,6 +287,8 @@ void LayoutBlockFlow::layoutBlock(bool relayoutChildren)
 
     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.
@@ -304,6 +306,8 @@ void LayoutBlockFlow::layoutBlock(bool relayoutChildren)
     // 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;