Make scroll anchoring adjustment when the bounds of a scroller is changed

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

Index: third_party/WebKit/Source/core/frame/FrameView.cpp
diff --git a/third_party/WebKit/Source/core/frame/FrameView.cpp b/third_party/WebKit/Source/core/frame/FrameView.cpp
index 9e64a983ed273135f768d12e98d0c4a98780eba9..2f1567b54aa5936428a1722e214109c461f59e91 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.cpp
+++ b/third_party/WebKit/Source/core/frame/FrameView.cpp
@@ -3647,9 +3647,16 @@ void FrameView::updateScrollbars()
 void FrameView::adjustScrollPositionFromUpdateScrollbars()
 {
     DoublePoint adjustedScrollPosition = clampScrollPosition(scrollPositionDouble());
-
     if (adjustedScrollPosition != scrollPositionDouble() || scrollOriginChanged()) {
-        ScrollableArea::setScrollPosition(adjustedScrollPosition, ProgrammaticScroll);
+        // Since the scroll bounds have changed and the scroller is at a position
+        // that is outside its new bounds. Calling ScrollAnchor::restore

[skobes, 2016/07/14 14:49:25]

If the scroll bounds have changed, we will relayout.  Why wouldn't we have
already restored by this point?

+        // ensures that no visible jump is seen when clamping.
+        if (shouldPerformScrollAnchoring() && m_scrollAnchor.hasScroller()) {
+            m_scrollAnchor.restore();
+            adjustedScrollPosition = clampScrollPosition(scrollPositionDouble());
+        }
+        if (adjustedScrollPosition != scrollPositionDouble())
+            ScrollableArea::setScrollPosition(adjustedScrollPosition, ProgrammaticScroll);
         resetScrollOriginChanged();
     }
 }