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..df30fbde50a018f0153d7606b648ddc7d4f91e31 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.cpp
+++ b/third_party/WebKit/Source/core/frame/FrameView.cpp
@@ -3646,10 +3646,19 @@ void FrameView::updateScrollbars()
 
 void FrameView::adjustScrollPositionFromUpdateScrollbars()
 {
-    DoublePoint adjustedScrollPosition = clampScrollPosition(scrollPositionDouble());
-
-    if (adjustedScrollPosition != scrollPositionDouble() || scrollOriginChanged()) {
-        ScrollableArea::setScrollPosition(adjustedScrollPosition, ProgrammaticScroll);
+    DoublePoint clamped = clampScrollPosition(scrollPositionDouble());
+    if (clamped != scrollPositionDouble() || scrollOriginChanged()) {
+        // Since the scroll bounds have changed and the scroller is at a position
+        // that is outside its new bounds. Calling ScrollAnchor::restore
+        // ensures that no visible jump is seen when clamping.

[skobes, 2016/07/15 20:17:42]

I think this comment should mention that we restore before clamping because
clamping clears the scroll anchor.

[ymalik, 2016/07/18 16:08:48]

Done.

+        // TODO(ymalik): This same logic exists in PaintLayerScrollableArea.
+        // Remove when root-layer-scrolls is enabled.
+        if (shouldPerformScrollAnchoring() && m_scrollAnchor.hasScroller()) {
+            m_scrollAnchor.restore();
+            clamped = clampScrollPosition(scrollPositionDouble());
+        }
+        if (clamped != scrollPositionDouble())
+            ScrollableArea::setScrollPosition(clamped, ProgrammaticScroll);
         resetScrollOriginChanged();
     }
 }