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

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

Index: third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
index 52b9388e24a8e3a1c000d9436060d890b878bdd8..cfc4185b17a6514fbfc6db8f091ac8eed0b4fe8c 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -752,14 +752,21 @@ void PaintLayerScrollableArea::clampScrollPositionsAfterLayout()
         return;
     }
 
-    DoublePoint clampedScrollPosition = clampScrollPosition(scrollPositionDouble());
-    if (clampedScrollPosition != scrollPositionDouble())
-        ScrollableArea::setScrollPosition(clampedScrollPosition, ProgrammaticScroll);
-    else if (scrollOriginChanged())
-        scrollPositionChanged(clampedScrollPosition, 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.
+        if (shouldPerformScrollAnchoring() && m_scrollAnchor.hasScroller()) {
+            m_scrollAnchor.restore();
+            clamped = clampScrollPosition(scrollPositionDouble());
+        }
+        if (clamped != scrollPositionDouble())
+            ScrollableArea::setScrollPosition(clamped, ProgrammaticScroll);
+        resetScrollOriginChanged();
+    }
 
     setNeedsScrollPositionClamp(false);
-    resetScrollOriginChanged();
     m_scrollbarManager.destroyDetachedScrollbars();
 }