Better scrolling fix

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 087eaa3ce71a05b843d9fc3e4dfbb19d7e495d8f..e40d3bc920c23b7226720a12b1ce82c5464b6cff 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -596,6 +596,9 @@ void PaintLayerScrollableArea::computeScrollDimensions()
     int scrollableLeftOverflow = m_overflowRect.x() - box().borderLeft() - (box().shouldPlaceBlockDirectionScrollbarOnLogicalLeft() ? box().verticalScrollbarWidth() : 0);
     int scrollableTopOverflow = m_overflowRect.y() - box().borderTop();
     setScrollOrigin(IntPoint(-scrollableLeftOverflow, -scrollableTopOverflow));
+    fprintf(stderr, "origin=%i,%i\n", scrollOrigin().x(), scrollOrigin().y());
+        fprintf(stderr, "........overflowRect x = %f width = %f layoutRect x = %f width = %f\n", m_overflowRect.x().toFloat(), m_overflowRect.width().toFloat(), box().location().x().toFloat(), box().size().width().toFloat());
+    fprintf(stderr, "offset=%f,%f\n", m_scrollOffset.width(), m_scrollOffset.height());
 }
 
 void PaintLayerScrollableArea::scrollToPosition(const DoublePoint& scrollPosition, ScrollOffsetClamping clamp, ScrollBehavior scrollBehavior, ScrollType scrollType)
@@ -607,6 +610,15 @@ void PaintLayerScrollableArea::scrollToPosition(const DoublePoint& scrollPositio
         ScrollableArea::setScrollPosition(newScrollPosition, scrollType, scrollBehavior);
 }
 
+void PaintLayerScrollableArea::clampPositionAfterLayout()
+{
+    // Layout may cause us to be at an invalid scroll position. In this case we need
+    // to pull our scroll offsets back to the max (or push them up to the min).
+    DoublePoint clampedScrollPosition = clampScrollPosition(scrollPositionDouble());
+    if (clampedScrollPosition != scrollPositionDouble())
+        scrollToPosition(clampedScrollPosition);
+}
+
 bool PaintLayerScrollableArea::updateAfterLayout(SubtreeLayoutScope* delayedLayoutScope)
 {
     ASSERT(box().hasOverflowClip());
@@ -624,18 +636,25 @@ bool PaintLayerScrollableArea::updateAfterLayout(SubtreeLayoutScope* delayedLayo
     IntPoint originalOrigin = scrollOrigin();
     computeScrollDimensions();
 
-    // Layout may cause us to be at an invalid scroll position. In this case we need
-    // to pull our scroll offsets back to the max (or push them up to the min).
-    DoublePoint clampedScrollPosition = clampScrollPosition(scrollPositionDouble());
-    if (clampedScrollPosition != scrollPositionDouble()) {
-        scrollToPosition(clampedScrollPosition);
-    } else if (originalOrigin != scrollOrigin()) {
+    fprintf(stderr, "updateAfterLayout -- origin is %i,%i offset %f,%f\n", scrollOrigin().x(), scrollOrigin().y(), scrollPositionDouble().x(), scrollPositionDouble().y());
+
+    IntPoint newOrigin(scrollOrigin());
+    if (originalOrigin != newOrigin) {
         // TODO: We should be able to use scrollOriginChanged() here, but we can't because
         // PaintLayerScrollableArea does not maintain that flag: it gets set, but it never
         // gets unset.  We should unset the flag after layout.
-        scrollPositionChanged(scrollPositionDouble(), ProgrammaticScroll);
+        DoubleSize originDiff(newOrigin - originalOrigin);
+        /*
+        if (originDiff.width() < 0)
+            originDiff.setWidth(0);
+        if (originDiff.height() < 0)
+            originDiff.setHeight(0);
+        */
+        scrollPositionChanged(scrollPositionDouble() + originDiff, ProgrammaticScroll);
+        fprintf(stderr, "    adjusting position by %f,%f\n", originDiff.width(), originDiff.height());
     }
 
+
     m_scrollbarManager.setCanDetachScrollbars(false);
 
     bool hasHorizontalOverflow = this->hasHorizontalOverflow();