Round anchor offsets to DIPs in ScrollAnchor::restore.

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

Index: third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
diff --git a/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp b/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
index 2d805f19d6f25979023b1f490169cb57cc4a0e4d..97f4d7d29a409e428d8000b96240ec4564cd1cf5 100644
--- a/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
+++ b/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
@@ -89,8 +89,13 @@ static LayoutRect relativeBounds(const LayoutObject* layoutObject, const Scrolla
     }
     LayoutRect relativeBounds = LayoutRect(layoutObject->localToAncestorQuad(
         FloatRect(localBounds), scrollerLayoutBox(scroller)).boundingBox());
+    // When the scroller is the FrameView, localToAncestorQuad returns document coords,
+    // so we must subtract scroll offset to get viewport coords. We discard the fractional
+    // part of the scroll offset so that the rounding in restore() matches the snapping of
+    // the anchor node to the pixel grid of the layer it paints into. For non-FrameView
+    // scrollers, we rely on the flooring behavior of LayoutBox::scrolledContentOffset.
     if (scroller->isFrameView())
-        relativeBounds.moveBy(-LayoutPoint(scroller->scrollPositionDouble()));
+        relativeBounds.moveBy(-flooredIntPoint(scroller->scrollPositionDouble()));

[eae, 2016/05/10 23:09:18]

We round scroll offsets in PaintLayerScrollableArea and floor them in
RootFrameViewport. Logically rounding seems more correct but as long as we're
consistent it doesn't really matter.

[skobes, 2016/05/10 23:19:00]

It's somewhat more subtle.

For the return value of relativeBounds the important thing is that the
fractional part matches the fractional position the paint code will see when it
paints the anchor node into the GraphicsLayer that holds the scrolling content. 
The way the ScrollableArea treats the fractional part of the scroll offset is
irrelevant.

If scroller is a PaintLayerScrollableArea, localToAncestorQuad has already given
us coordinates relative to the scroller's viewport based on a floored scroll
offset (in LayoutBox::scrolledContentOffset).  Flooring the FrameView's offset
is consistent with that.

     return relativeBounds;
 }
 
@@ -181,21 +186,27 @@ void ScrollAnchor::restore()
     if (!m_anchorObject)
         return;
 
-    LayoutSize adjustment = computeRelativeOffset(m_anchorObject, m_scroller, m_corner) - m_savedRelativeOffset;
+    // The anchor node can report fractional positions, but it is DIP-snapped when
+    // painting (crbug.com/610805), so we must round the offsets to determine the
+    // visual delta. If we scroll by the delta in LayoutUnits, the snapping of the
+    // anchor node may round differently from the snapping of the scroll position.
+    // (For example, anchor moving from 2.4px -> 2.6px is really 2px -> 3px, so we
+    // should scroll by 1px instead of 0.2px.) This is true regardless of whether
+    // the ScrollableArea actually uses fractional scroll positions.
+    IntSize adjustment =
+        roundedIntSize(computeRelativeOffset(m_anchorObject, m_scroller, m_corner)) -
+        roundedIntSize(m_savedRelativeOffset);
     if (!adjustment.isZero()) {
+        DoublePoint desiredPos = m_scroller->scrollPositionDouble() + adjustment;
         ScrollAnimatorBase* animator = m_scroller->existingScrollAnimator();
         if (!animator || !animator->hasRunningAnimation()) {
-            m_scroller->setScrollPosition(
-                m_scroller->scrollPositionDouble() + DoubleSize(adjustment),
-                AnchoringScroll);
+            m_scroller->setScrollPosition(desiredPos, AnchoringScroll);
         } else {
             // If in the middle of a scroll animation, stop the animation, make
             // the adjustment, and continue the animation on the pending delta.
             FloatSize pendingDelta = animator->desiredTargetPosition() - FloatPoint(m_scroller->scrollPositionDouble());
             animator->cancelAnimation();
-            m_scroller->setScrollPosition(
-                m_scroller->scrollPositionDouble() + DoubleSize(adjustment),
-                AnchoringScroll);
+            m_scroller->setScrollPosition(desiredPos, AnchoringScroll);
             animator->userScroll(ScrollByPixel, pendingDelta);
         }
         // Update UMA metric.