Fix fixed-background offset issue

Source/core/paint/BackgroundImageGeometry.cpp

Index: Source/core/paint/BackgroundImageGeometry.cpp
diff --git a/Source/core/paint/BackgroundImageGeometry.cpp b/Source/core/paint/BackgroundImageGeometry.cpp
index d4a3bedff08bf0af15358b19fe13c291f3f5f1f4..aa1151313dcfd83af950c2162dc3eb2171556dd6 100644
--- a/Source/core/paint/BackgroundImageGeometry.cpp
+++ b/Source/core/paint/BackgroundImageGeometry.cpp
@@ -156,7 +156,7 @@ void BackgroundImageGeometry::clip(const IntRect& clipRect)
 
 void BackgroundImageGeometry::calculate(const LayoutBoxModelObject& obj, const LayoutBoxModelObject* paintContainer,
     const GlobalPaintFlags globalPaintFlags, const FillLayer& fillLayer, const LayoutRect& paintRect,
-    LayoutObject* backgroundObject)
+    const IntPoint& accumulatedScrollOffsetSincePaintContainer, LayoutObject* backgroundObject)
 {
     LayoutUnit left = 0;
     LayoutUnit top = 0;
@@ -223,10 +223,13 @@ void BackgroundImageGeometry::calculate(const LayoutBoxModelObject& obj, const L
         setHasNonLocalGeometry();
 
         IntRect viewportRect = pixelSnappedIntRect(obj.viewRect());
-        if (fixedBackgroundPaintsInLocalCoordinates(obj, globalPaintFlags))
+        if (fixedBackgroundPaintsInLocalCoordinates(obj, globalPaintFlags)) {
             viewportRect.setLocation(IntPoint());
-        else if (FrameView* frameView = obj.view()->frameView())
-            viewportRect.setLocation(frameView->scrollPosition());
+        } else {
+            if (FrameView* frameView = obj.view()->frameView()) {
+                viewportRect.setLocation(frameView->scrollPosition());
+            viewportRect.moveBy(accumulatedScrollOffsetSincePaintContainer);
+        }
 
         if (paintContainer) {
             IntPoint absoluteContainerOffset = roundedIntPoint(paintContainer->localToAbsolute(FloatPoint()));

[chrishtr, 2015/09/09 17:05:41]

localToAbsolute includes scroll offsets...why isn't it happening here?

[Xianzhu, 2015/09/09 17:09:09]

It includes scroll offsets from root to the paint container, not including those
from the paint container to the current painting object.

[chrishtr, 2015/09/09 17:29:16]

Oh right.

How about replacing your code with mapLocalToContainer from the local object to
the paintContainer? Then this code can at least be localized to fixed-position
backgrounds.

[Xianzhu, 2015/09/09 20:40:03]

This seems not to work because we only need to compensate scroll offsets for
which we issued translations, but not other offsets in paint offset.

[chrishtr, 2015/09/09 20:48:15]

Maybe just make a one-off method that accumulates just scroll offsets then? SP
phase 2 should
allow us to fix this more cleanly by specifying the root transform space, once
trchen's
work is done. In the meantime I think this should be as targeted of a fix as
possible.

[Xianzhu, 2015/09/09 22:04:06]

Good idea! Done.