Move sticky position constraints update to the pre-paint tree walk.

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

Index: third_party/WebKit/Source/core/paint/PrePaintTreeWalk.cpp
diff --git a/third_party/WebKit/Source/core/paint/PrePaintTreeWalk.cpp b/third_party/WebKit/Source/core/paint/PrePaintTreeWalk.cpp
index f122464d2b1cb1e038e3cd68bd66596feaefbf86..1b548b8c4bf78a2167536801ab7c2325427869cf 100644
--- a/third_party/WebKit/Source/core/paint/PrePaintTreeWalk.cpp
+++ b/third_party/WebKit/Source/core/paint/PrePaintTreeWalk.cpp
@@ -10,18 +10,27 @@
 #include "core/layout/LayoutMultiColumnSpannerPlaceholder.h"
 #include "core/layout/LayoutPart.h"
 #include "core/layout/LayoutView.h"
+#include "core/paint/PaintLayer.h"
 
 namespace blink {
 
 struct PrePaintTreeWalkContext {
-  PrePaintTreeWalkContext() : paintInvalidatorContext(treeBuilderContext) {}
+  PrePaintTreeWalkContext()
+      : paintInvalidatorContext(treeBuilderContext),
+        ancestorOverflowPaintLayer(nullptr) {}
   PrePaintTreeWalkContext(const PrePaintTreeWalkContext& parentContext)
       : treeBuilderContext(parentContext.treeBuilderContext),
         paintInvalidatorContext(treeBuilderContext,
-                                parentContext.paintInvalidatorContext) {}
+                                parentContext.paintInvalidatorContext),
+        ancestorOverflowPaintLayer(parentContext.ancestorOverflowPaintLayer) {}
 
   PaintPropertyTreeBuilderContext treeBuilderContext;
   PaintInvalidatorContext paintInvalidatorContext;
+
+  // The ancestor in the PaintLayer tree which has overflow clip, or

[pdr., 2016/12/09 00:04:10]

The PrePaintTreeWalk is shared by paint invalidation, property tree building,
and compositing thingers. Unfortunately, there is a little paint property logic
spilling over (paint offset diffing) but otherwise we've kept real logic in
other classes. In the interest of keeping the prepaint tree walk as simple as
possible, WDYT of moving this into a helper file?

[chrishtr, 2016/12/09 01:22:54]

Are you suggesting making an AuxiliaryObjectPropertiesContext struct, and moving
the updateAuxiliaryObjectProperties() into that file?

+  // is the root layer. Note that it is tree ancestor, not containing
+  // block or stacking ancestor.
+  PaintLayer* ancestorOverflowPaintLayer;
 };
 
 void PrePaintTreeWalk::walk(FrameView& rootFrame) {
@@ -62,6 +71,36 @@ bool PrePaintTreeWalk::walk(FrameView& frameView,
   return descendantsFullyUpdated;
 }
 
+static void updateAuxiliaryObjectProperties(
+    const LayoutObject& object,
+    PrePaintTreeWalkContext& localContext) {
+  PaintLayer* paintLayer = nullptr;
+
+  if (object.isBoxModelObject() && object.hasLayer())
+    paintLayer = object.enclosingLayer();
+
+  if (paintLayer) {
+    paintLayer->updateAncestorOverflowLayer(
+        localContext.ancestorOverflowPaintLayer);
+  }
+
+  if (object.styleRef().position() == StickyPosition && paintLayer) {
+    paintLayer->layoutObject()->updateStickyPositionConstraints();
+
+    // Sticky position constraints and ancestor overflow scroller affect
+    // the sticky layer position, so we need to update it again here.
+    // TODO(flackr): This should be refactored in the future to be clearer
+    // (i.e. update layer position and ancestor inputs updates in the
+    // same walk)
+    paintLayer->updateLayerPosition();
+  }
+
+  if (object.hasOverflowClip() || (paintLayer && paintLayer->isRootLayer())) {
+    DCHECK(paintLayer);
+    localContext.ancestorOverflowPaintLayer = paintLayer;
+  }
+}
+
 bool PrePaintTreeWalk::walk(const LayoutObject& object,
                             const PrePaintTreeWalkContext& context) {
   PrePaintTreeWalkContext localContext(context);
@@ -118,6 +157,8 @@ bool PrePaintTreeWalk::walk(const LayoutObject& object,
   m_propertyTreeBuilder.updatePropertiesForChildren(
       object, localContext.treeBuilderContext);
 
+  updateAuxiliaryObjectProperties(object, localContext);
+
   bool descendantsFullyUpdated = true;
   for (const LayoutObject* child = object.slowFirstChild(); child;
        child = child->nextSibling()) {