Don't invalidate rect if it's covered by fully invalidated parent's visual rect

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

Index: third_party/WebKit/Source/core/paint/ObjectPaintInvalidator.cpp
diff --git a/third_party/WebKit/Source/core/paint/ObjectPaintInvalidator.cpp b/third_party/WebKit/Source/core/paint/ObjectPaintInvalidator.cpp
index eedca8d87b2e4e5ed0df5364011f1a75c4de5a89..0dfc5589a62a1a6e8c936c0d8f78ea2e6771f05f 100644
--- a/third_party/WebKit/Source/core/paint/ObjectPaintInvalidator.cpp
+++ b/third_party/WebKit/Source/core/paint/ObjectPaintInvalidator.cpp
@@ -455,15 +455,37 @@ void ObjectPaintInvalidatorWithContext::fullyInvalidatePaint(
   // the other.
   if (!newVisualRect.contains(oldVisualRect)) {
     LayoutRect invalidationRect = oldVisualRect;
-    invalidatePaintUsingContainer(*m_context.paintInvalidationContainer,
-                                  invalidationRect, reason);
+    invalidatePaintRectangleWithContext(invalidationRect, reason);
 
     if (invalidationRect.contains(newVisualRect))
       return;
   }
 
-  invalidatePaintUsingContainer(*m_context.paintInvalidationContainer,
-                                newVisualRect, reason);
+  invalidatePaintRectangleWithContext(newVisualRect, reason);
+}
+
+void ObjectPaintInvalidatorWithContext::invalidatePaintRectangleWithContext(
+    const LayoutRect& rect,
+    PaintInvalidationReason reason) {
+  if (rect.isEmpty())
+    return;
+
+  // If the parent has fully invalidated and its visual rect covers this object
+  // on the same backing, skip the invalidation.
+  if (m_object.parent() && m_context.parentContext &&
+      m_context.parentContext->paintInvalidationContainer ==
+          m_context.paintInvalidationContainer &&
+      // This is a quick test to exclude the case that parent and child are not
+      // on the same backing of the paint invalidation container.
+      m_object.parent() != m_context.paintInvalidationContainer &&

[wkorman, 2017/02/23 01:26:25]

I wanted to better understand how checking whether parent !=
paint-invalidation-container relates to whether they're on the same backing?

I figure this is correct and I just need to hear you explain it a bit more to
me, but perhaps we should also make it clearer in the comment since I wondered.
It is hard for me to quickly understand "exclude the case that they are not",
when in fact if we pass this conditional test, what it means is that we will
proceed in considering application of the optimization.

[Xianzhu, 2017/02/23 04:38:49]

The idea was that the paint invalidation container itself may paint into
multiple backings. The parent may have been fully invalidated on the scrolling
container layer, while the current object may be invalidating on the scrolling
contents layer.

I have extracted the conditions into
ObjectPaintInvalidator::parentFullyInvalidatedOnSameBacking() and added
comments.

+      isImmediateFullPaintInvalidationReason(
+          m_object.parent()->fullPaintInvalidationReason()) &&
+      (m_context.parentContext->oldVisualRect.contains(rect) ||
+       m_context.parentContext->newVisualRect.contains(rect)))
+    return;
+
+  invalidatePaintUsingContainer(*m_context.paintInvalidationContainer, rect,
+                                reason);
 }
 
 DISABLE_CFI_PERF