Tweak priorities of paint invalidation reasons

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

Index: third_party/WebKit/Source/core/paint/BoxPaintInvalidator.cpp
diff --git a/third_party/WebKit/Source/core/paint/BoxPaintInvalidator.cpp b/third_party/WebKit/Source/core/paint/BoxPaintInvalidator.cpp
index a1625c3af3e9e00c5a9f149d6868482d0e95d497..04b3f8d23f47280a96b2335b2a011b749bfccc8f 100644
--- a/third_party/WebKit/Source/core/paint/BoxPaintInvalidator.cpp
+++ b/third_party/WebKit/Source/core/paint/BoxPaintInvalidator.cpp
@@ -105,7 +105,7 @@ PaintInvalidationReason BoxPaintInvalidator::computePaintInvalidationReason()
 {
     PaintInvalidationReason reason = ObjectPaintInvalidator(m_box, m_context).computePaintInvalidationReason();
 
-    if (reason != PaintInvalidationDelayedFull && isFullPaintInvalidationReason(reason))
+    if (isImmediateFullPaintInvalidationReason(reason) || reason == PaintInvalidationNone)
         return reason;
 
     if (m_box.mayNeedPaintInvalidationAnimatedBackgroundImage() && !m_box.backgroundIsKnownToBeObscured())
@@ -164,11 +164,6 @@ PaintInvalidationReason BoxPaintInvalidator::computePaintInvalidationReason()
     if (oldBorderBoxSize == newBorderBoxSize)
         return reason;
 
-    // LayoutBox::incrementallyInvalidatePaint() depends on positionFromPaintInvalidationBacking
-    // which is not available when slimmingPaintOffsetCachingEnabled.
-    if (RuntimeEnabledFeatures::slimmingPaintInvalidationEnabled() && (style.hasBoxDecorations() || style.hasBackground()))
-        return PaintInvalidationBorderBoxChange;
-
     // See another hasNonCompositedScrollbars() callsite above.
     if (m_box.hasNonCompositedScrollbars())
         return PaintInvalidationBorderBoxChange;
@@ -190,7 +185,7 @@ PaintInvalidationReason BoxPaintInvalidator::computePaintInvalidationReason()
     if (oldBorderBoxSize.height() != newBorderBoxSize.height() && m_box.mustInvalidateBackgroundOrBorderPaintOnHeightChange())
         return PaintInvalidationBorderBoxChange;
 
-    if (reason == PaintInvalidationNone && (style.hasBackground() || style.hasBoxDecorations()))
+    if (reason == PaintInvalidationProbablyNone && (style.hasBackground() || style.hasBoxDecorations()))

[pdr., 2016/08/15 23:37:42]

Could we just return PaintInvalidationIncremental directly instead of modifying
the invalidation reason here?

I don't really understand the PaintInvalidationProbablyNone invalidation reason.
Can you think of a name that is more concrete?

[Xianzhu, 2016/08/16 01:13:54]

Added comments in PaintInvalidationReason.h.

For the following cases (assuming the object is a box and
shouldDoFullPaintInvalidation() is false):
1. both old bounds and new bounds are empty,
2. old bounds and new bounds are non-empty and equal,
  2.1. old and new border boxes equals,
  2.2. old and new border box don't equal,
    2.2.1 the box has background or box decorations,
    2.2.2 the box doesn't have background or box decorations,
3. old bounds and new bounds have the same location but don't equal:

Case   ObjectPaintInvalidator         BoxPaintInvalidator
1      PaintInvalidationNone          PaintInvalidationNone
2.1    PaintInvalidationProbablyNone  PaintInvalidationProbablyNone
2.2.1  PaintInvalidationProbablyNone  PaintInvalidationProbablyNone
2.2.2  PaintInvalidationProbablyNone  PaintInvalidationIncremental
3      PaintInvalidationIncremental   PaintInvalidationIncremental

Previously ObjectPaintInvalidator::computePaintInvalidationReason() returned
PaintInvalidationNone for both case 1 and case 2 though the function can already
be sure no paint invalidation is needed for case 1.

         reason = PaintInvalidationIncremental;
 
     return reason;