[SP] Don't invalidate the whole scrolled tree

Source/core/paint/DeprecatedPaintLayerScrollableArea.cpp

Index: Source/core/paint/DeprecatedPaintLayerScrollableArea.cpp
diff --git a/Source/core/paint/DeprecatedPaintLayerScrollableArea.cpp b/Source/core/paint/DeprecatedPaintLayerScrollableArea.cpp
index e22534da688657165f6e1e08a74ed66aac68d8f3..b04e90d779ee783168fff559d18f391fcfe7aa77 100644
--- a/Source/core/paint/DeprecatedPaintLayerScrollableArea.cpp
+++ b/Source/core/paint/DeprecatedPaintLayerScrollableArea.cpp
@@ -424,13 +424,8 @@ void DeprecatedPaintLayerScrollableArea::setScrollOffset(const DoublePoint& newS
             requiresPaintInvalidation = true;
     }
 
-    // For slimming paint we need to fully invalidate during scrolling.
-    if (requiresPaintInvalidation && RuntimeEnabledFeatures::slimmingPaintEnabled()) {

[chrishtr, 2015/06/03 18:46:08]

Doesn't the culling code for scrolled content still try to be smart about the
clip of the scroller?

[Xianzhu, 2015/06/03 19:06:59]

Do you mean the 'if (paintInfo.rect.intersects()) return' checkings in the
painters, where the rect is passed as the clip rect of the scroller? If yes,
this doesn't actually matter:
- For non-slimming paint, the scrolled contents are covered by the invalidation
at line 435 below.
- For slimming paint, we'll just treat the scroller in the same way as
composited scrolling, though for now the clip rect is much smaller than
composited scrolling. The merge algorithm can handle appearing and disappeared
display items because of scrolling. This doesn't need any invalidation.

-        DisablePaintInvalidationStateAsserts disabler;
-        box().invalidatePaintIncludingNonCompositingDescendants();
-    }
-
     // Just schedule a full paint invalidation of our object.
+    // FIXME: This invalidation will be unnecessary in slimming paint phase 2.
     if (requiresPaintInvalidation)
         box().setShouldDoFullPaintInvalidation();