[SP] Don't invalidate the whole scrolled tree

Source/core/paint/DeprecatedPaintLayerScrollableArea.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 406 matching lines @@
         if (usesCompositedScrolling() || onlyScrolledCompositedLayers)
             requiresPaintInvalidation = false;
     }
 
     // Only the root layer can overlap non-composited fixed-position elements.
     if (!requiresPaintInvalidation && layer()->isRootLayer() && frameView->hasViewportConstrainedObjects()) {
         if (!frameView->invalidateViewportConstrainedObjects())
             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();
 
     // Schedule the scroll DOM event.
     if (box().node())
         box().node()->document().enqueueScrollEventForNode(box().node());
 
     if (AXObjectCache* cache = box().document().existingAXObjectCache())
         cache->handleScrollPositionChanged(&box());
 }
@@ skipping 959 matching lines @@
 void DeprecatedPaintLayerScrollableArea::setTopmostScrollChild(DeprecatedPaintLayer* scrollChild)
 {
     // We only want to track the topmost scroll child for scrollable areas with
     // overlay scrollbars.
     if (!hasOverlayScrollbars())
         return;
     m_nextTopmostScrollChild = scrollChild;
 }
 
 } // namespace blink