Reduce invalidation on children-needs-layout containers

Source/core/rendering/LayoutRepainter.cpp

Index: Source/core/rendering/LayoutRepainter.cpp
diff --git a/Source/core/rendering/LayoutRepainter.cpp b/Source/core/rendering/LayoutRepainter.cpp
index 0e1f7791ce75f65c0d9aed0d2a2077bdb2f42991..14d59aff3494348cbdefa1c83561364ca4c5e03d 100644
--- a/Source/core/rendering/LayoutRepainter.cpp
+++ b/Source/core/rendering/LayoutRepainter.cpp
@@ -26,6 +26,7 @@
 #include "config.h"
 #include "core/rendering/LayoutRepainter.h"
 
+#include "core/rendering/RenderBlock.h"
 #include "core/rendering/RenderLayer.h"
 #include "core/rendering/RenderObject.h"
 
@@ -50,11 +51,27 @@ LayoutRepainter::LayoutRepainter(RenderObject& object, bool checkForRepaint)
     }
 }
 
+bool LayoutRepainter::skipInvalidationWhenLayingOutChildren() const

[eseidel, 2014/03/21 00:37:09]

I would have pushed this check onto the RenderObject itself.

I'm also surprised that this is only useful in the children-changed case. A
similar "is self painting" check could be used to avoid repaints for empty divs
changing size, etc.

[Julien - ping for review, 2014/03/24 18:00:01]

That wouldn't work currently as changing size triggers a full layout
(selfNeedsLayout() == true) so we have to be conservative. This is one of the
reasons I am advocating for more granular layout phases.

If we had a way of detecting this case, then yes this code should be reused as
you mentioned. I will add a FIXME about that.

+{
+    if (!m_object.needsLayoutBecauseOfChildren())
+        return false;
+
+    // FIXME: We don't skip invalidations for SVG renderers for now.

[pdr., 2014/03/21 00:25:06]

This is fine, we'll never need this.

[Julien - ping for review, 2014/03/24 18:00:01]

Done.

+    // RenderBlock with line boxes are responsible to invalidate them so we can't ignore them.
+    if (m_object.isSVG() || (m_object.isRenderBlock() && toRenderBlock(&m_object)->firstLineBox()))
+        return false;
+
+    return !m_object.style()->hasVisualOverflowingEffect() && !m_object.style()->hasBorder() && !m_object.style()->hasBackground();
+}
+
 bool LayoutRepainter::repaintAfterLayout()
 {
     if (RuntimeEnabledFeatures::repaintAfterLayoutEnabled())
         return false;
 
+    if (skipInvalidationWhenLayingOutChildren())
+        return false;
+
     // Hits in compositing/video/video-controls-layer-creation.html
     DisableCompositingQueryAsserts disabler;