Avoid repainting non needs-self-layout container

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..b6f3851ba199ca51c0770333c08264013ca6d010 100644
--- a/Source/core/rendering/LayoutRepainter.cpp
+++ b/Source/core/rendering/LayoutRepainter.cpp
@@ -26,15 +26,20 @@
 #include "config.h"
 #include "core/rendering/LayoutRepainter.h"
 
+#include "core/rendering/RenderBlockFlow.h"
+#include "core/rendering/RenderBox.h"
 #include "core/rendering/RenderLayer.h"
-#include "core/rendering/RenderObject.h"
 
 namespace WebCore {
 
+static const int gUnsetValue = -1;
+
 LayoutRepainter::LayoutRepainter(RenderObject& object, bool checkForRepaint)
     : m_object(object)
     , m_repaintContainer(0)
     , m_checkForRepaint(checkForRepaint)
+    , m_oldLogicalWidth(gUnsetValue)
+    , m_oldLogicalHeight(gUnsetValue)
 {
     if (RuntimeEnabledFeatures::repaintAfterLayoutEnabled())
         return;
@@ -47,6 +52,11 @@ LayoutRepainter::LayoutRepainter(RenderObject& object, bool checkForRepaint)
             m_oldBounds = m_object.clippedOverflowRectForRepaint(m_repaintContainer);
         }
         m_oldOutlineBox = m_object.outlineBoundsForRepaint(m_repaintContainer);
+        if (m_object.isBox()) {
+            const RenderBox& box = *toRenderBox(&m_object);
+            m_oldLogicalWidth = box.logicalWidth();
+            m_oldLogicalHeight = box.logicalHeight();
+        }
     }
 }
 
@@ -55,10 +65,58 @@ bool LayoutRepainter::repaintAfterLayout()
     if (RuntimeEnabledFeatures::repaintAfterLayoutEnabled())
         return false;
 
+    if (!m_checkForRepaint)
+        return false;
+
+    if (canSkipRepaint())
+        return false;
+
     // Hits in compositing/video/video-controls-layer-creation.html
     DisableCompositingQueryAsserts disabler;
 
-    return m_checkForRepaint ? m_object.repaintAfterLayoutIfNeeded(m_repaintContainer, m_object.selfNeedsLayout(), m_oldBounds, m_oldOutlineBox) : false;
+    return m_object.repaintAfterLayoutIfNeeded(m_repaintContainer, m_object.selfNeedsLayout(), m_oldBounds, m_oldOutlineBox);
+}
+
+static bool hasChildrenNeedingRepaintFromContainingBlock(const RenderBlock& containingBlock)
+{
+    // FIXME: We currently disable this optimization for floats as they need to be
+    // correctly repainted by their (not marked for self-layout) block-flow container.
+    return containingBlock.containsFloats();
+}
+
+bool LayoutRepainter::canSkipRepaint() const
+{
+    // If we needed layout, our content could have changed so we have to repaint.
+    if (m_object.selfNeedsLayout())
+        return false;
+
+    // FIXME: We disable the optimization for SVG text renderers as they
+    // are the one repainting their inline content.
+    if (m_object.isSVGText())
+        return false;
+
+    if (m_oldLogicalWidth == gUnsetValue) {
+        ASSERT(m_oldLogicalHeight == gUnsetValue);
+        return false;
+    }
+
+    ASSERT(m_oldLogicalWidth != gUnsetValue);
+    ASSERT(m_oldLogicalHeight != gUnsetValue);
+
+    // We don't repaint box containers that are not marked for self-layout and didn't change size.
+    // This is valid because:
+    // - if their content changed, they would be marked for self-layout.
+    // - their descendant(s) that actually changed should repaint themselves correctly.
+    // FIXME: It should be possible to avoid repainting size changes but we have to be careful with
+    // some cases (e.g. gradient or repeated backgrounds).
+    const RenderBox& box = *toRenderBox(&m_object);
+    if (m_oldLogicalWidth != box.logicalWidth() || m_oldLogicalHeight != box.logicalHeight())
+        return false;
+
+    if (!m_object.isRenderBlock())
+        return true;
+
+    return !hasChildrenNeedingRepaintFromContainingBlock(*toRenderBlock(&box));
 }
 
 } // namespace WebCore