Avoid using forced layout to trigger paint invalidation for SVG containers

third_party/WebKit/Source/core/layout/svg/LayoutSVGViewportContainer.cpp

Index: third_party/WebKit/Source/core/layout/svg/LayoutSVGViewportContainer.cpp
diff --git a/third_party/WebKit/Source/core/layout/svg/LayoutSVGViewportContainer.cpp b/third_party/WebKit/Source/core/layout/svg/LayoutSVGViewportContainer.cpp
index d0d69377816a2f37d2f07ea860ee979d49ee2d61..edd346104c2246b95f78aa32038576b5ba09cea9 100644
--- a/third_party/WebKit/Source/core/layout/svg/LayoutSVGViewportContainer.cpp
+++ b/third_party/WebKit/Source/core/layout/svg/LayoutSVGViewportContainer.cpp
@@ -64,14 +64,29 @@ void LayoutSVGViewportContainer::calcViewport()
     }
 }
 
-bool LayoutSVGViewportContainer::calculateLocalTransform()
+void LayoutSVGViewportContainer::setNeedsTransformUpdate()
+{
+    setMayNeedPaintInvalidationSubtree();
+    m_needsTransformUpdate = true;
+}
+
+static std::pair<double, double> scaleReference(const AffineTransform& transform)
+{
+    return std::make_pair(transform.xScaleSquared(), transform.yScaleSquared());
+}
+
+unsigned LayoutSVGViewportContainer::calculateLocalTransform()
 {
     if (!m_needsTransformUpdate)
-        return false;
+        return NoTransformChange;
 
+    std::pair<double, double> oldScale = scaleReference(m_localToParentTransform);
     m_localToParentTransform = AffineTransform::translation(m_viewport.x(), m_viewport.y()) * viewportTransform();
     m_needsTransformUpdate = false;
-    return true;
+
+    unsigned changeFlags = TransformChange;
+    changeFlags |= scaleReference(m_localToParentTransform) != oldScale ? ScaleChange : 0;
+    return changeFlags;
 }
 
 AffineTransform LayoutSVGViewportContainer::viewportTransform() const