Strength-reduce the "scale-factor changed" condition in LayoutSVGRoot

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

Index: third_party/WebKit/Source/core/layout/svg/LayoutSVGRoot.cpp
diff --git a/third_party/WebKit/Source/core/layout/svg/LayoutSVGRoot.cpp b/third_party/WebKit/Source/core/layout/svg/LayoutSVGRoot.cpp
index 7cd271a0483aeeac1d94181c48fa79978e263e4c..3fbc2df6a26356626559a7146c09d3d8ff2a0978 100644
--- a/third_party/WebKit/Source/core/layout/svg/LayoutSVGRoot.cpp
+++ b/third_party/WebKit/Source/core/layout/svg/LayoutSVGRoot.cpp
@@ -145,23 +145,34 @@ void LayoutSVGRoot::layout() {
   updateLogicalWidth();
   updateLogicalHeight();
 
-  buildLocalToBorderBoxTransform();
-  // TODO(fs): Temporarily, needing a layout implies that the local transform
-  // has changed. This should be updated to be more precise and factor in the
-  // actual (relevant) changes to the computed user-space transform.
-  m_didScreenScaleFactorChange = selfNeedsLayout();
+  // The local-to-border-box transform is a function with the following as
+  // input:
+  //
+  //  * effective zoom
+  //  * contentWidth/Height
+  //  * viewBox
+  //  * border + padding
+  //  * currentTranslate
+  //  * currentScale
+  //
+  // Which means that |transformChange| will notice a change to the scale from
+  // any of these.
+  SVGTransformChange transformChange = buildLocalToBorderBoxTransform();
+
+  // The scale factor from the local-to-border-box transform is all that our
+  // scale-dependent descendants care about.
+  m_didScreenScaleFactorChange = transformChange == SVGTransformChange::Full;
 
   SVGLayoutSupport::layoutResourcesIfNeeded(this);
 
   // selfNeedsLayout() will cover changes to one (or more) of viewBox,
-  // current{Scale,Translate} and decorations.
+  // current{Scale,Translate}, decorations and 'overflow'.
   const bool viewportMayHaveChanged = selfNeedsLayout() || oldSize != size();
 
-  // The scale of one or more of the SVG elements may have changed, or new
-  // content may have been exposed, so mark the entire subtree as needing paint
-  // invalidation checking. (It is only somewhat by coincidence that this
-  // condition happens to be the same as the one for viewport changes.)
-  if (viewportMayHaveChanged) {
+  // The scale of one or more of the SVG elements may have changed, content
+  // (the entire SVG) could have moved or new content may have been exposed, so
+  // mark the entire subtree as needing paint invalidation checking.
+  if (transformChange != SVGTransformChange::None || viewportMayHaveChanged) {
     setMayNeedPaintInvalidationSubtree();
     if (RuntimeEnabledFeatures::slimmingPaintInvalidationEnabled())
       setNeedsPaintPropertyUpdate();
@@ -333,21 +344,23 @@ PositionWithAffinity LayoutSVGRoot::positionForPoint(const LayoutPoint& point) {
 
 // LayoutBox methods will expect coordinates w/o any transforms in coordinates
 // relative to our borderBox origin.  This method gives us exactly that.
-void LayoutSVGRoot::buildLocalToBorderBoxTransform() {
+SVGTransformChange LayoutSVGRoot::buildLocalToBorderBoxTransform() {
+  SVGTransformChangeDetector changeDetector(m_localToBorderBoxTransform);
   SVGSVGElement* svg = toSVGSVGElement(node());
   ASSERT(svg);
   float scale = style()->effectiveZoom();
-  FloatPoint translate = svg->currentTranslate();
-  LayoutSize borderAndPadding(borderLeft() + paddingLeft(),
-                              borderTop() + paddingTop());
   m_localToBorderBoxTransform = svg->viewBoxToViewTransform(
       contentWidth() / scale, contentHeight() / scale);
 
+  FloatPoint translate = svg->currentTranslate();
+  LayoutSize borderAndPadding(borderLeft() + paddingLeft(),
+                              borderTop() + paddingTop());
   AffineTransform viewToBorderBoxTransform(
       scale, 0, 0, scale, borderAndPadding.width() + translate.x(),
       borderAndPadding.height() + translate.y());
   viewToBorderBoxTransform.scale(svg->currentScale());
   m_localToBorderBoxTransform.preMultiply(viewToBorderBoxTransform);
+  return changeDetector.computeChange(m_localToBorderBoxTransform);
 }
 
 AffineTransform LayoutSVGRoot::localToSVGParentTransform() const {