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

Spend some cycles examining the difference between the old and new
local-to-border-box transforms, and only signal scale-factor changes
if that part of the transform changed.
This also means that we now detect changes scale that we previously
didn't (like [1].)

[1] paint/invalidation/svg/absolute-sized-content-with-resources.xhtml

BUG=603956,664961

Review-Url: https://codereview.chromium.org/2559123003
Cr-Commit-Position: refs/heads/master@{#437767}

third_party/WebKit/LayoutTests/paint/invalidation/svg/absolute-sized-content-with-resources-expected.txt

@@ -19,7 +19,7 @@
         {
           "object": "LayoutSVGRect rect",
           "rect": [42, 117, 334, 268],
-          "reason": "bounds change"
+          "reason": "SVG resource change"
         },
         {
           "object": "LayoutSVGRoot svg",
@@ -54,7 +54,7 @@
         {
           "object": "LayoutSVGRect rect",
           "rect": [17, 217, 84, 68],
-          "reason": "bounds change"
+          "reason": "SVG resource change"
         },
         {
           "object": "LayoutSVGRoot svg",
@@ -109,7 +109,7 @@
     },
     {
       "object": "LayoutSVGRect rect",
-      "reason": "bounds change"
+      "reason": "SVG resource change"
     }
   ]
 }

third_party/WebKit/LayoutTests/platform/linux/paint/invalidation/svg/absolute-sized-content-with-resources-expected.txt

@@ -19,7 +19,7 @@
         {
           "object": "LayoutSVGRect rect",
           "rect": [42, 119, 334, 268],
-          "reason": "bounds change"
+          "reason": "SVG resource change"
         },
         {
           "object": "LayoutSVGRoot svg",
@@ -54,7 +54,7 @@
         {
           "object": "LayoutSVGRect rect",
           "rect": [17, 219, 84, 68],
-          "reason": "bounds change"
+          "reason": "SVG resource change"
         },
         {
           "object": "LayoutSVGRoot svg",
@@ -109,7 +109,7 @@
     },
     {
       "object": "LayoutSVGRect rect",
-      "reason": "bounds change"
+      "reason": "SVG resource change"
     }
   ]
 }

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 {

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

@@ -28,6 +28,7 @@
 namespace blink {
 
 class SVGElement;
+enum class SVGTransformChange;
 
 class CORE_EXPORT LayoutSVGRoot final : public LayoutReplaced {
  public:
@@ -156,7 +157,7 @@ class CORE_EXPORT LayoutSVGRoot final : public LayoutReplaced {
   void descendantIsolationRequirementsChanged(DescendantIsolationState) final;
 
   void updateCachedBoundaries();
-  void buildLocalToBorderBoxTransform();
+  SVGTransformChange buildLocalToBorderBoxTransform();
 
   PositionWithAffinity positionForPoint(const LayoutPoint&) final;