Bug 1523343 - Should not round translation for SVG frame r=longsonr

When computing transform matrix, translation in SVG should not be rounded.
Checking !IsSVGTransformed() isn't the correct way, since an SVG frame may
have CSS transform applied.

Differential Revision: https://phabricator.services.mozilla.com/D26271

--HG--
extra : moz-landing-system : lando

layout/painting/nsDisplayList.cpp

@@ -7783,6 +7783,12 @@ Matrix4x4 nsDisplayTransform::GetResultingTransformMatrixInternal(
   bool hasSVGTransforms =
       frame &&
       frame->IsSVGTransformed(&svgTransform, &parentsChildrenOnlyTransform);
+
+  // An SVG frame should not have its translation rounded.
+  // Note it's possible that the SVG frame doesn't have an SVG
+  // transform but only has a CSS transform.
+  bool shouldRound = !(frame && frame->IsFrameOfType(nsIFrame::eSVG));
+
   /* Transformed frames always have a transform, or are preserving 3d (and might
    * still have perspective!) */
   if (aProperties.HasTransform()) {
@@ -7852,7 +7858,7 @@ Matrix4x4 nsDisplayTransform::GetResultingTransformMatrixInternal(
     // space.
     if (frame->IsTransformed()) {
       nsLayoutUtils::PostTranslate(result, frame->GetPosition(),
-                                   aAppUnitsPerPixel, !hasSVGTransforms);
+                                   aAppUnitsPerPixel, shouldRound);
     }
     Matrix4x4 parent = GetResultingTransformMatrixInternal(
         props, nsPoint(0, 0), aAppUnitsPerPixel, flags, nullptr);
@@ -7861,7 +7867,7 @@ Matrix4x4 nsDisplayTransform::GetResultingTransformMatrixInternal(
 
   if (aFlags & OFFSET_BY_ORIGIN) {
     nsLayoutUtils::PostTranslate(result, aOrigin, aAppUnitsPerPixel,
-                                 !hasSVGTransforms);
+                                 shouldRound);
   }
 
   return result;

layout/reftests/svg/css-transform-svg-ref.html

@@ -0,0 +1,10 @@
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<style>
+</style>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 20 20">
+    <rect id="a" x="0.49" y="0.51" width="2.5" height="2.5"/>
+    <rect id="b" x="3.5" y="3.5" width="2.5" height="2.5"/>
+</svg>

layout/reftests/svg/css-transform-svg.html

@@ -0,0 +1,13 @@
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<style>
+#a {
+  transform: scaleY(1);
+}
+</style>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 20 20">
+    <rect id="a" x="0.49" y="0.51" width="2.5" height="2.5"/>
+    <rect id="b" x="3.5" y="3.5" width="2.5" height="2.5"/>
+</svg>

layout/reftests/svg/reftest.list

@@ -490,6 +490,7 @@ random-if(/^Windows\x20NT\x206\.1/.test(http.oscpu)) == textPath-line-01.svg tex
 == thin-stroke-01.svg pass.svg
 
 == transform-outer-svg-01.svg transform-outer-svg-01-ref.svg
+== css-transform-svg.html css-transform-svg-ref.html
 
 == tspan-dxdy-01.svg tspan-dxdy-ref.svg
 == tspan-dxdy-02.svg tspan-dxdy-ref.svg