Bug 1815061 - Fix handling of startOffset and stroke-dasharray when the pathLength attribute is zero r=emilio

See https://github.com/w3c/svgwg/issues/81

Differential Revision: https://phabricator.services.mozilla.com/D168899

dom/svg/SVGContentUtils.cpp

@@ -184,7 +184,7 @@ static DashState GetStrokeDashData(
       pathScale =
           static_cast<SVGGeometryElement*>(aElement)->GetPathLengthScale(
               SVGGeometryElement::eForStroking);
-      if (pathScale <= 0) {
+      if (pathScale <= 0 || !IsFinite(pathScale)) {
         return eContinuousStroke;
       }
     }

dom/svg/SVGGeometryElement.cpp

@@ -253,7 +253,7 @@ float SVGGeometryElement::GetPathLengthScale(PathLengthScaleForType aFor) {
   MOZ_ASSERT(aFor == eForTextPath || aFor == eForStroking, "Unknown enum");
   if (mPathLength.IsExplicitlySet()) {
     float authorsPathLengthEstimate = mPathLength.GetAnimValue();
-    if (authorsPathLengthEstimate > 0) {
+    if (authorsPathLengthEstimate >= 0) {
       RefPtr<Path> path = GetOrBuildPathForMeasuring();
       if (!path) {
         // The path is empty or invalid so its length must be zero and

layout/svg/SVGTextFrame.cpp

@@ -4605,12 +4605,18 @@ gfxFloat SVGTextFrame::GetStartOffset(nsIFrame* aTextPathFrame) {
       &tp->mLengthAttributes[SVGTextPathElement::STARTOFFSET];
 
   if (length->IsPercentage()) {
+    if (!IsFinite(GetOffsetScale(aTextPathFrame))) {
+      // Either pathLength="0" for this path or the path has 0 length.
+      return 0.0;
+    }
     RefPtr<Path> data = GetTextPath(aTextPathFrame);
     return data ? length->GetAnimValInSpecifiedUnits() * data->ComputeLength() /
                       100.0
                 : 0.0;
   }
-  return length->GetAnimValue(tp) * GetOffsetScale(aTextPathFrame);
+  float lengthValue = length->GetAnimValue(tp);
+  // If offsetScale is infinity we want to return 0 not NaN
+  return lengthValue == 0 ? 0.0 : lengthValue * GetOffsetScale(aTextPathFrame);
 }
 
 void SVGTextFrame::DoTextPathLayout() {

testing/web-platform/meta/svg/path/distance/pathLength-zero-percentage.svg.ini

@@ -1,3 +0,0 @@
-[pathLength-zero-percentage.svg]
-  expected:
-    FAIL

testing/web-platform/meta/svg/path/distance/pathLength-zero.svg.ini

@@ -1,3 +0,0 @@
-[pathLength-zero.svg]
-  expected:
-    FAIL

testing/web-platform/meta/svg/path/distance/pathlength-path-zero.svg.ini

@@ -1,2 +0,0 @@
-[pathlength-path-zero.svg]
-  expected: FAIL