Bug 1117514 - lines with zero length dashes are not rendered. r=jwatt

dom/svg/SVGContentUtils.cpp

@@ -138,12 +138,24 @@ GetStrokeDashData(SVGContentUtils::AutoStrokeOptions* aStrokeOptions,
     totalLengthOfGaps += origTotalLengthOfDashes;
   }
 
-  if (totalLengthOfDashes <= 0 || totalLengthOfGaps <= 0) {
-    if (totalLengthOfGaps > 0 && totalLengthOfDashes <= 0) {
-      return eNoStroke;
-    }
+  // Stroking using dashes is much slower than stroking a continuous line
+  // (see bug 609361 comment 40), and much, much slower than not stroking the
+  // line at all. Here we check for cases when the dash pattern causes the
+  // stroke to essentially be continuous or to be nonexistent in which case
+  // we can avoid expensive stroking operations (the underlying platform
+  // graphics libraries don't seem to optimize for this).
+  if (totalLengthOfDashes <= 0 && totalLengthOfGaps <= 0) {
+    return eNoStroke;
+  }
+  if (totalLengthOfGaps <= 0) {
     return eContinuousStroke;
   }
+  // We can only return eNoStroke if the value of stroke-linecap isn't
+  // adding caps to zero length dashes.
+  if (totalLengthOfDashes <= 0 &&
+      aStyleSVG->mStrokeLinecap == NS_STYLE_STROKE_LINECAP_BUTT) {
+    return eNoStroke;
+  }
 
   if (aContextPaint && aStyleSVG->mStrokeDashoffsetFromObject) {
     aStrokeOptions->mDashOffset = Float(aContextPaint->GetStrokeDashOffset());

layout/reftests/svg/stroke-linecap-round-w-zero-length-segs-01.svg

@@ -144,4 +144,12 @@ path.coverer {
     <path class="coverer" d="M20,20 L30,30 M70,70 L80,80"/>
   </g>
 
+  <!-- Column 5: test stroke-dasharray -->
+
+  <g transform="translate(525,25)">
+    <circle cy="0" r="8"/>
+    <circle cy="40" r="8"/>
+    <circle cy="80" r="8"/>
+    <path class="circles-expected" d="M0,0v81" stroke-dasharray="0 40" />
+  </g>
 </svg>