Bug 1539366 - Avoid calling AreOnSameLine once we know we've already changed lines since it can be expensive to compute. r=mats

Differential Revision: https://phabricator.services.mozilla.com/D26650

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

layout/painting/nsCSSRendering.cpp

@@ -349,6 +349,7 @@ struct InlineBackgroundData {
     // Start with the previous flow frame as our continuation point
     // is the total of the widths of the previous frames.
     nsIFrame* inlineFrame = GetPrevContinuation(aFrame);
+    bool changedLines = false;
     while (inlineFrame) {
       if (!mPIStartBorderData.mFrame &&
           !(mVertical ? inlineFrame->GetSkipSides().Top()
@@ -357,8 +358,9 @@ struct InlineBackgroundData {
       }
       nsRect rect = inlineFrame->GetRect();
       mContinuationPoint += mVertical ? rect.height : rect.width;
-      if (mBidiEnabled && !AreOnSameLine(aFrame, inlineFrame)) {
+      if (mBidiEnabled && (changedLines || !AreOnSameLine(aFrame, inlineFrame))) {
         mLineContinuationPoint += mVertical ? rect.height : rect.width;
+        changedLines = true;
       }
       mUnbrokenMeasure += mVertical ? rect.height : rect.width;
       mBoundingBox.UnionRect(mBoundingBox, rect);