Bug 429899. Always treat the first character of a ligature as the start of a cluster when dividing ligature width among clusters. r+a=vlad

gfx/thebes/public/gfxFont.h

@@ -1271,8 +1271,8 @@ public:
         // when the part is as the end of the ligature
         gfxFloat mPartWidth;
         
-        PRPackedBool mPartIsStartOfLigature;
-        PRPackedBool mPartIsEndOfLigature;
+        PRPackedBool mClipBeforePart;
+        PRPackedBool mClipAfterPart;
     };
 
 #ifdef DEBUG

gfx/thebes/src/gfxFont.cpp

@@ -1191,7 +1191,10 @@ gfxTextRun::ComputeLigatureData(PRUint32 aPartStart, PRUint32 aPartEnd,
     PRUint32 partClusterIndex = 0;
     PRUint32 partClusterCount = 0;
     for (i = result.mLigatureStart; i < result.mLigatureEnd; ++i) {
-        if (charGlyphs[i].IsClusterStart()) {
+        // Treat the first character of the ligature as the start of a
+        // cluster for our purposes of allocating ligature width to its
+        // characters.
+        if (i == result.mLigatureStart || charGlyphs[i].IsClusterStart()) {
             ++totalClusterCount;
             if (i < aPartStart) {
                 ++partClusterIndex;
@@ -1202,8 +1205,20 @@ gfxTextRun::ComputeLigatureData(PRUint32 aPartStart, PRUint32 aPartEnd,
     }
     result.mPartAdvance = ligatureWidth*partClusterIndex/totalClusterCount;
     result.mPartWidth = ligatureWidth*partClusterCount/totalClusterCount;
-    result.mPartIsStartOfLigature = partClusterIndex == 0;
-    result.mPartIsEndOfLigature = partClusterIndex + partClusterCount == totalClusterCount;
+
+    if (partClusterCount == 0) {
+        // nothing to draw
+        result.mClipBeforePart = result.mClipAfterPart = PR_TRUE;
+    } else {
+        // Determine whether we should clip before or after this part when
+        // drawing its slice of the ligature.
+        // We need to clip before the part if any cluster is drawn before
+        // this part.
+        result.mClipBeforePart = partClusterIndex > 0;
+        // We need to clip after the part if any cluster is drawn after
+        // this part.
+        result.mClipAfterPart = partClusterIndex + partClusterCount < totalClusterCount;
+    }
 
     if (aProvider && (mFlags & gfxTextRunFactory::TEXT_ENABLE_SPACING)) {
         gfxFont::Spacing spacing;
@@ -1310,16 +1325,14 @@ static void
 ClipPartialLigature(gfxTextRun *aTextRun, gfxFloat *aLeft, gfxFloat *aRight,
                     gfxFloat aXOrigin, gfxTextRun::LigatureData *aLigature)
 {
-    if (!aLigature->mPartIsStartOfLigature) {
-        // need to clip the ligature before the part
+    if (aLigature->mClipBeforePart) {
         if (aTextRun->IsRightToLeft()) {
             *aRight = PR_MIN(*aRight, aXOrigin);
         } else {
             *aLeft = PR_MAX(*aLeft, aXOrigin);
         }
     }
-    if (!aLigature->mPartIsEndOfLigature) {
-        // need to clip the ligature after the part
+    if (aLigature->mClipAfterPart) {
         gfxFloat endEdge = aXOrigin + aTextRun->GetDirection()*aLigature->mPartWidth;
         if (aTextRun->IsRightToLeft()) {
             *aLeft = PR_MAX(*aLeft, endEdge);