--- a/gfx/thebes/gfxFT2Fonts.cpp
+++ b/gfx/thebes/gfxFT2Fonts.cpp
@@ -50,17 +50,18 @@ gfxFT2Font::ShapeText(gfxContext     *aC
                       int32_t         aScript,
                       bool            aVertical,
                       gfxShapedText  *aShapedText)
 {
     if (!gfxFont::ShapeText(aContext, aText, aOffset, aLength, aScript,
                             aVertical, aShapedText)) {
         // harfbuzz must have failed(?!), just render raw glyphs
         AddRange(aText, aOffset, aLength, aShapedText);
-        PostShapingFixup(aContext, aText, aOffset, aLength, aShapedText);
+        PostShapingFixup(aContext, aText, aOffset, aLength, aVertical,
+                         aShapedText);
     }
 
     return true;
 }
 
 void
 gfxFT2Font::AddRange(const char16_t *aText, uint32_t aOffset,
                      uint32_t aLength, gfxShapedText *aShapedText)

--- a/gfx/thebes/gfxFont.cpp
+++ b/gfx/thebes/gfxFont.cpp
@@ -2485,33 +2485,39 @@ gfxFont::ShapeText(gfxContext      *aCon
             mHarfBuzzShaper = new gfxHarfBuzzShaper(this);
         }
         ok = mHarfBuzzShaper->ShapeText(aContext, aText, aOffset, aLength,
                                         aScript, aVertical, aShapedText);
     }
 
     NS_WARN_IF_FALSE(ok, "shaper failed, expect scrambled or missing text");
 
-    PostShapingFixup(aContext, aText, aOffset, aLength, aShapedText);
+    PostShapingFixup(aContext, aText, aOffset, aLength, aVertical,
+                     aShapedText);
 
     return ok;
 }
 
 void
 gfxFont::PostShapingFixup(gfxContext      *aContext,
                           const char16_t *aText,
                           uint32_t         aOffset,
                           uint32_t         aLength,
+                          bool             aVertical,
                           gfxShapedText   *aShapedText)
 {
     if (IsSyntheticBold()) {
-        float synBoldOffset =
-                GetSyntheticBoldOffset() * CalcXScale(aContext);
-        aShapedText->AdjustAdvancesForSyntheticBold(synBoldOffset,
-                                                    aOffset, aLength);
+        const Metrics& metrics =
+            GetMetrics(aVertical ? eVertical : eHorizontal);
+        if (metrics.maxAdvance > metrics.aveCharWidth) {
+            float synBoldOffset =
+                    GetSyntheticBoldOffset() * CalcXScale(aContext);
+            aShapedText->AdjustAdvancesForSyntheticBold(synBoldOffset,
+                                                        aOffset, aLength);
+        }
     }
 }
 
 #define MAX_SHAPING_LENGTH  32760 // slightly less than 32K, trying to avoid
                                   // over-stressing platform shapers
 #define BACKTRACK_LIMIT     16 // backtrack this far looking for a good place
                                // to split into fragments for separate shaping
 

--- a/gfx/thebes/gfxFont.h
+++ b/gfx/thebes/gfxFont.h
@@ -1876,16 +1876,17 @@ protected:
 
     // Helper to adjust for synthetic bold and set character-type flags
     // in the shaped text; implementations of ShapeText should call this
     // after glyph shaping has been completed.
     void PostShapingFixup(gfxContext      *aContext,
                           const char16_t *aText,
                           uint32_t         aOffset, // position within aShapedText
                           uint32_t         aLength,
+                          bool             aVertical,
                           gfxShapedText   *aShapedText);
 
     // Shape text directly into a range within a textrun, without using the
     // font's word cache. Intended for use when the font has layout features
     // that involve space, and therefore require shaping complete runs rather
     // than isolated words, or for long strings that are inefficient to cache.
     // This will split the text on "invalid" characters (tab/newline) that are
     // not handled via normal shaping, but does not otherwise divide up the

--- a/gfx/thebes/gfxMacFont.cpp
+++ b/gfx/thebes/gfxMacFont.cpp
@@ -138,17 +138,18 @@ gfxMacFont::ShapeText(gfxContext     *aC
     // so we ignore RequiresAATLayout if vertical is requested.
     if (static_cast<MacOSFontEntry*>(GetFontEntry())->RequiresAATLayout() &&
         !aVertical) {
         if (!mCoreTextShaper) {
             mCoreTextShaper = new gfxCoreTextShaper(this);
         }
         if (mCoreTextShaper->ShapeText(aContext, aText, aOffset, aLength,
                                        aScript, aVertical, aShapedText)) {
-            PostShapingFixup(aContext, aText, aOffset, aLength, aShapedText);
+            PostShapingFixup(aContext, aText, aOffset, aLength, aVertical,
+                             aShapedText);
             return true;
         }
     }
 
     return gfxFont::ShapeText(aContext, aText, aOffset, aLength, aScript,
                               aVertical, aShapedText);
 }