Expand WidthCache for complex text width queries

Source/platform/fonts/Font.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2003, 2006, 2010, 2011 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 133 matching lines @@
         // The complex path is more restrictive about returning fallback fonts than the simple path, so we need an explicit test to make their behaviors match.
         if (!FontPlatformFeatures::canReturnFallbackFontsForComplexText())
             fallbackFonts = 0;
         // The simple path can optimize the case where glyph overflow is not observable.
         if (codePathToUse != SimpleWithGlyphOverflowPath && (glyphOverflow && !glyphOverflow->computeBounds))
             glyphOverflow = 0;
     }
 
     bool hasKerningOrLigatures = fontDescription().typesettingFeatures() & (Kerning | Ligatures);
     bool hasWordSpacingOrLetterSpacing = fontDescription().wordSpacing() || fontDescription().letterSpacing();
-    float* cacheEntry = m_fontFallbackList->widthCache().add(run, std::numeric_limits<float>::quiet_NaN(), hasKerningOrLigatures, hasWordSpacingOrLetterSpacing, glyphOverflow);
+    bool isCacheable = (codePathToUse == ComplexPath || hasKerningOrLigatures)
+        && !hasWordSpacingOrLetterSpacing // Word spacing and letter spacing can change the width of a word.
+        && !glyphOverflow // Since this is just a width cache, we don't have enough information to satisfy glyph queries.
+        && !run.allowTabs(); // If we allow tabs and a tab occurs inside a word, the width of the word varies based on its position on the line.
+
+    float* cacheEntry = isCacheable
+        ? m_fontFallbackList->widthCache().add(run, std::numeric_limits<float>::quiet_NaN())
+        : 0;
     if (cacheEntry && !std::isnan(*cacheEntry))
         return *cacheEntry;
 
     float result;
     if (codePathToUse == ComplexPath)
         result = floatWidthForComplexText(run, fallbackFonts, glyphOverflow);
     else
         result = floatWidthForSimpleText(run, fallbackFonts, glyphOverflow);
 
     if (cacheEntry && (!fallbackFonts || fallbackFonts->isEmpty()))
@@ skipping 655 matching lines @@
                 if (delta <= 0)
                     break;
             }
         }
     }
 
     return offset;
 }
 
 }