diff --git a/gfx/thebes/public/gfxTextRunWordCache.h b/gfx/thebes/public/gfxTextRunWordCache.h
index 9375192f6bb..8a712f693e1 100644
--- a/gfx/thebes/public/gfxTextRunWordCache.h
+++ b/gfx/thebes/public/gfxTextRunWordCache.h
@@ -20,6 +20,8 @@
  *
  * Contributor(s):
  *   Vladimir Vukicevic <vladimir@pobox.com>
+ *   Ehsan Akhgari <ehsan.akhgari@gmail.com>
+ *   Jonathan Kew <jfkthame@gmail.com>
  *
  * Alternatively, the contents of this file may be used under the terms of
  * either the GNU General Public License Version 2 or later (the "GPL"), or
@@ -48,12 +50,18 @@ class THEBES_API gfxTextRunWordCache {
 public:
     enum {
       TEXT_IN_CACHE = 0x10000000,
+
+      /**
+       * nsTextFrameThebes sets these, but they're defined here rather than in
+       * nsTextFrameUtils.h because the cache also needs to check the _INCOMING flag
+       */
+      TEXT_TRAILING_ARABICCHAR = 0x20000000,
       /**
        * When set, the previous character for this textrun was an Arabic
        * character.  This is used for the context detection necessary for
        * bidi.numeral implementation.
        */
-      TEXT_INCOMING_ARABICCHAR = 0x10000000
+      TEXT_INCOMING_ARABICCHAR = 0x40000000
     };
 
     /**
diff --git a/layout/generic/nsTextFrameThebes.cpp b/layout/generic/nsTextFrameThebes.cpp
index bff14f371c0..85559e6fc87 100644
--- a/layout/generic/nsTextFrameThebes.cpp
+++ b/layout/generic/nsTextFrameThebes.cpp
@@ -1136,7 +1136,7 @@ void BuildTextRunsScanner::FlushFrames(PRBool aFlushLineBreaks, PRBool aSuppress
   if (!mSkipIncompleteTextRuns && mCurrentFramesAllSameTextRun &&
       ((mCurrentFramesAllSameTextRun->GetFlags() & nsTextFrameUtils::TEXT_INCOMING_WHITESPACE) != 0) ==
       ((mCurrentRunContextInfo & nsTextFrameUtils::INCOMING_WHITESPACE) != 0) &&
-      ((mCurrentFramesAllSameTextRun->GetFlags() & nsTextFrameUtils::TEXT_INCOMING_ARABICCHAR) != 0) ==
+      ((mCurrentFramesAllSameTextRun->GetFlags() & gfxTextRunWordCache::TEXT_INCOMING_ARABICCHAR) != 0) ==
       ((mCurrentRunContextInfo & nsTextFrameUtils::INCOMING_ARABICCHAR) != 0) &&
       IsTextRunValidForMappedFlows(mCurrentFramesAllSameTextRun)) {
     // Optimization: We do not need to (re)build the textrun.
@@ -1149,7 +1149,7 @@ void BuildTextRunsScanner::FlushFrames(PRBool aFlushLineBreaks, PRBool aSuppress
     if (textRun->GetFlags() & nsTextFrameUtils::TEXT_TRAILING_WHITESPACE) {
       mNextRunContextInfo |= nsTextFrameUtils::INCOMING_WHITESPACE;
     }
-    if (textRun->GetFlags() & nsTextFrameUtils::TEXT_TRAILING_ARABICCHAR) {
+    if (textRun->GetFlags() & gfxTextRunWordCache::TEXT_TRAILING_ARABICCHAR) {
       mNextRunContextInfo |= nsTextFrameUtils::INCOMING_ARABICCHAR;
     }
   } else {
@@ -1485,7 +1485,7 @@ BuildTextRunsScanner::BuildTextRunForFrames(void* aTextBuffer)
     textFlags |= nsTextFrameUtils::TEXT_INCOMING_WHITESPACE;
   }
   if (mCurrentRunContextInfo & nsTextFrameUtils::INCOMING_ARABICCHAR) {
-    textFlags |= nsTextFrameUtils::TEXT_INCOMING_ARABICCHAR;
+    textFlags |= gfxTextRunWordCache::TEXT_INCOMING_ARABICCHAR;
   }
 
   nsAutoTArray<PRInt32,50> textBreakPoints;
@@ -1658,7 +1658,7 @@ BuildTextRunsScanner::BuildTextRunForFrames(void* aTextBuffer)
     textFlags |= nsTextFrameUtils::TEXT_TRAILING_WHITESPACE;
   }
   if (mNextRunContextInfo & nsTextFrameUtils::INCOMING_ARABICCHAR) {
-    textFlags |= nsTextFrameUtils::TEXT_TRAILING_ARABICCHAR;
+    textFlags |= gfxTextRunWordCache::TEXT_TRAILING_ARABICCHAR;
   }
   // ContinueTextRunAcrossFrames guarantees that it doesn't matter which
   // frame's style is used, so use the last frame's
diff --git a/layout/generic/nsTextFrameUtils.h b/layout/generic/nsTextFrameUtils.h
index 1d8905e8d08..92b8bb7b5e6 100644
--- a/layout/generic/nsTextFrameUtils.h
+++ b/layout/generic/nsTextFrameUtils.h
@@ -79,9 +79,12 @@ public:
     // We normally don't use this break opportunity because the following text
     // will have a break opportunity at the start, but it's useful for line
     // layout to know about it in case the following content is not text
-    TEXT_HAS_TRAILING_BREAK  = 0x4000000,
-    TEXT_TRAILING_ARABICCHAR = 0x8000000,
-    TEXT_INCOMING_ARABICCHAR = 0x10000000
+    TEXT_HAS_TRAILING_BREAK  = 0x4000000
+
+    // The following are defined by gfxTextRunWordCache rather than here,
+    // so that it also has access to the _INCOMING flag
+    // TEXT_TRAILING_ARABICCHAR
+    // TEXT_INCOMING_ARABICCHAR
   };
 
   // These constants are used in TransformText to represent context information