diff --git a/mozilla/layout/generic/nsBlockFrame.cpp b/mozilla/layout/generic/nsBlockFrame.cpp
index 08871532822..f73645e4d29 100644
--- a/mozilla/layout/generic/nsBlockFrame.cpp
+++ b/mozilla/layout/generic/nsBlockFrame.cpp
@@ -665,6 +665,7 @@ nsBlockFrame::GetMinWidth(nsIRenderingContext *aRenderingContext)
         }
         // XXX Bug NNNNNN Should probably handle percentage text-indent.
 
+      data.line = &line;
         nsIFrame *kid = line->mFirstChild;
         for (PRInt32 i = 0, i_end = line->GetChildCount(); i != i_end;
              ++i, kid = kid->GetNextSibling()) {
@@ -736,6 +737,7 @@ nsBlockFrame::GetPrefWidth(nsIRenderingContext *aRenderingContext)
         }
         // XXX Bug NNNNNN Should probably handle percentage text-indent.
 
+      data.line = &line;
         nsIFrame *kid = line->mFirstChild;
         for (PRInt32 i = 0, i_end = line->GetChildCount(); i != i_end;
              ++i, kid = kid->GetNextSibling()) {
@@ -4798,12 +4800,19 @@ nsBlockFrame::AddFrames(nsIFrame* aFrameList,
       prevSibLine->SetChildCount(prevSibLine->GetChildCount() - rem);
       prevSibLine->MarkDirty();
     }
+    // Force the lines next to where we're inserting content to regenerate
+    // their textruns
+    prevSibLine->SetInvalidateTextRuns(PR_TRUE);
+    if (prevSibLine.next() != end_lines()) {
+      prevSibLine.next()->SetInvalidateTextRuns(PR_TRUE);
+    }
 
     // Now (partially) join the sibling lists together
     aPrevSibling->SetNextSibling(aFrameList);
   }
   else if (! mLines.empty()) {
     prevSiblingNextFrame = mLines.front()->mFirstChild;
+    mLines.front()->SetInvalidateTextRuns(PR_TRUE);
   }
 
   // Walk through the new frames being added and update the line data
@@ -5212,6 +5221,10 @@ found_frame:;
     NS_ERROR("can't find deleted frame in lines");
     return NS_ERROR_FAILURE;
   }
+  
+  if (line != mLines.front()) {
+    line.prev()->SetInvalidateTextRuns(PR_TRUE);
+  }
 
   if (prevSibling && !prevSibling->GetNextSibling()) {
     // We must have found the first frame in the overflow line list. So
@@ -5224,6 +5237,8 @@ found_frame:;
     NS_ASSERTION(this == aDeletedFrame->GetParent(), "messed up delete code");
     NS_ASSERTION(line->Contains(aDeletedFrame), "frame not in line");
 
+    line->SetInvalidateTextRuns(PR_TRUE);
+
     // If the frame being deleted is the last one on the line then
     // optimize away the line->Contains(next-in-flow) call below.
     PRBool isLastFrameOnLine = (1 == line->GetChildCount() ||
@@ -5370,6 +5385,10 @@ found_frame:;
       }
     }
   }
+  
+  if (line.next() != line_end) {
+    line.next()->SetInvalidateTextRuns(PR_TRUE);
+  }
 
 #ifdef DEBUG
   VerifyLines(PR_TRUE);
@@ -6104,10 +6123,16 @@ nsBlockFrame::ChildIsDirty(nsIFrame* aChild)
     // otherwise we have an empty line list, and ReflowDirtyLines
     // will handle reflowing the bullet.
   } else {
-    // Mark the line containing the child frame dirty.
+    // Mark the line containing the child frame dirty. We would rather do this
+    // in MarkIntrinsicWidthsDirty but that currently won't tell us which
+    // child is being dirtied.
     line_iterator fline = FindLineFor(aChild);
-    if (fline != end_lines())
+    if (fline != end_lines()) {
+      // An inline descendant might have been added or removed, so we should
+      // reconstruct textruns.
+      fline->SetInvalidateTextRuns(PR_TRUE);
       MarkLineDirty(fline);
+    }
   }
 
   nsBlockFrameSuper::ChildIsDirty(aChild);
diff --git a/mozilla/layout/generic/nsIFrame.h b/mozilla/layout/generic/nsIFrame.h
index 66f8ca36dde..a61c8a675d3 100644
--- a/mozilla/layout/generic/nsIFrame.h
+++ b/mozilla/layout/generic/nsIFrame.h
@@ -92,6 +92,7 @@ class nsDisplayListSet;
 class nsDisplayList;
 class gfxSkipChars;
 class gfxSkipCharsIterator;
+class nsLineList_iterator;
 
 struct nsPeekOffsetStruct;
 struct nsPoint;
@@ -1137,12 +1138,16 @@ public:
    */
   struct InlineIntrinsicWidthData {
     InlineIntrinsicWidthData()
-      : prevLines(0)
+      : line(nsnull)
+      , prevLines(0)
       , currentLine(0)
       , skipWhitespace(PR_TRUE)
       , trailingWhitespace(0)
     {}
 
+    // The line. This may be null if the inlines are not associated with a block.
+    const nsLineList_iterator* line;
+
     // The maximum intrinsic width for all previous lines.
     nscoord prevLines;
 
diff --git a/mozilla/layout/generic/nsLineBox.h b/mozilla/layout/generic/nsLineBox.h
index faa97e01ec2..bb032a4a0af 100644
--- a/mozilla/layout/generic/nsLineBox.h
+++ b/mozilla/layout/generic/nsLineBox.h
@@ -305,6 +305,15 @@ public:
     return mFlags.mLineWrapped;
   }
 
+  // mInvalidateTextRuns bit
+  void SetInvalidateTextRuns(PRBool aOn) {
+    NS_ASSERTION((PR_FALSE==aOn || PR_TRUE==aOn), "somebody is playing fast and loose with bools and bits!");
+    mFlags.mInvalidateTextRuns = aOn;
+  }
+  PRBool GetInvalidateTextRuns() const {
+    return mFlags.mInvalidateTextRuns;
+  }
+
   // mResizeReflowOptimizationDisabled bit
   void DisableResizeReflowOptimization() {
     mFlags.mResizeReflowOptimizationDisabled = PR_TRUE;
@@ -487,6 +496,7 @@ public:
     PRUint32 mBlock : 1;
     PRUint32 mImpactedByFloat : 1;
     PRUint32 mLineWrapped: 1;
+    PRUint32 mInvalidateTextRuns : 1;
     PRUint32 mResizeReflowOptimizationDisabled: 1;  // default 0 = means that the opt potentially applies to this line. 1 = never skip reflowing this line for a resize reflow
     PRUint32 mEmptyCacheValid: 1;
     PRUint32 mEmptyCacheState: 1;
diff --git a/mozilla/layout/generic/nsTextFrameThebes.cpp b/mozilla/layout/generic/nsTextFrameThebes.cpp
index 0b907023ba9..2b394ad6e7e 100644
--- a/mozilla/layout/generic/nsTextFrameThebes.cpp
+++ b/mozilla/layout/generic/nsTextFrameThebes.cpp
@@ -1206,6 +1206,7 @@ BuildTextRuns(nsIRenderingContext* aRC, nsTextFrame* aForFrame,
     line = forwardIterator.GetLine();
     if (line->IsBlock())
       break;
+    line->SetInvalidateTextRuns(PR_FALSE);
     scanner.SetAtStartOfLine();
     scanner.SetCommonAncestorWithLastFrame(nsnull);
     nsIFrame* child = line->mFirstChild;
@@ -1966,7 +1967,7 @@ nsTextFrame::EnsureTextRun(nsIRenderingContext* aRC, nsIFrame* aLineContainer,
                            const nsLineList::iterator* aLine,
                            PRUint32* aFlowEndInTextRun)
 {
-  if (mTextRun) {
+  if (mTextRun && (!aLine || !(*aLine)->GetInvalidateTextRuns())) {
     if (mTextRun->GetExpirationState()->IsTracked()) {
       gTextRuns->MarkUsed(mTextRun);
     }
@@ -5001,7 +5002,7 @@ nsTextFrame::AddInlineMinWidthForFlow(nsIRenderingContext *aRenderingContext,
 {
   PRUint32 flowEndInTextRun;
   gfxSkipCharsIterator iter =
-    EnsureTextRun(aRenderingContext, nsnull, nsnull, &flowEndInTextRun);
+    EnsureTextRun(aRenderingContext, nsnull, aData->line, &flowEndInTextRun);
   if (!mTextRun)
     return;
 
@@ -5099,7 +5100,7 @@ nsTextFrame::AddInlinePrefWidthForFlow(nsIRenderingContext *aRenderingContext,
 {
   PRUint32 flowEndInTextRun;
   gfxSkipCharsIterator iter =
-    EnsureTextRun(aRenderingContext, nsnull, nsnull, &flowEndInTextRun);
+    EnsureTextRun(aRenderingContext, nsnull, aData->line, &flowEndInTextRun);
   if (!mTextRun)
     return;