--- a/layout/generic/nsTextFrame.cpp
+++ b/layout/generic/nsTextFrame.cpp
@@ -4471,16 +4471,25 @@ nsTextFrame::CharacterDataChanged(Charac
   }
 
   return NS_OK;
 }
 
 /* virtual */ void
 nsTextFrame::DidSetStyleContext(nsStyleContext* aOldStyleContext)
 {
+  // A belt-and-braces check just in case we never get the
+  // MarkIntrinsicISizesDirty call from the style system.
+  if (StyleText()->mTextTransform == NS_STYLE_TEXT_TRANSFORM_CAPITALIZE &&
+      mTextRun &&
+      !(mTextRun->GetFlags() & nsTextFrameUtils::TEXT_IS_TRANSFORMED)) {
+    NS_ERROR("the current textrun doesn't match the style");
+    // The current textrun is now of the wrong type.
+    ClearTextRuns();
+  }
   nsFrame::DidSetStyleContext(aOldStyleContext);
 }
 
 class nsDisplayTextGeometry : public nsDisplayItemGenericGeometry
 {
 public:
   nsDisplayTextGeometry(nsDisplayItem* aItem, nsDisplayListBuilder* aBuilder)
     : nsDisplayItemGenericGeometry(aItem, aBuilder)