--- a/layout/base/SelectionCarets.cpp
+++ b/layout/base/SelectionCarets.cpp
@@ -295,26 +295,26 @@ IsRightToLeft(nsIFrame* aFrame)
   MOZ_ASSERT(aFrame);
 
   return aFrame->IsFrameOfType(nsIFrame::eLineParticipant) ?
     (nsBidiPresUtils::GetFrameEmbeddingLevel(aFrame) & 1) :
     aFrame->StyleVisibility()->mDirection == NS_STYLE_DIRECTION_RTL;
 }
 
 /*
- * Reduce rect to 1 app unit width along either left or right edge base on
+ * Reduce rect to 1 css pixel width along either left or right edge base on
  * aToRightEdge parameter.
  */
 static void
 ReduceRectToVerticalEdge(nsRect& aRect, bool aToRightEdge)
 {
   if (aToRightEdge) {
-    aRect.x = aRect.XMost() - 1;
+    aRect.x = aRect.XMost() - AppUnitsPerCSSPixel();
   }
-  aRect.width = 1;
+  aRect.width = AppUnitsPerCSSPixel();
 }
 
 static nsIFrame*
 FindFirstNodeWithFrame(nsIDocument* aDocument,
                        nsRange* aRange,
                        nsFrameSelection* aFrameSelection,
                        bool aBackward,
                        int& aOutOffset)