--- a/layout/generic/nsIFrame.h
+++ b/layout/generic/nsIFrame.h
@@ -729,31 +729,49 @@ public:
    */
   void SetRect(mozilla::WritingMode aWritingMode,
                const mozilla::LogicalRect& aRect,
                nscoord aContainerWidth) {
     SetRect(aRect.GetPhysicalRect(aWritingMode, aContainerWidth));
   }
 
   /**
-   * Set this frame's size from a logical size in its own writing direction
+   * Set this frame's size from a logical size in its own writing direction.
+   * This leaves the frame's logical position unchanged, which means its
+   * physical position may change (for right-to-left modes).
    */
   void SetSize(const mozilla::LogicalSize& aSize) {
     SetSize(GetWritingMode(), aSize);
   }
   /*
-   * Set this frame's size from a logical size in a different writing direction
+   * Set this frame's size from a logical size in a different writing direction.
+   * This leaves the frame's logical position in the given mode unchanged,
+   * which means its physical position may change (for right-to-left modes).
    */
   void SetSize(mozilla::WritingMode aWritingMode,
-               const mozilla::LogicalSize& aSize) {
-    SetSize(aSize.GetPhysicalSize(aWritingMode));
+               const mozilla::LogicalSize& aSize)
+  {
+    if ((!aWritingMode.IsVertical() && !aWritingMode.IsBidiLTR()) ||
+        aWritingMode.IsVerticalRL()) {
+      nscoord oldWidth = mRect.width;
+      SetSize(aSize.GetPhysicalSize(aWritingMode));
+      mRect.x -= mRect.width - oldWidth;
+    } else {
+      SetSize(aSize.GetPhysicalSize(aWritingMode));
+    }
   }
+
+  /**
+   * Set this frame's physical size. This leaves the frame's physical position
+   * (topLeft) unchanged.
+   */
   void SetSize(const nsSize& aSize) {
     SetRect(nsRect(mRect.TopLeft(), aSize));
   }
+
   void SetPosition(const nsPoint& aPt) { mRect.MoveTo(aPt); }
   void SetPosition(mozilla::WritingMode aWritingMode,
                    const mozilla::LogicalPoint& aPt,
                    nscoord aContainerWidth) {
     // We subtract mRect.width from the container width to account for
     // the fact that logical origins in RTL coordinate systems are at
     // the top right of the frame instead of the top left.
     mRect.MoveTo(aPt.GetPhysicalPoint(aWritingMode,