--- a/layout/tools/reftest/reftest-content.js
+++ b/layout/tools/reftest/reftest-content.js
@@ -645,19 +645,36 @@ function WaitForTestEnd(contentRootEleme
     function AfterPaintListener(event) {
         LogInfo("AfterPaintListener in " + event.target.document.location.href);
         if (event.target.document != currentDoc) {
             // ignore paint events for subframes or old documents in the window.
             // Invalidation in subframes will cause invalidation in the toplevel document anyway.
             return;
         }
 
+        // true if rectA contains rectB
+        function Contains(rectA, rectB) {
+            return (rectA.left <= rectB.left && rectB.right <= rectA.right && rectA.top <= rectB.top && rectB.bottom <= rectA.bottom);
+        }
+        // true if some rect in rectList contains rect
+        function ContainedIn(rectList, rect) {
+            for (let i = 0; i < rectList.length; ++i) {
+                if (Contains(rectList[i], rect)) {
+                    return true;
+                }
+            }
+            return false;
+        }
+
         updateCanvasPending = true;
         for (let i = 0; i < event.clientRects.length; ++i) {
             let r = event.clientRects[i];
+            if (ContainedIn(updateCanvasRects, r)) {
+                continue;
+            }
 
             // Copy the rect; it's content and we are chrome, which means if the
             // document goes away (and it can in some crashtests) our reference
             // to it will be turned into a dead wrapper that we can't acccess.
             updateCanvasRects.push({ left: r.left, top: r.top, right: r.right, bottom: r.bottom });
         }
 
         if (waitingForAnAfterPaint) {