Bug 1593170. Avoid useless painting work in queued up rects to paint by ignoring new rects that are already covered. r=mattwoodrow

I don't think this is strictly necessary but it lets us avoid a bunch of useless work, especially with webrender where these rects are always the full window size.

Differential Revision: https://phabricator.services.mozilla.com/D51346

--HG--
extra : moz-landing-system : lando

layout/tools/reftest/reftest-content.js

@@ -650,9 +650,26 @@ function WaitForTestEnd(contentRootElement, inPrintMode, spellCheckedElements, f
             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