Bug 1437295 - Ensure that waitUntilApzStable also flushes the parent process. r=botond

With e10s enabled, we need to make sure that not only has the content
process layer tree reached the compositor, but also that the parent
process layer tree with the correct RefLayer has reached the compositor.
This is important for some APZ tests which proceed on the assumption
that the content process has been composited.

MozReview-Commit-ID: D0peZsJMHNT

--HG--
extra : rebase_source : 487a11a5478416e275013941282ff8f7636fb67c

gfx/layers/apz/test/mochitest/apz_test_utils.js

@@ -281,6 +281,60 @@ function pushPrefs(prefs) {
 }
 
 async function waitUntilApzStable() {
+  if (!SpecialPowers.isMainProcess()) {
+    // We use this waitUntilApzStable function during test initialization
+    // and for those scenarios we want to flush the parent-process layer
+    // tree to the compositor and wait for that as well. That way we know
+    // that not only is the content-process layer tree ready in the compositor,
+    // the parent-process layer tree in the compositor has the appropriate
+    // RefLayer pointing to the content-process layer tree.
+
+    // Sadly this helper function cannot reuse any code from other places because
+    // it must be totally self-contained to be shipped over to the parent process.
+    function parentProcessFlush() {
+      addMessageListener("apz-flush", function() {
+        ChromeUtils.import("resource://gre/modules/Services.jsm");
+        var topWin = Services.wm.getMostRecentWindow("navigator:browser");
+        var topUtils = topWin.QueryInterface(Ci.nsIInterfaceRequestor)
+                             .getInterface(Ci.nsIDOMWindowUtils);
+
+        var repaintDone = function() {
+          Services.obs.removeObserver(repaintDone, "apz-repaints-flushed");
+          // send message back to content process
+          sendAsyncMessage("apz-flush-done", null);
+        };
+        var flushRepaint = function() {
+          if (topUtils.isMozAfterPaintPending) {
+            topWin.addEventListener("MozAfterPaint", flushRepaint, { once: true });
+            return;
+          }
+
+          Services.obs.addObserver(repaintDone, "apz-repaints-flushed");
+          if (topUtils.flushApzRepaints()) {
+            dump("Parent process: flushed APZ repaints, waiting for callback...\n");
+          } else {
+            dump("Parent process: flushing APZ repaints was a no-op, triggering callback directly...\n");
+            repaintDone();
+          }
+        }
+
+        // Flush APZ repaints, but wait until all the pending paints have been
+        // sent.
+        flushRepaint();
+      });
+    }
+
+    // This is the first time waitUntilApzStable is being called, do initialization
+    if (typeof waitUntilApzStable.chromeHelper == "undefined") {
+      waitUntilApzStable.chromeHelper = SpecialPowers.loadChromeScript(parentProcessFlush);
+      ApzCleanup.register(() => { waitUntilApzStable.chromeHelper.destroy(); });
+    }
+
+    // Actually trigger the parent-process flush and wait for it to finish
+    waitUntilApzStable.chromeHelper.sendAsyncMessage("apz-flush", null);
+    await waitUntilApzStable.chromeHelper.promiseOneMessage("apz-flush-done");
+  }
+
   await SimpleTest.promiseFocus(window);
   await promiseAllPaintsDone();
   await promiseApzRepaintsFlushed();