Bug 1933958 - Add a test for the widget opaque region. r=gfx-reviewers,aosmond

This will prevent regressions like bug 1933952.

Differential Revision: https://phabricator.services.mozilla.com/D230523

browser/base/content/test/performance/browser.toml

@@ -39,6 +39,8 @@ skip-if = [
 
 ["browser_hiddenwindow_existence.js"]
 
+["browser_opaque_region.js"]
+
 ["browser_panel_vsync.js"]
 support-files = ["!/browser/components/downloads/test/browser/head.js"]
 

browser/base/content/test/performance/browser_opaque_region.js

@@ -0,0 +1,36 @@
+/* Any copyright is dedicated to the Public Domain.
+   https://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+add_task(async function test_opaque_region() {
+  // Ensure we've painted.
+  await new Promise(r => requestAnimationFrame(() => requestAnimationFrame(r)));
+
+  let contentRect = document
+    .getElementById("tabbrowser-tabbox")
+    .getBoundingClientRect();
+  let opaqueRegion = window.windowUtils.getWidgetOpaqueRegion();
+
+  info(`Got opaque region: ${JSON.stringify(opaqueRegion)}`);
+  isnot(opaqueRegion.length, 0, "Should have some part of the window opaque");
+
+  let anyContainsContentRect = false;
+  let containsContentRect = opaqueRect => {
+    return (
+      opaqueRect.x <= contentRect.x &&
+      opaqueRect.y <= contentRect.y &&
+      opaqueRect.width >= contentRect.width &&
+      opaqueRect.height >= contentRect.height
+    );
+  };
+
+  for (let opaqueRect of opaqueRegion) {
+    anyContainsContentRect |= containsContentRect(opaqueRect);
+  }
+
+  ok(
+    anyContainsContentRect,
+    "The browser area should be considered opaque by widget"
+  );
+});