DEV: Allow afterFramePaint to be used in tests (#27231)

We need to register a waiter so that `settled()` will wait for `runAfterFramePaint()` callbacks to be run before proceeding.

Re-lands 63b7b598cb, but wrapped with `isTesting()` to avoid production errors.

app/assets/javascripts/discourse/app/lib/after-frame-paint.js

@@ -1,8 +1,18 @@
+import DEBUG from "@glimmer/env";
+import { registerWaiter } from "@ember/test";
+import { isTesting } from "discourse-common/config/environment";
+
 /**
  * Runs `callback` shortly after the next browser Frame is produced.
  * ref: https://webperf.tips/tip/measuring-paint-time
  */
 export default function runAfterFramePaint(callback) {
+  let done = false;
+
+  if (DEBUG && isTesting()) {
+    registerWaiter(() => done);
+  }
+
   // Queue a "before Render Steps" callback via requestAnimationFrame.
   requestAnimationFrame(() => {
     // MessageChannel is one of the highest priority task queues
@@ -10,7 +20,10 @@ export default function runAfterFramePaint(callback) {
     const messageChannel = new MessageChannel();
 
     // Setup the callback to run in a Task
-    messageChannel.port1.onmessage = callback;
+    messageChannel.port1.onmessage = () => {
+      done = true;
+      callback();
+    };
 
     // Queue the Task on the Task Queue
     messageChannel.port2.postMessage(undefined);

app/assets/javascripts/discourse/tests/unit/lib/after-frame-paint-test.js

@@ -0,0 +1,16 @@
+import { settled } from "@ember/test-helpers";
+import { setupTest } from "ember-qunit";
+import { module, test } from "qunit";
+import runAfterFramePaint from "discourse/lib/after-frame-paint";
+
+module("Unit | Lib | afterFramePaint", function (hooks) {
+  setupTest(hooks);
+
+  test("should run callback correctly", async function (assert) {
+    let callbackDone = false;
+    runAfterFramePaint(() => (callbackDone = true));
+    assert.false(callbackDone, "callback was not run immediately");
+    await settled();
+    assert.true(callbackDone, "callback was run before settled resolved");
+  });
+});