Replace async act in cleanup to still flush microtasks without swallowing missing act warnings

Author: mpeyper

src/cleanup.js

@@ -1,9 +1,9 @@
-import { act } from 'react-test-renderer'
+import flushMicroTasks from './flush-microtasks'
 
 let cleanupCallbacks = []
 
 async function cleanup() {
-  await act(async () => {})
+  await flushMicroTasks()
   cleanupCallbacks.forEach((cb) => cb())
   cleanupCallbacks = []
 }

src/flush-microtasks.js

@@ -0,0 +1,48 @@
+/* istanbul ignore file */
+// the part of this file that we need tested is definitely being run
+// and the part that is not cannot easily have useful tests written
+// anyway. So we're just going to ignore coverage for this file
+/**
+ * copied from React's enqueueTask.js
+ * copied again from React Testing Library's flush-microtasks.js
+ */
+
+let didWarnAboutMessageChannel = false
+let enqueueTask
+try {
+  // read require off the module object to get around the bundlers.
+  // we don't want them to detect a require and bundle a Node polyfill.
+  const requireString = `require${Math.random()}`.slice(0, 7)
+  const nodeRequire = module && module[requireString]
+  // assuming we're in node, let's try to get node's
+  // version of setImmediate, bypassing fake timers if any.
+  enqueueTask = nodeRequire('timers').setImmediate
+} catch (_err) {
+  // we're in a browser
+  // we can't use regular timers because they may still be faked
+  // so we try MessageChannel+postMessage instead
+  enqueueTask = (callback) => {
+    if (didWarnAboutMessageChannel === false) {
+      didWarnAboutMessageChannel = true
+      // eslint-disable-next-line no-console
+      console.error(
+        typeof MessageChannel !== 'undefined',
+        'This browser does not have a MessageChannel implementation, ' +
+          'so enqueuing tasks via await act(async () => ...) will fail. ' +
+          'Please file an issue at https://github.com/facebook/react/issues ' +
+          'if you encounter this warning.'
+      )
+    }
+    const channel = new MessageChannel()
+    channel.port1.onmessage = callback
+    channel.port2.postMessage(undefined)
+  }
+}
+
+export default function flushMicroTasks() {
+  return {
+    then(resolve) {
+      enqueueTask(resolve)
+    }
+  }
+}