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diff --git a/remote/cdp/test/browser/input/browser_dispatchKeyEvent_race.js b/remote/cdp/test/browser/input/browser_dispatchKeyEvent_race.js
new file mode 100644
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+++ b/remote/cdp/test/browser/input/browser_dispatchKeyEvent_race.js
@@ -0,0 +1,92 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Here we test that the `dispatchKeyEvent` API resolves after all the synchronous event
+// handlers from the content page have been flushed.
+//
+// Say the content page has an event handler such as:
+//
+//   el.addEventListener("keyup", () => {
+//     doSomeVeryLongProcessing(); // <- takes a long time but is synchronous!
+//     window.myVariable = "newValue";
+//   });
+//
+// And imagine this is tested via:
+//
+//   await Input.dispatchKeyEvent(...);
+//   const myVariable = await Runtime.evaluate({ expression: "window.myVariable" });
+//   equals(myVariable, "newValue");
+//
+// In order for this to work, we need to be sure that `await Input.dispatchKeyEvent`
+// resolves only after the content page flushed the event handlers (and
+// `window.myVariable = "newValue"` was executed).
+//
+// This can be racy because Input.dispatchKeyEvent and window.myVariable = "newValue" run
+// in different processes.
+
+const PAGE_URL =
+  "https://example.com/browser/remote/cdp/test/browser/input/doc_dispatchKeyEvent_race.html";
+
+add_task(async function ({ client }) {
+  await loadURL(PAGE_URL);
+
+  const { Input, Runtime } = client;
+
+  // Need an enabled Runtime domain to run evaluate.
+  info("Enable the Runtime domain");
+  await Runtime.enable();
+  const { context } = await Runtime.executionContextCreated();
+
+  info("Focus the input on the page");
+  await SpecialPowers.spawn(gBrowser.selectedBrowser, [], function () {
+    const input = content.document.querySelector("input");
+    input.focus();
+    is(input, content.document.activeElement, "Input should be focused");
+  });
+
+  // See doc_input_dispatchKeyEvent_race.html
+  // The page listens to `input` events to update a property on window.
+  // We will check that the value is updated as soon dispatchKeyEvent has resolved.
+  await checkWindowTestValue("initial-value", context.id, Runtime);
+
+  info("Write 'hhhhhh' ('h' times 6)");
+  for (let i = 0; i < 6; i++) {
+    await dispatchKeyEvent(Input, "h", 72, "keyDown");
+    await dispatchKeyEvent(Input, "h", 72, "keyUp");
+  }
+  await checkWindowTestValue("hhhhhh", context.id, Runtime);
+
+  info("Write 'aaaaaa' with 6 consecutive keydown and one keyup");
+  await Promise.all([
+    dispatchKeyEvent(Input, "a", 65, "keyDown"),
+    dispatchKeyEvent(Input, "a", 65, "keyDown"),
+    dispatchKeyEvent(Input, "a", 65, "keyDown"),
+    dispatchKeyEvent(Input, "a", 65, "keyDown"),
+    dispatchKeyEvent(Input, "a", 65, "keyDown"),
+    dispatchKeyEvent(Input, "a", 65, "keyDown"),
+  ]);
+  await dispatchKeyEvent(Input, "a", 65, "keyUp");
+  await checkWindowTestValue("hhhhhhaaaaaa", context.id, Runtime);
+});
+
+function dispatchKeyEvent(Input, key, keyCode, type, modifiers = 0) {
+  info(`Send ${type} for key ${key}`);
+  return Input.dispatchKeyEvent({
+    type,
+    modifiers,
+    windowsVirtualKeyCode: keyCode,
+    key,
+  });
+}
+
+async function checkWindowTestValue(expected, contextId, Runtime) {
+  info("Retrieve the value of `window.testValue` in the test page");
+  const { result } = await Runtime.evaluate({
+    contextId,
+    expression: "window.testValue",
+  });
+
+  is(result.value, expected, "Content window test value is correct");
+}