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diff --git a/devtools/server/tests/chrome/test_suspendTimeouts.js b/devtools/server/tests/chrome/test_suspendTimeouts.js
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+"use strict";
+
+// The debugger uses nsIDOMWindowUtils::suspendTimeouts and ...::resumeTimeouts
+// to ensure that content event handlers do not run while a JavaScript
+// invocation is stepping or paused at a breakpoint. If a worker thread sends
+// messages to the content while the content is paused, those messages must not
+// run until the JavaScript invocation interrupted by the debugger has completed.
+//
+// Bug 1426467 is that calling nsIDOMWindowUtils::resumeTimeouts actually
+// delivers deferred messages itself, calling the content's 'onmessage' handler.
+// But the debugger calls suspend/resume around each individual interruption of
+// the debuggee -- each step, say -- meaning that hitting the "step into" button
+// causes you to step from the debuggee directly into an onmessage handler,
+// since the onmessage handler is the next function call the debugger sees.
+//
+// In other words, delivering deferred messages from resumeTimeouts, as it is
+// used by the debugger, breaks the run-to-completion rule. They must not be
+// delivered until after the JavaScript invocation at hand is complete. That's
+// what this test checks.
+//
+// For this test to detect the bug, the following steps must take place in
+// order:
+//
+// 1) The content page must call suspendTimeouts.
+// 2) A runnable conveying a message from the worker thread must attempt to
+//    deliver the message, see that the content page has suspended such things,
+//    and hold the message for later delivery.
+// 3) The content page must call resumeTimeouts.
+//
+// In a correct implementation, the message from the worker thread is delivered
+// only after the main thread returns to the event loop after calling
+// resumeTimeouts in step 3). In the buggy implementation, the onmessage handler
+// is called directly from the call to resumeTimeouts, so that the onmessage
+// handlers run in the midst of whatever JavaScript invocation resumed timeouts
+// (say, stepping in the debugger), in violation of the run-to-completion rule.
+//
+// In this specific bug, the handlers are called from resumeTimeouts, but
+// really, running them any time before that invocation returns to the main
+// event loop would be a bug.
+//
+// Posting the message and calling resumeTimeouts take place in different
+// threads, but if 2) and 3) don't occur in that order, the worker's message
+// will never be delayed and the test will pass spuriously. But the worker
+// can't communicate with the content page directly, to let it know that it
+// should proceed with step 3): the purpose of suspendTimeouts is to pause
+// all such communication.
+//
+// So instead, the content page creates a MessageChannel, and passes one
+// MessagePort to the worker and the other to this mochitest (which has its
+// own window, separate from the one calling suspendTimeouts). The worker
+// notifies the mochitest when it has posted the message, and then the
+// mochitest calls into the content to carry out step 3).
+
+// To help you follow all the callbacks and event handlers, this code pulls out
+// event handler functions so that control flows from top to bottom.
+
+window.onload = function() {
+  // This mochitest is not complete until we call SimpleTest.finish. Don't just
+  // exit as soon as we return to the main event loop.
+  SimpleTest.waitForExplicitFinish();
+
+  const iframe = document.createElement("iframe");
+  iframe.src =
+    "http://mochi.test:8888/chrome/devtools/server/tests/chrome/suspendTimeouts_content.html";
+  iframe.onload = iframe_onload_handler;
+  document.body.appendChild(iframe);
+
+  function iframe_onload_handler() {
+    const content = iframe.contentWindow.wrappedJSObject;
+
+    const windowUtils = iframe.contentWindow.windowUtils;
+
+    // Hand over the suspend and resume functions to the content page, along
+    // with some testing utilities.
+    content.suspendTimeouts = function() {
+      SimpleTest.info("test_suspendTimeouts", "calling suspendTimeouts");
+      windowUtils.suspendTimeouts();
+    };
+    content.resumeTimeouts = function() {
+      windowUtils.resumeTimeouts();
+      SimpleTest.info("test_suspendTimeouts", "resumeTimeouts called");
+    };
+    content.info = function(message) {
+      SimpleTest.info("suspendTimeouts_content.js", message);
+    };
+    content.ok = SimpleTest.ok;
+    content.finish = finish;
+
+    SimpleTest.info(
+      "Disappointed with National Tautology Day? Well, it is what it is."
+    );
+
+    // Once the worker has sent a message to its parent (which should get delayed),
+    // it sends us a message directly on this channel.
+    const workerPort = content.create_channel();
+    workerPort.onmessage = handle_worker_echo;
+
+    // Have content send the worker a message that it should echo back to both
+    // content and us. The echo to content should get delayed; the echo to us
+    // should cause our handle_worker_echo to be called.
+    content.start_worker();
+
+    function handle_worker_echo({ data }) {
+      info(`mochitest received message from worker: ${data}`);
+
+      // As it turns out, it's not correct to assume that, if the worker posts a
+      // message to its parent via the global `postMessage` function, and then
+      // posts a message to the mochitest via the MessagePort, those two
+      // messages will be delivered in the order they were sent.
+      //
+      // - Messages sent via the worker's global's postMessage go through two
+      //   ThrottledEventQueues (one in the worker, and one on the parent), and
+      //   eventually find their way into the thread's primary event queue,
+      //   which is a PrioritizedEventQueue.
+      //
+      // - Messages sent via a MessageChannel whose ports are owned by different
+      //   threads are passed as IPDL messages.
+      //
+      // There's basically no reliable way to ensure that delivery to content
+      // has been attempted and the runnable deferred; there are too many
+      // variables affecting the order in which things are processed. Delaying
+      // for a second is the best I could think of.
+      //
+      // Fortunately, this tactic failing can only cause spurious test passes
+      // (the runnable never gets deferred, so things work by accident), not
+      // spurious failures. Without some kind of trustworthy notification that
+      // the runnable has been deferred, perhaps via some special white-box
+      // testing API, we can't do better.
+      setTimeout(() => {
+        content.resume_timeouts();
+      }, 1000);
+    }
+
+    function finish(message) {
+      SimpleTest.info("suspendTimeouts_content.js", "called finish");
+      SimpleTest.finish();
+    }
+  }
+};