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diff --git a/devtools/server/tests/xpcshell/test_promises_run_to_completion.js b/devtools/server/tests/xpcshell/test_promises_run_to_completion.js
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+// Bug 1145201:  Promise then-handlers can still be executed while the debugger is paused.
+//
+// When a promise is resolved, for each of its callbacks, a microtask is queued
+// to run the callback. At various points, the HTML spec says the browser must
+// "perform a microtask checkpoint", which means to draw microtasks from the
+// queue and run them, until the queue is empty.
+//
+// The HTML spec is careful to perform a microtask checkpoint directly after
+// each invocation of an event handler or DOM callback, so that code using
+// promises can trust that its promise callbacks run promptly, in a
+// deterministic order, without DOM events or other outside influences
+// intervening.
+//
+// When the JavaScript debugger interrupts the execution of debuggee content
+// code, it naturally must process events for its own user interface and promise
+// callbacks. However, it must not run any debuggee microtasks. The debuggee has
+// been interrupted in the midst of executing some other code, and the
+// JavaScript spec promises developers: "Once execution of a Job is initiated,
+// the Job always executes to completion. No other Job may be initiated until
+// the currently running Job completes." [1] This promise would be broken if the
+// debugger's own event processing ran debuggee microtasks during the
+// interruption.
+//
+// Looking at things from the other side, a microtask checkpoint must be
+// performed before returning from a debugger callback, rather than being put
+// off until the debuggee performs its next microtask checkpoint, so that
+// debugger microtasks are not interleaved with debuggee microtasks. A debuggee
+// microtask could hit a breakpoint or otherwise re-enter the debugger, which
+// might be quite surprised to see a new debugger callback begin before its
+// previous promise callbacks could finish.
+//
+// [1]: https://www.ecma-international.org/ecma-262/9.0/index.html#sec-jobs-and-job-queues
+
+"use strict";
+
+const Debugger = require("Debugger");
+
+function test_promises_run_to_completion() {
+  const g = createTestGlobal(
+    "test global for test_promises_run_to_completion.js"
+  );
+  const dbg = new Debugger(g);
+  g.Assert = Assert;
+  const log = [""];
+  g.log = log;
+
+  dbg.onDebuggerStatement = function handleDebuggerStatement(frame) {
+    dbg.onDebuggerStatement = undefined;
+
+    // Exercise the promise machinery: resolve a promise and perform a microtask
+    // queue. When called from a debugger hook, the debuggee's microtasks should not
+    // run.
+    log[0] += "debug-handler(";
+    Promise.resolve(42).then(v => {
+      Assert.equal(
+        v,
+        42,
+        "debugger callback promise handler got the right value"
+      );
+      log[0] += "debug-react";
+    });
+    log[0] += "(";
+    force_microtask_checkpoint();
+    log[0] += ")";
+
+    Promise.resolve(42).then(v => {
+      // The microtask running this callback should be handled as we leave the
+      // onDebuggerStatement Debugger callback, and should not be interleaved
+      // with debuggee microtasks.
+      log[0] += "(trailing)";
+    });
+
+    log[0] += ")";
+  };
+
+  // Evaluate some debuggee code that resolves a promise, and then enters the debugger.
+  Cu.evalInSandbox(
+    `
+    log[0] += "eval(";
+    Promise.resolve(42).then(function debuggeePromiseCallback(v) {
+      Assert.equal(v, 42, "debuggee promise handler got the right value");
+      // Debugger microtask checkpoints must not run debuggee microtasks, so
+      // this callback should run at the next microtask checkpoint *not*
+      // performed by the debugger.
+      log[0] += "eval-react";
+    });
+
+    log[0] += "debugger(";
+    debugger;
+    log[0] += "))";
+  `,
+    g
+  );
+
+  // Let other microtasks run. This should run the debuggee's promise callback.
+  log[0] += "final(";
+  force_microtask_checkpoint();
+  log[0] += ")";
+
+  Assert.equal(
+    log[0],
+    `\
+eval(\
+debugger(\
+debug-handler(\
+(debug-react)\
+)\
+(trailing)\
+))\
+final(\
+eval-react\
+)`,
+    "microtasks ran as expected"
+  );
+
+  run_next_test();
+}
+
+function force_microtask_checkpoint() {
+  // Services.tm.spinEventLoopUntilEmpty only performs a microtask checkpoint if
+  // there is actually an event to run. So make one up.
+  let ran = false;
+  Services.tm.dispatchToMainThread(() => {
+    ran = true;
+  });
+  Services.tm.spinEventLoopUntil(
+    "Test(test_promises_run_to_completion.js:force_microtask_checkpoint)",
+    () => ran
+  );
+}
+
+add_test(test_promises_run_to_completion);