Don't require action within a Promise reaction microtask.

index.bs

@@ -1966,6 +1966,40 @@ See also:
 
 * <a href="https://www.w3.org/2001/tag/doc/promises-guide">Writing Promise-Using Specifications</a>
 
+<h4 id="sync-callbacks">When to use callback functions instead</h4>
+
+<p>If an algorithm requires that callback functions respond
+synchronously (e.g., to take or veto some action), then that is a
+synchronous API, and should not be modeled with a Promise. Instead,
+use an explicit callback or event.</p>
+
+<p>In [Promise reaction callbacks](https://tc39.es/ecma262/multipage/control-abstraction-objects.html#sec-promisereaction-records),
+you may not require action within
+a fixed number of microtasks.</p>
+
+<p>This protects Promise composition: patterns like <code>await</code>,
+wrapping helper functions, logging, <code>Promise.race()</code>, or multiple
+<code>.then()</code> branches all queue an extra microtask and must
+not significantly change the behavior of the code they wrap.</p>
+
+<div class=example>
+If developer-supplied code needs to be able to call <code>event.{{Event/preventDefault()}}</code>
+or choose a response object with
+<code>fetchEvent.{{FetchEvent/respondWith()}}</code>,
+it needs to be a callback rather than a reaction passed to <code>Promise.{{Promise/then()}}</code>.
+</div>
+<div class="example">
+A non-event example is the <code>captureController.{{CaptureController/setFocusBehavior()}}</code>
+method, which pushes a window the user has chosen to screen-capture to
+the front. Applications can decide this based on the window chosen.
+But any delay risks click-jacking.
+
+Instead of requiring the method be called synchronously on the
+reaction microtask of the promise from `getDisplayMedia()`, the
+solution was to instead require the method be called on the
+same promise reaction task, with a one second timeout.
+</div>
+
 <h3 id="aborting">Cancel asynchronous APIs/operations using AbortSignal</h3>
 
 If an asynchronous method can be cancelled,
@@ -2132,6 +2166,10 @@ Follow the <a href="https://www.w3.org/2001/tag/doc/promises-guide#one-time-even
 in the <strong><a href="https://www.w3.org/2001/tag/doc/promises-guide">Writing
 Promise-Using Specifications</a></strong> guideline.
 
+<h3 id="promises-and-reaction">Don't use promises for cancelable events</h3>
+
+See <a href="#sync-callbacks">when to use callback functions instead</a>.
+
 <h3 id="promises-and-events">Events should fire before related Promises resolve</h3>
 
 If a Promise-based asynchronous algorithm dispatches events,