In `avoid-accidental-submissions`, focus primary button instead of next field #4462

If you recall, in my first ever comment about having a feature like this, I had written that:

GitHub allows in the Issues and Discussions section of repos:

• to submit a new issue/discussion with just 1 character as title and no body, and
• to submit a comment with just 1 character .

I just noticed that in Discussions this is no longer the case: if you try to submit an empty discussion, all empty elements (dropdown/input/textarea) get a red border and text balloons that "category/title/body can't be blank" :

  So, I thought of the following alternative approach: maybe we could duplicate or re-use the style (colored border + balloon text, maybe blue instead of red) and display your suggested message in that text: A submission via Enter has been prevented. You press enter again or use ctrl-enter next time maintaining the focus to the title input element the first time you press Enter, thus avoid creating a temporary Confirm? button below the title, as in your mockup?

Yeah maybe a little notice is enough, no focus change. Chrome has a simple solution to avoid accidental closures of the app via CMD+Q: It just shows a temporary overlay for a couple of second, during which you can just press the buttons again to close the app.

In this case:

1. Prevent enter
2. Show message somewhere
3. Allow a second enter on the field, either at any time later or just within the next few seconds

Adding a plain <p> as pictured before might be the easiest solution that doesn't require us to deal with error-looking styles on fields that might not support them.

This is easily implemented as:

+ const previouslyPressedEnter = new WeakSet<HTMLFormElement>();
function onKeyDown(event: delegate.Event<KeyboardEvent, HTMLInputElement>): void {
+    const {form} = event.delegateTarget;
    if (
        event.key !== 'Enter'
        || event.ctrlKey
        || event.metaKey
        || event.isComposing // #4323
        || select.exists('.suggester', form) // GitHub’s autocomplete dropdown
+        || previouslyPressedEnter.has(form)
    ) {
        return;
    }
+    previouslyPressedEnter.add(form);
+    form.prepend(
+        <p className="github’s notice classname I don't remember which">My message here</p>
+    )

Actually we can skip the WeakSet and just add a class to the existing select.exists check: .suggester, .rgh-accidental-blah-blah-notice

I just made PR #4556 based on your instructions, to facilitate closing this. You deserve all the credit of course 👍