Close watchers: history-action user activation is consumed even for the first close watcher

[domenic]

What is the issue with the HTML Standard?

Consider a case like

button.onclick = () => {
  const cw = new CloseWatcher();
  cw.oncancel = e => e.preventDefault();
  cw.requestClose();
};

or

button.onclick = () => {
  dialog.showModal();
  dialog.oncancel = e => e.preventDefault();
  // now the user issues a close request
};

The intent of the close request and close watcher infrastructure introduced in #9462 was that the cancel event would fire for these close watchers, consuming the history user-action user activation that was created on clicking the button. Basically, close watchers can be used to extend the required number of back gestures/presses by 1; see this section of the explainer.

However, the spec currently consumes the history user-activation created by clicking the button when establishing the CloseWatcher: https://html.spec.whatwg.org/#establish-a-close-watcher .

We should swap steps 3 and 4 in this algorithm, so that we only consume history user-activation if we've already used up the "one free close watcher" slot.

This will help ameliorate issues like https://bugs.chromium.org/p/chromium/issues/detail?id=1512224, where at least two web developers have managed to fall into the 0.000015% of page views cases that we determined was OK to break when merging #9462. (See also my summary comment on that bug.)

/cc @josepharhar @lukewarlow