DOM mutations suppress selection-change event

[luin]

We notice when DOM mutations are made inside the handler of document's selectionchange event synchronously, Quill won't fire a selection-change event.

Steps for Reproduction

1. Visit https://codepen.io/luin/pen/OJzBGXM
2. Open the dev tool and observe console logs.
3. Click on "CLICK ME".
4. Observe the logs.
5. Click on other text.
6. Observe the logs.

CleanShot.2022-04-15.at.17.12.59.mp4

Expected behavior:
Each click should trigger a selection-change log.

Actual behavior:
Nothing is logged.

Platforms:

Chrome, Firefox, and Safari.

Version: 2.0 beta

The breakdown of what happened:

1. When clicking on "CLICK ME", a mutation of adding a class to the "CLICK ME" node is made.
2. Scroll#update is triggered, and https://github.com/quilljs/quill/blob/b212f9eaea7c12efb255830f45edb41a2addb5b6/core/selection.js#L53 selection is updated silently as a result of the mutation.
3. On mouse up, Quill calls selection.update('user'), but nothing happens as the selection is already updated.