♻️ Change flag opening/closing logic

[fyvfyv]

There is the issue with using CSS for opening/closing logic for flags. It's broken on touch devices (which don't trigger the :hover styling) (Issue #77). To fix it, we moved all the logic to JS:

• Changed z-index to «move» a cursor behind the text. It resolves the issue, when a cursor prevent clicking on the text (even if a user clicks exactly on cursor)
• Added listeners for mousemove and touchstart with a callback, which shows/closes flags depends on a point position

To achieve it there are added a couple of callbacks to update cursors position and store it in the quill-cursors module. It should be enough to cover all cases for touch and non-touch devices. Also, there is a small throttling for mousemove events to improve performance and decrease amount of callbacks executions

[alecgibson]

@fyvfyv thanks for your patience! I've done a bit of tinkering, and I've potentially got an avenue we can explore that doesn't require constantly listening for mousemove on the entire Quill root. There's a rough proof-of-concept here, where we toggle a class in JS on hover, but also allow clicking through to the underlying link.

The basic principles are:

1. Initially listen for mouseover on the cursor only, which vastly restricts the number of events we're handling
2. When we get a mouseover, then we apply pointer-events: none (and any hover state we want) — this then lets us click the underlying element.
3. We now start a small loop of one-off mouseover handlers on the document itself: when the mouse moves, we check if it's still intersecting the cursor. If it is, then we add another one-off handler. If it's not, then we remove the hover state and reset pointer-events, which lets us pick up a hover again.

This potentially handles a lot of events if a client makes extremely small movements within the cursor (thus staying in the manual mouseout check), but given that the cursor is so small, I think this is very unlikely.

What do you think?

[fyvfyv]

@alecgibson Yeah, it might work. I'm trying to implement it in this repo and let's check