Selection check infinite loop kicks in too often

[Reinmar]

1. Go to https://ckeditor5.github.io/
2. Put selection in the first heading.
3. Start pressing up, down, up, down arrows quickly.

If you do it frequently enough and repeat enough times you got:

selectionchange-infinite-loop: Selection change observer detected an infinite rendering loop. undefined

and the seleciton stops refreshing (the label of the headings dropdown stops updating).

Now, god bless our marvellous idea to log stuff like this to the console!!! Imagine the hours spent on debugging this security check if it ended up with a silent errror.

[Reinmar]

This is even easier to trigger (which makes it pretty critical). Just keep pressing backspace for a longer time. The warning will be logged pretty quickly.

[scofalik]

After fixing #3701 selection infinite loop kicks in also happen at the edge of <strong> element.

[scofalik]

This is even easier to trigger (which makes it pretty critical). Just keep pressing backspace for a longer time. The warning will be logged pretty quickly.

I can't reproduce this. However in this case:

Go to https://ckeditor5.github.io/
Put selection in the first heading.
Start pressing up, down, up, down arrows quickly.

The warning/check kicks in incorrectly. When you comment out the whole if nothing breaks or crashes (no infinite loop).

[scofalik]

Okay it appears that the _loopbackCounter in SelectionObserver is not cleared on user action which is just wrong. You can trigger infinite loop by pressing left and right arrow key and you don't even have to do it fast. The infinite loop condition should kick in only if there were multiple dom selection changes without user interaction. Do you have an idea how to check it without having interaction between other observers?

Maybe clear it on timeout? Or add keydown and mousedown event listeners here: https://github.com/ckeditor/ckeditor5-engine/blob/master/src/view/observer/selectionobserver.js#L104 that will clear it.

[Reinmar]

I think that what we want to check is the number of selection changes in a period of time. Like whether 100 last changes happened in a second. If that's true, then we have loop.

The first way to implement this which comes to my mind is having an array of last change timestamps. You add the current one and checks the one that you remove (like in a FIFO queue). I guess there may be smarter ways too :D.

Anyway, this should not fail us. In fact, this is how I thought it worked.

[Reinmar]

PS. The numbers must be chosen in such a way that it won't fail in automated tests :D. But it should be fine if in every beforeEach() we create a new observer. If not, then this is test what's broken.

[Reinmar]

Fixed by ckeditor/ckeditor5-engine#671.