Mutations in UIElements should not be propagated / force render

[mlewand]

Suggested merge commit message (convention)

Fix: Change in UIElement view will no longer force a view render nor will it trigger mutations event on the document. Closes ckeditor/ckeditor5#5600.

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Additional information

The tests are passing, but this change needs to be carefully tested manually.

• image resize doesn't play well with it (toolbar is stuck in place during resize) - that's fine as we're going to hide it during resize (It should be possible to hide/disable a widget toolbar ckeditor5#5964)

I suggest few testing scenario:

• image style (aligning them, making sure that the toolbar moves properly)
• inline toolbar for table
• Cloud Features that use inline toolbar
• not sure if it might get with toolbar rotator?

Also it's good that we'll be able to merge it at the beginning of iteration, so there's a bigger chance that we'll catch nuances if any appear.

[Reinmar]

I think it's much more complicated to test than what you described, Marek. This PR is about changes that are not part of the editing pipeline, but instead come from the external logic and go directly to UIElements or changes that do not translate into mutations on the view tree. The problem is – it can be pretty much everything and it can happen at random moments because it may be done by the browser.

Because of that, I'd merge this ASAP and just live with it for some time and see.

[Reinmar]

Of course, Kacper and Filip, you can play with a couple of things but I think there are many more things that can go wrong so keep your eyes open :)

[Reinmar]

One thing's missing – a note in UIElement#render() that when you want to update the UI after re-rendering your element, you should call editor.ui.update().

[Reinmar]

As commented – misses one note in the UIElement#render().

[mlewand]

Pushed the changes. I'm not entirely sure though if the wording is right so please double check this.