Selection doesn't work in Safari 15 with Shadow Dom

[azvoncov-smartling]

Description
Bug is similar to this #3749
But the fix does not work in the latest version of Safari

Recording

Sandbox
https://codesandbox.io/s/slate-reproductions-forked-wzjois

For some reason sandbox doesn't want to register the webcomponent, but you can run this code locally

Steps
To reproduce the behavior:

1. Run Slate inside Shadow Root in Safari
2. Try to type something
3. The caret jumps to the beginning

Expectation
Same behavior as in Chrome

Environment

• Slate Version: any
• Operating System: mac
• Browser: Safari

Context
I think we can use these developments to repeat the work of getSelection
https://github.com/ProseMirror/prosemirror-view/pull/142/files

[AlekseyManetov]

@dylans can anyone look on this issue please? Currently it's impossible to use editor with shadow dom in Safari and FF browser. It looks like critical issue for such setup.

[dylans]

@AlekseyManetov I personally haven't had time for this one, but I want to support getting it fixed.

The most logical first step if possible would be to write failing tests cases so we can track this down more easily and hopefully not regress in the future. I realistically won't have time to address this on my own without failing test cases.

There are a few people who have been making good contributions to Plate under a bounty program, so that might be something to consider. Or I'm happy to review PRs.