[IME][Safari] Navigating through IME panel randomly breaks composition inside inline element

[f1ames]

Is this a bug report or feature request? (choose one)

🐞 Bug report

💻 Version of CKEditor

CKEditor 5 (v10.0.1)

📋 Steps to reproduce

Extracted from ckeditor/ckeditor5-engine#861 (see ckeditor/ckeditor5-engine#861 (comment)).

1. Go to https://ckeditor5.github.io.
2. Switch to Japanese Hiragana keyboard.
3. Start composition inside some inline element (strong for example).
4. When IME panel appears navigate with up/down arrows.

✅ Expected result

It is possible to navigate through IME panel without breaking it.

❎ Actual result

Composition stops/breaks eventually and IME panel disappears.

📃 Other details that might be useful

Happens only in Safari (11.0.3 (13604.5.6)).

Side note: I checked it while checking how https://github.com/ckeditor/ckeditor5-engine/issues/1417 behaves. There were some differences, it happened there only inside empty inline element (I suspect it was related to inline filler - and the fact that it gets removed when selection is changing inside composition panel). But at the current state, I'm not sure what is the exact cause.

[Mgsy]

This bug isn't reproducible anymore.