Unsupported characters (specifically emojis) break the textbox

I realize Emoji aren’t supported yet, but I’d expect the � to act appropriately. I’d also worry that other characters besides emojis could cause this.

Note: This crashes instead after upgrading XamlControlsGallery to the latest CI., so this may have been fixed but I can’t be sure until #4754 is fixed.

Environment

Observed in XamlControlsGallery (which is using 0.10.0-preview5) while trying to reproduce #4754 with other Avalonia apps.

Windows 10 2004

Steps to reproduce

1. Use Win+. to insert some emoji or copy paste these ones into the example text box: 😂😂😂
2. Try clicking in the middle of the ��� rendered as a placeholder for the emoji
3. Try pressing backspace a few times to delete the emoji
4. Try pressing delete a few times to forward-delete the emoji
5. Press home
6. Type aaa
7. Press end
8. Type aaa again
9. Press Ctrl+A and then backspace
10. Paste the emoji again
11. Press home
12. Press right arrow once
13. Type aaa

Expected behavior

Sanity

Actual behavior

Insanity

1. Works mostly as expected, you see ��� which is what you’d expect until https://github.com/AvaloniaUI/Avalonia/issues/1817 is done. However, the text cursor is at the beginning of the text box rather than the end.
2. You can’t set the cursor into the middle of the emoji (it always renders as if it’s at the start
3. You can’t remove the emoji (maybe makes sense since the text cursor is at the start.)
4. You can’t remove the emoji (doesn’t make sense.)
5. ✅
6. ✅
7. Text cursor appears between aaa and ��� instead of at the end
8. The aaa appears at the end as expected, but the text cursor position is totally wrong.

1. ✅ The textbox is cleared even though deleting the emojis failed earlier
2. Same as 1
3. ✅
4. The text cursor doesn’t advance (not a surprise at this point)
5. The text stops rendering entirely and the text cursor is floating in the textbox:

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This is possibly a duplicate of https://github.com/AvaloniaUI/Avalonia/issues/4754 since it seems to largely deal with the text measuring going haywire.

[Gillibald]

Y TextBox needs to properly handle surrogate pairs and grapheme clusters. That isn’t currently the case. I will start rewriting the TextBox in October.

[PathogenDavid]

That definitely aligns with my experiences today. I think it’s probably a law of the universe that text input controls are either terrible to maintain or shatter in the face of edge cases. There is no in-between.