Composing text shouldn't be part of undo/redo

[justinmc]

When using a composing IME, composing text should not be counted as a part of the undo/redo stack.

Before (from issue)	After

CC @tgucio

Fixes #106473

[LongCatIsLooong]

When using a composing IME, composing text should not be counted as a part of the undo/redo stack.

That depends on how the IME uses the composing region? Gboard puts everything in a composing region so this would prevent undo-redo from working if the user is using gboard.

[LongCatIsLooong]

can composing be null? The next expression wouldn't have passed the analyzer if it could?

[justinmc]

Good catch, I feel like that should be caught by the analyzer...

[justinmc]

@LongCatIsLooong Shucks good point.

It looks like Gboard does not have undo/redo capabilities [0]. I tried hooking up a keyboard to my Android phone and ctrl-z does indeed work on both native and Flutter. If I go so far as to type a bunch of text with Gboard, then plug in a keyboard and start pressing ctrl-z, Flutter's coalescing happens only between words, while native sometimes undoes only part of a word. That's incorrect but a pretty subtle and niche difference.

Do you think it's better if I check the platform and allow coalescing during composing on mobile, or another approach? Or is the current way best?

[0] The Google Keep app has its own undo/redo buttons, and it does coalesce partially composed words. A Flutter dev could implement that if they wanted, though. Other apps have no way to undo/redo with Gboard.

[justinmc]

@LongCatIsLooong I updated this PR to allow coalescing of composing text on Android only. This isn't perfect but it seems to handle the more common use case:

Scenario	This PR as-is, which doesn't coalesce composing text	Only coalesce composing text on Android, not elsewhere
Typing English on Android on a hardware keyboard with Gboard enabled	❌ Coalesces whole words.	✅ Coalesces partial words
Typing CJK on Android on a hardware keyboard with Gboard enabled	✅ Coalesces final characters.	✅ Coalesces final characters.
Typing CJK on Android on a hardware keyboard with a software keyboard that shows composing text in the field	✅ Coalesces final characters.	❌ Coalesces composing text.
Typing English everywhere else	✅ Coalesces partial words.	✅ Coalesces partial words.
Typing CJK everywhere else	✅ Coalesces final characters.	✅ Coalesces final characters.

Let me know what you think.

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