IME reconversion (再変換) is not supported in AvaloniaEdit TextBox (Japanese IME)

[hisui3393]

It seems my previous explanation was unclear, so I would like to clarify the issue more precisely.

Environment:

• OS: Windows 11
• Framework: Avalonia (used in Subtitle Edit 5 beta 17)
• Control: AvaloniaEdit TextBox
• IME: Microsoft IME, Google Japanese Input

Issue:
IME reconversion (再変換) does not work in AvaloniaEdit TextBox.

Steps to reproduce:

1. Type Japanese text in the TextBox

2. Confirm the text (確定)

3. Press reconversion key:

   • Henkan key (変換キー), or
   • Ctrl + Backspace (depending on IME settings)

Expected behavior:

• The confirmed text should re-enter conversion mode (reconversion)

Actual behavior:

• Nothing happens
• Sometimes focus is lost or moved

Comparison:

• Works correctly in native Windows applications (e.g., Notepad)
• Worked correctly in Subtitle Edit 4 (WinForms)
• Reproducible with both Microsoft IME and Google Japanese Input

Important note:
This is not about basic IME input (which works), but specifically about reconversion (再変換).

Impact:
Reconversion is a standard and frequently used feature in Japanese text editing.
Lack of this functionality significantly reduces usability for Japanese users.

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If reconversion is currently not supported, is there any plan to support it?