Repro steps:

1. In a textfield such as in https://github.com/cbracken/jp_flutter switch input mode to Japanese, then enter the text 日本語 (type: 'nihonngo').
2. Press backspace.

Expected result: Textfield contains the text にほん
Actual result: Textfield contains the text にほん日本

Taking a look at this under lldb:

1. The character 語 is deleted from the end of the string
2. The composing region is incorrectly collapsed to (0,0)
3. TextInputModel::UpdateComposingText is called with the new composing text (にほん)

As a result, the new composing text is injected into the string at (0,0) rather than replacing the text 日本語.

/cc @dkwingsmt since I'm wondering if this may be related to the new direct keypress handling.

Part of #30660