[Bug 55055] New: Japanese IME fails to clear composition string when deleting the only remaining character

[WineHQ Bugzilla]

https://bugs.winehq.org/show_bug.cgi?id=55055

Bug ID: 55055
           Summary: Japanese IME fails to clear composition string when
                    deleting the only remaining character
           Product: Wine
           Version: 8.10
          Hardware: x86-64
                OS: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: imm32
          Assignee: [e-mail]
          Reporter: [e-mail]
      Distribution: ---

Created attachment 74617
  --> https://bugs.winehq.org/attachment.cgi?id=74617
PoC patch

Environment: Linux, LANG=ja_JP.UTF-8, ATOK X3 (XIM)

Since Wine 8.9 (and possibly 8.8, but not in 8.7), if XIM input results in a
composition string being reduced to zero length, the previous composition
string remains displayed until further input.  For example, using Japanese:

1) Press 'A' -> composition string "あ" -> program displays "あ"
2) Press backspace -> composition string empty -> program still displays "あ"
3) Press 'I' -> composition string "I" -> program displays "い"

In step 2, the program should no longer display a composition string, but the
"あ" from step 1 remains displayed until further input in step 3 (at which point
the displayed composition string is correct again).

I first observed this in Final Fantasy XIV, but it reproduces in Notepad as
well.

Changing dlls/imm32/ime.c:ImeToAsciiEx() to send WM_IME_ENDCOMPOSITION when
compstr->dwCompStrLen is zero (see attached patch) seems to fix the bug without
any side effects in FFXIV and Notepad, but I have no idea what other effects
this might have, or if there needs to be some sort of deeper state tracking for
a proper fix.