- Win32: This works fine.
- macOS: The caret is always at the last of the preedit text.
- X11:
- over-the-spot: The callback is not used.
- on-the-spot: In IBus, the caret is always at the last of the preedit
text, although the actual position can be changed. On-the-spot on X11
has many other unstable behaviors, so allow this problem for now.