PreeditHandler: update preedit string when it's empty

[liangqi]

See also https://bugreports.qt.io/browse/QTBUG-127995 , Deleted character appears again when switching languages (Mozc / IBus)

Description

Deleted character appears again when switching languages (Mozc / IBus).

Issue IDs

N/A

Steps to test new behaviors (if any)

• OS: [Ubuntu 24.04 x11]
• Steps:
  1. Japanese - Mozc, Heragana, a -> あ, for example, 3 times, then backspace 3 times
  2. Super+Space to switch to English, for example, the last あ was committed.

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[liangqi]

CLA fixed with c61b828.

[hiroyuki-komatsu]

Thank you for your PR with references. They are very helpful for us.

[hiroyuki-komatsu]

I'd like to have an explicit path when output does not have preedit.
How about this one?

bool PreeditHandler::Update(IbusEngineWrapper *engine,
                            const commands::Output &output) {
  if (!output.has_preedit()) {
    IbusTextWrapper empty_text("");
    engine->UpdatePreeditTextWithMode(&empty_text, 0);
    engine->HidePreeditText();
    return true;
  }
  IbusTextWrapper text = ComposePreeditText(output.preedit());
  engine->UpdatePreeditTextWithMode(&text, CursorPos(output));
  // |text| is released by ibus_engine_update_preedit_text.
  return true;
}

[liangqi]

Thanks, done in 6c1ddfa .