m key input gets deleted when fcitx5 IME (Shuangpin/Ziranma) is active in prompt input

Description

When fcitx5 IME is active (Chinese pinyin / Shuangpin "Ziranma" scheme) and typing into the prompt input, pressing the m key causes the character to appear and then immediately be deleted (a backspace is swallowed by the input). This only affects m (and similar pure-rhyme keys), and only when the IME is in Chinese mode.

Environment

• OpenCode version: v1.18.16 (also reproduced on current dev)
• OS: Linux (Ubuntu-based), X11
• Terminal: Ghostty 1.3.1 (GTK)
• IME: fcitx5 5.1.7, Shuangpin (Ziranma/Natural Code) scheme

Root cause analysis

In the Ziranma Shuangpin layout, m is the key for the rhyme ian. When the IME is active, fcitx5 intercepts m to start a composition (preedit). Since a standalone rhyme cannot form a valid syllable, fcitx5 sends a backspace to clear the preedit — but that backspace lands in the prompt input, deleting the just-typed m.

Keys that are initials or standalone vowels don't trigger this, which is why only m (and other pure-rhyme keys like l=ai, z=ei...) misbehave.

Related issues

• #39632 (IME composition breaks on the first keystroke)
• #41678, #41647

Expected behavior

The prompt input should properly handle IME composition/preedit so the backspace issued by the IME to manage preedit state is not treated as user input.

Repro steps

1. Set fcitx5 IME to Chinese pinyin (Shuangpin/Ziranma)
2. Focus the opencode prompt input
3. Press m — character appears then is deleted
4. Switch IME to English, press m again — works fine