Popup command input corrupts multi-byte UTF-8 typed via fcitx5 IME (orphan lead bytes, dropped chars) on 0.9.1

Environment

• herdr 0.9.1 stable (herdr-bin package, Arch Linux)
• Wayland (Hyprland), host terminal: foot
• IME: fcitx5 (Chinese, Rime), XMODIFIERS=@im=fcitx, QT_IM_MODULE=fcitx
• LANG=en_US.UTF-8

Setup

A custom command keybinding opens a popup running a trivial bash script that reads one line and logs the raw bytes it received on stdin:

[[keys.command]]
key = "prefix+/"
type = "popup"
command = "bash /path/to/log-input.sh"
width = "60%"
height = 10

#!/usr/bin/env bash
IFS= read -r line
printf 'read rc=%s len=%s hex=%s\n' "$?" "${#line}" \
  "$(printf '%s' "$line" | od -An -tx1 | tr -d ' \n')"

Symptom

Chinese text typed into the popup via fcitx5 frequently arrives on the script's stdin as invalid UTF-8: orphan lead bytes mid-string (bytes dropped from the middle of multi-byte sequences), and occasionally wrong-but-valid characters (bytes dropped/reordered rather than truncated).

Three captures from one session (expected text vs received bytes):

Each of these lines was typed normally (compose with fcitx5, commit, continue typing, Enter). Backspace during IME composition reproduces it most reliably; longer sentences hit it even without visible backspace. Pasting the same text into the popup (terminal paste) delivered clean UTF-8 in 3/3 attempts. The same IME setup types flawless Chinese into regular panes.

Downstream impact

Forwarding the line to herdr agent start -- <args> fails herdr's own argv validation:

error: argument 12 is not valid UTF-8

Arbitrary popup scripts that don't validate will silently consume corrupt input.

Notes

• Possibly related family: #3178 (remote rendering mojibake, fixed), #3499 (Korean IME drops syllable in a pane, fixed), #2894 (Chinese IME over direct SSH). This report is about the local popup command input path on 0.9.1.
• Workaround we use: sanitize with iconv -f UTF-8 -t UTF-8 -c (note it exits non-zero even with -c while still emitting the cleaned prefix), or paste instead of typing.

Happy to provide full hex dumps or test a preview build.