manaflow-ai/cmux

CJK (Korean) text output is clipped/truncated in cmux but renders correctly in standalone Ghostty

Open

#338 opened on Feb 23, 2026

View on GitHub
 (4 comments) (1 reaction) (0 assignees)Swift (16,966 stars) (1,325 forks)batch import
help wanted

Description

Description

Korean (CJK) text is clipped/truncated character-by-character when rendered inside cmux, while the same text renders correctly in standalone Ghostty with identical configuration.

The issue occurs during streaming output (e.g., Claude Code responses) — Korean characters appear cut off, as if each wide character's second column is being dropped or miscalculated.

Steps to Reproduce

  1. Open standalone Ghostty → run echo "가나다라마바사아자차카타파하" → renders correctly
  2. Open cmux → run the same command → characters appear clipped/truncated
  3. The issue is most visible during streaming output from CLI tools (e.g., Claude Code)

Expected Behavior

Korean text should render identically to standalone Ghostty, since cmux uses libghostty for terminal rendering.

Actual Behavior

Korean characters are clipped — each character appears to lose its trailing column, causing text to be truncated or misaligned.

Environment

  • macOS: 26.2 (arm64, Apple Silicon)
  • cmux: 0.59.0
  • Ghostty config font: JetBrainsMono Nerd Font Mono + Sarasa Mono K (CJK fallback)
  • Locale: en_US.UTF-8
  • TERM: xterm-ghostty

Ghostty Config (relevant sections)

font-family = "JetBrainsMono Nerd Font Mono"
font-family = "Sarasa Mono K"
font-size = 14
font-thicken = true
adjust-cell-height = 15%

Notes

  • Disabling adjust-cell-height does not fix the issue in cmux (still clipped), while standalone Ghostty handles it fine either way
  • Previously ran tmux inside cmux — removed it, but the issue persists
  • Korean IME input was fixed in #118 / PR #125, but this is an output rendering issue, not input

Contributor guide

Tech stack
swift
Domain
cli
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Swiftterminal rendering conceptsCJK character handling
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
50
Research direction
Investigate how cmux's rendering of wide characters differs from standalone Ghostty. Examine the cell width calculation in libghostty integration, particularly in the rendering pipeline. Check if character widths are correctly propagated from Ghostty's pty layer to cmux's cell grid. Review commit history for any recent changes to cell width handling. Also verify that the font fallback is correctly configured in cmux's terminal renderer.