Weird font behavior in Tmux

[Yappaholic]

I encountered very strange behavior with Tmux inside Ghostty.

ghostty +version
Ghostty 1.1.3-main+efc1b10

Version
  - version: 1.1.3-main+efc1b10
  - channel: tip
Build Config
  - Zig version: 0.13.0
  - build mode : builtin.OptimizeMode.ReleaseFast
  - app runtime: apprt.Runtime.gtk
  - font engine: font.main.Backend.fontconfig_freetype
  - renderer   : renderer.OpenGL
  - libxev     : io_uring
  - desktop env: other
  - GTK version:
    build      : 4.16.12
    runtime    : 4.16.12
  - libadwaita : enabled
    build      : 1.6.4
    runtime    : 1.6.4
  - libX11     : enabled
  - libwayland : enabled

When I open Tmux, everything seems fine, but then if I start typing in russian, it ouputs underscores instead of actual characters. This is exclusive to Tmux, because in Ghostty typing in russian does work properly.

Also in Neovim, when I use indent-blankline.nvim and lualine.nvim, they show underscores instead of actual icons.

This is Neovim without Tmux:

And this is Neovim in Tmux:

I'm not sure why this happens, so I would be glad to know and help, if possible.

[pluiedev]

In general if it happens only with tmux and doesn't happen with just Ghostty, then it's likely a bug with tmux (or Neovim). How tmux functions is to add its own interpretation layer between the program and the terminal and that layer can add bugs entirely outside our control, since we don't even have access to the actual program in this case.

[Yappaholic]

In general if it happens only with tmux and doesn't happen with just Ghostty, then it's likely a bug with tmux (or Neovim). How tmux functions is to add its own interpretation layer between the program and the terminal and that layer can add bugs entirely outside our control, since we don't even have access to the actual program in this case.

I get it, though it is strange how Ghostty works with tmux in comparison to other similar terminals (Kitty, Wezterm). For me tmux is not that necessary in Ghostty because of the features provided by the terminal, but it's still worth to observe and maybe fix this behavior if possible.

[00-kat]

xref Discord help post “how to fix this underscore symbol in tmux”.

Try launching tmux with the -u flag; if it works then, the issue is that your locale configuration is incorrect. Make sure your $LANG environment variable is of a UTF-8 locale (such as en_US.UTF-8).