Macos – Weird behaviour/rendering on tmux in osx

macosterminaltmux

I've been having this issue for some time now, and it is really annoying, but I can't figure out what's wrong. When I start a tmux session on my terminal, no matter if it is on Terminal.app or iTerm2, I always get a weird space between the prompt and the text I type, roughly 3 times the space I get outside tmux. This wouldn't be a big issue by itself, but when I try to press backspace to delete something or use vi mode (zsh) to modify something, the cursor displays as if the text was in the right position (the way it displays outside tmux) but the text is on the wrong place, so it is impossible to know what I'm actually modifying. Also, when I press enter, I can see the text blink to the right position, but this isn't really useful at that point.

I've tried reporting the $TERM type with different options when I start tmux, but none of them seem to make it better.

Does anyone know what the issue could be?

EDIT: This is my $PROMPT/$PS1: %{$fg_bold[red]%}➜ %{$fg_bold[green]%}%p %{$fg[cyan]%}%c %{$fg_bold[blue]%}$(git_prompt_info)%{$fg_bold[blue]%} % %{$reset_color%}

Best Answer

I have seen a problem like this when using multi-byte (UTF-8 encoded) characters in the prompt, but only when tmux’s utf8 window option is turned off.

To identify such characters look for any “fancy” shapes, special symbols, or accented characters that are outside the ASCII repertoire (i.e. any Unicode codepoint beyond U+007F).

To check whether the utf8 option is active use this command in a window that is having the problem:

tmux show-option -gw utf8 \; show-option -w utf8

(or at a tmux : prompt without the leading tmux and without the backslash)

If it shows two values, then the second one is the active value for that window. The first (or only) value is the global value that will be used if there is no window-local value established.

If you are seeing a different value from the one established by your configuration file, then you probably need to restart your server (or iff the changes made by your configuration file are idempotent, then you can source it: tmux source ~/.tmux.conf).

The problem arises because (without utf8 enabled) tmux and your external terminal emulator have a different idea of how many columns are used to display each multibyte character. tmux expects a multibyte character to take up multiple columns (one for each byte), but your external emulator is probably configured to recognize it as a single UTF-8 character and thus will render it in a single column.

The problem is especially visible in a zsh prompt because zsh is careful to track where the cursor is so that is knows when the line will break and what it needs to do to properly redraw the prompt. Unfortunately, the mismatch between (non-utf8) tmux and the (UTF-8 configured) external emulator causes tmux to report a cursor position that is several columns to the right of the expected position (one extra space for each extra byte in every multibyte character in the prompt).

Enabling utf8 in tmux fixes the problem because both tmux and the external emulator will recognize the sequences for each multibyte UTF-8 codepoint as taking up a single column. Reconfiguring your external emulator to use a single-byte encoding would also fix the discrepancy, but at the cost of not being able to properly view UTF-8 encoded data (although such a view does basically show you what tmux is “thinking” when utf8 is off).

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