Regression: setting fish_color_user in fish_prompt causes the prompt to be re-drawn repeatedly forever #12233

fish, version 4.3.1
macOS 26.1

This does not occur with sh -c 'env HOME=$(mktemp -d) XDG_CONFIG_HOME= XDG_DATA_DIRS= fish'

I have the following prompt config, which I've been using in more or less the same form for over a year (and without any real modifications for months). It worked just fine in fish 4.2.1.

https://github.com/lgarron/dotfiles/blob/23948cb93d9cc0091629a261f293afe2261266df/dotfiles/fish/.config/fish/functions/fish_prompt.fish

This is what it looks like in the built-in macOS Terminal.app — it results in the prompt getting stuck in a loop and refusing input:

In iTerm.app, this also causes prompt flickering.

If I change prompt.fish to a blank text file, I successfully get the default prompt.

If I change it to the following, then I get a > prompt as I would expect:

function fish_prompt
    echo "> "
end

However, even setting the prompt to this minimal example results in a terminal where I cannot input anything:

set -g _FISH_PROMPT_LCARS_HEADER_COLOR F19E4C

function fish_prompt
    set -l fish_color_user $_FISH_PROMPT_LCARS_HEADER_COLOR
end

The terminal looks like this no matter what I try to type:

This also reproduces with cargo build --release on HEAD (fish, version 4.3.1-15-gb31387416d).

While trying to debug different reproductions, I've also run intermittently into the following:

• Extreme input lag — perhaps on the order of a second per keystroke.
• The following message:

warning: fish could not read response to Primary Device Attribute query after waiting for 2 seconds. This is often due to a missing feature in your terminal. See 'help terminal-compatibility' or 'man fish-terminal-compatibility'. This fish process will no longer wait for outstanding queries, which disables some optional features.

I'm going to attempt a bisect on master to see if I can identify anything.