How to avoid curly underline falling back to straight underline

[krobelus]

Commit bc0e527 (Support for extended underline styles on terminals .. 2018-10-18)
added support for things like curly underline (\e[4:3m)
but if terminfo does not advertise support for this (Su/Smulx),
tmux will output a straight underline instead.

Similar behavior exists in Emacs ansi-term and emacs-vterm, GNU screen and Vim's :terminal.
But not in XTerm and most other "non-nested" terminals that don't support curly underlines yet - they simply ignore \e[4:3m.
I wonder why tmux' behavior is desirable? I guess it seems like helpful for other underline styles but not so much for the case where curly underlines should indicate a syntax error.

We're looking for a way to circumvent it, see fish-shell/fish-shell#11388 (comment)
Our goal is to not emit any underline if tmux' underlying terminal does not support it.
Are we supposed to use tmux display-message -p '#{client_termname}' and read its terminfo?
Or does tmux expose the value of tty_term_has(tty->term, TTYC_SMULX)?

There are also passthrough sequences which we could use to by pass tmux but I don't think that tmux enables them by default.
I'm mainly interested in things that can work out-of-the-box.

[nicm]

You should probably use them and let tmux decide what to do with them depending on the features of the outside terminals.

[nicm]

Falling back to plain underscore to show something is there seems more useful than not showing anything at all, but I don't feel particularly strongly about it and I doubt any users will care, so I could be convinced (ideally by a PR)...

I do think we should do the same thing for all the extra underscore styles though and not a mix of behaviours.

[nicm]

If I ask tmux "Does this terminal support Smulx?", I expect it to answer yes iff both tmux as well as the parent terminal have the Smulx capability.

No, this is not how tmux works. What if there are a mixture of outside terminals with different capabilities? What if the user detaches from a terminal without a capability after starting your program, and later attaches to one with, or vice versa? tmux's job is to offer a superset of capabilities and then map that to outside terminals depending on their capabilities.

[krobelus]

Makes sense. I wonder which other commands face this issue.
I'll try to take a look at the source and create a PR if I'm still convinced.

This shouldn't even be a problem on terminals like gnome-terminal/konsole that do support curly underlines but terminfo gives the wrong answer to tmux.
fish no longer reads terminfo (except for indn). I don't expect any practical issue with this.
But of course tmux needs to have a way to query for capabilties

[nicm]

I don't mind adding the way to query capabilities but they will always be tmux's capabilities rather than the outside terminals'.

I don't really mind whether we downgrade styled underscores down to normal underscores or discard them on terminals which don't support them.

[krobelus]

Thanks. Maybe there should be a query that tells the user whether the terminal is a multiplexer.
To allow the user to be more conservative in special cases like this one.
Not sure what would be an obvious interface for that.

[nicm]

You can use XDA (\033[q) and check if it is tmux.