Respond to OSC 10 (fg color), OSC 11 (bg color) and OSC 4 (256-color) queries

[malsyned]

• Add do_osc_internal() for handling certain OSC commands within libtsm instead of delegating them to vte->osc_cb().
• Respond to OSC 10 (fg color) and OSC 11 (bg color) queries

Fix #53

[kdj0c]

Thanks for your contribution.
That's really good, I don't have much comments, you even added some unit tests.
I will merge it next Monday to give a chance to other to review.

Do you have an application that uses this, to check that it works?

[malsyned]

Do you have an application that uses this, to check that it works?

Yup! I have a Python + curses TUI 3-way merge tool I'm developing that uses this functionality to pick a suitable selection highlight color. Here's a before and after running it in GtkStm:

Before

tuimerge fails to read the background color with OSC 11, so doesn't try to add a background highlight and just highlights the gutter indicator.

After

tuimerge succeeds at querying the background color, so computes a subtle highlight color from it.

And a quick hack to color_palette_legacy to show it reacting to the different background color. (It looks better in terminals that allow assigning colors to palette entries, but it's doing its best with the 216-color indexed color palette).

Other examples

Here is that same code, adapting to the background color of Ghostty and modifying the palette entry for a closer match, to show that the code that works after this PR also works on other terminals:

[malsyned]

And why not, a GtkTsm example where it detects a light-mode theme and uses a darker highlight instead of a lighter one.

[malsyned]

Just pushed a few small style cleanups. Works exactly the same, tests still pass.

[malsyned]

I was just looking over the source for msedit and found that it uses OSC 4 as well to query the colors of the 16 SGR colors. I'm going to add support for that as well to this PR just to round out the capabilities.

[malsyned]

Just pushed a couple of commits that add support for OSC 4 queries. Here's GtkTsm and various other terminals responding to these queries, to show that they respond in the same way:

GtkTsm

XTerm

Ghostty

VTE (gnome-terminal)

[malsyned]

Adding OSC 4 ? support seems to give msedit whatever it's looking for to enable it to match its theme to the terminal's palette. Here's a before-and-after with the legacy TSM_COLOR_BACKGROUND set to [60, 20, 30]:

libtsm/main

malsyned/issue-53-osc-query

[malsyned]

Woke up this morning and realized I didn't love the structure of my OSC4 parser, so I made some tweaks. I'm done with this for now, pending change requests.