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?

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:

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

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

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.

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)

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

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.