Report color scheme should happen synchronously

[mitchellh]

Discussed in #5796

^{Originally posted by JakeWharton February 15, 2025}
Right now, querying theme state is async, and is usually delivered after responses to subsequent VT sequences.

❯ echo -ne '\e[?996n\e[5n`'; cat
`^[[0n^[[?997;1n^C⏎

❯ echo -ne '\e[?996n\e[5n`'; cat
`^[[?997;1n^[[0n^C

Here two executions of a theme query followed by DSR show that the theme response can come before the DSR or after, and it is usually after in my testing.

This is because the code is effectively double dispatching. The parser for the theme request delegates the query to the surface:

ghostty/src/termio/stream_handler.zig

Line 773 in 9a5bc65

.color_scheme => self.surfaceMessageWriter(.{ .report_color_scheme = true }),

The surface, upon receiving the dispatch:

ghostty/src/Surface.zig

Line 951 in 9a5bc65

.report_color_scheme => |force| self.reportColorScheme(force),

Enqueues a response:

ghostty/src/Surface.zig

Lines 1000 to 1005 in 9a5bc65

	const output = switch (self.config_conditional_state.theme) {
	.light => "\x1B[?997;2n",
	.dark => "\x1B[?997;1n",
	};
	self.io.queueMessage(.{ .write_stable = output }, .unlocked);

And this races the subsequent DSR parse and response.

This behavior is in contrast with something like focus, which as of #2103 responds synchronously with the current value:

ghostty/src/termio/stream_handler.zig

Lines 648 to 650 in 9a5bc65

	.focus_event => if (enabled) self.messageWriter(.{
	.focused = self.terminal.flags.focused,
	}),

❯ echo -ne '\e[?1004h\e[5n'; cat
^[[I^[[0n^C⏎

Is it possible to push theme state into the terminal so that it can respond to queries synchronously? I am working on a TUI library whose startup process sometimes sees the system theme query return before our DSR marker reply to be reflected in the initial state correctly, but more often than not sees the DSR marker reply before the system theme query response causing an unnecessary frame of potentially incorrect rendering.

Implementation Note

We can update termio.DerivedConfig to pull the Config._conditional_state so that we don't need to go back to the core Surface for this information. That way, we can avoid the async.

We should remove the report_color_scheme action from Surface if its otherwise unused.