Add option 'focus-at-startup' to focus a chosen output on start

[lualeet]

Adds a per-output focus-at-startup option to focus a chosen output when niri starts.

This is implemented by adding a focus_default_monitor method to State and calling it in State::new. If the option is set to null or no monitor is successfully matched, centers the cursor on the first monitor by name. Otherwise, it focuses the first detected and matching monitor by order of declaration.

The contents of the focus_default_monitor aren't nice to look at, but I couldn't get it any simpler without a temp variable dropped too early or a borrow checker upset.

[bbb651]

I can't find the issue/discussion but I remember someone was asking to have the mouse centered on startup, since move_cursor_to_output also centers the mouse maybe it should also be done for the default output when it's set to null?

[lualeet]

I can't find the issue/discussion but I remember someone was asking to have the mouse centered on startup, since move_cursor_to_output also centers the mouse maybe it should also be done for the default output when it's set to null?

Sure, that's a good idea. The sorting logic to pick a single output is duplicated a few times by now, but I don't think it should be an issue.

[YaLTeR]

Suggested change

	let config_temp = self.niri.config.clone();
	let config = config_temp.borrow();
	let config = self.niri.config.borrow();

[lualeet]

I tried that. Unfortunately,

let config = self.niri.config.borrow();

leads to E0502: cannot borrow `*self` as mutable because it is also borrowed as immutable as the immutable reference to self.config cannot be dropped before the loop is done iterating and self must be mutably borrowed to call move_cursor_to_output

Inlining it to

let config = self.niri.config.clone().borrow();

leads to E0716: temporary value dropped while borrowed

[YaLTeR]

Simplified it a bit, good to merge now.

[YaLTeR]

Thanks