Per-output layout settings

[YaLTeR]

It is sometimes useful to change some layout properties per output. For example, one might want a smaller default column width on a bigger monitor. Or always-centering mode restricted to a particular output (#381).

The obvious way to represent it in the config is to just add the layout section to the outputs:

output "eDP-1" {
    layout {
        default-column-width { proportion 0.75; }

        border {
            on; // These need an extra "on" flag.
        }
    }
}

niri-config-wise, we need to implement merging for the layout structs (similar to window rules vs. resolved window rules), which will also be useful in the future for config include. Also, some way to deny the on flag in the global layout section without duplicating all of the structures involved.

[Stugeh]

Love the project and I'm looking forward to it getting to the point I can replace Hvprland with this. I've begun work on this feature in a way that slightly deviates from the original idea in your description of the issue. Any feedback on the approach so far? I haven't had the chance to use Rust in a 'real' project before.

All the tangible work I've done so far is present in the 2 commits above that reference this issue. I will continue adding a reference to all my commits when I get around to progressing this issue.

[YaLTeR]

Hey,

Creating new types for the Output layouts because it makes handling the
layout overriding much simpler down stream by allowing us to just ignore
fields with None instead of writing some custom solution to detect which
fields are/ aren't set by the user since the regular Layout struct
hierarchy has set deafaults for all sub fields in the hierarchy.

I suspect we may need to do the Option refactor either way for the ability to split a config into separate files. Might be worth looking into refactoring the Layout config itself to use Options. Similar to how window rules work, together with ResolvedWindowRules.

[Stugeh]

I suspect we may need to do the Option refactor either way for the ability to split a config into separate files. Might be worth looking into refactoring the Layout config itself

I'll have a look if I can reasonably fit the refactor into this feature. I suspect there might be enough work involved that it could be worth being a separate issue from this one or as a part of the multi file config issue.