Calculate x and y of usable_area, not just width and heigth.
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struct wlr_box usable_area = { 0 };
struct wlr_box usable_area = *wlr_output_layout_get_box(output_layout, m->wlr_output);
wlr_output_effective_resolution(m->wlr_output, &usable_area.width, &usable_area.height);
wlr_output_effective_resolution(m->wlr_output,
&usable_area.width, &usable_area.height);