If you don't recalculate the monitor's geometry before arranging, clients get arranged in the first monitor. I don't understand why this fixes the bug since tile() uses m->w rather than m->m, nor why it needs to be recalculated after creating the monitor but sway does it too.

Although not necessary to fix the bug I also made arrangelayer() do like sway again and recalculate usable_area instead of reusing m->m, since m->m seems to be incorrect until it gets recalculated shortly after in arrange(), so I suspect that leaving usable_area = m->m will cause issues under certain circumstances.

Someone with a multi-monitor setup or better knowledge of Wayland may be able to figure out the cause of the bug. For now, this makes layer shell work.