When a monitor is created or removed, the geometries of the old ones
must be updated. This is also more efficient than before since we
calculate the monitor geometries only when creating and destroying
monitors. arrangelayers() is needed to recalculate m->w. arrange() is so
clients don't move to the left monitor when plugging or unplugging
monitors (clients keep the same coordinates but the field below them
changes).
The bug was caused by usable_area's x and y not being set in
arrangelayers. For example if on a 2nd HD monitor, x should be 1920
while the first one ends at 1919. So I don't see why m->m should be
recalculated after creating the monitor.
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.
When a layer surface is destroyed focus should be returned to the last
client. Luckily if there are multiple overlays the previous overlay
still gets focused.
