To reproduce the problem, create layout
H[view V[view view view-focused]], then switch to another workspace and
have the previously focused view in the vsplit close (eg. using
criteria, or an mpv video finishing). Return to the workspace using
`$mod+<num>` and the entire vsplit would be focused. This happens
because handle_seat_node_destroy would only set a new focus if the
currently focused view or a parent was being destroyed. To fix it, it
needs to set a sibling of the destroying container to focus_inactive
regardless of the current focus, then restore current focus if needed.
This patch changes the function accordingly. Additionally:
* The function now makes an early return if the node being destroyed is
a workspace.
* set_focus has been renamed to needs_new_focus. This variable is true
if the head focus needs to be changed.
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