Callers arriving after the window is published but before the creator
has drained the queue took the existing-window fast path and could run
ahead of older queued operations, so a newer SetURL could be overwritten
by an older one. withWindow now queues every caller while the creating
flag is set and clears the flag only once the queue is seen empty under
the lock.
A factory panic or a nil window left the creating flag set and the slot
dead; creation and drain now reset that state on early exit.
hideOtherWindows records the windows it hid only when no restore ran
in between, tracked by a generation counter, and re-shows them otherwise,
so a restore racing the hide cannot strand hidden windows.
Zoltán Papp
61b501984f