Freeze command menu in place when task leaves n-window

[MetRonnie]

I found one "quirk" when a task disappears from the n-window and is removed from the view, any open menu will jump to the top-left of the screen (because the target node has been removed).

Would be beneficial if we could disable this behaviour, but the menu remains functional so not a biggie.

Originally posted by @oliver-sanders in https://github.com/cylc/cylc-ui/pull/1775#pullrequestreview-2070816459