The OSC calls this "tooltip" (and although a general mechanism, there's
only one instance using it). One particular problem was that with the
default OSC layout, moving the mouse down and out of the window, the
tooltip stuck around, because the returned mouse position was the last
pixel row in the window, which still overlaps with the seek bar.
Instead of introducing mouse_in_window, you could check last_mouse_X for
nil, but I think this is clearer.
This returns (-1, -1) to the caller if the mouse is outside. Kind of
random, but works.
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