Another always-on-top window, or a shell action that hid ours, could
leave a bubble covered or hidden until restart. The fullscreen check
thread now shows every ready bubble again and puts it back at the top of
the always-on-top band once a minute, unless the user or a fullscreen
app has them hidden. Windows re-asserts HWND_TOPMOST without activating;
other platforms drop and re-set always-on-top.
A 1.5 s thread asks the platform whether the foreground window is a
fullscreen window of another process and feeds the result through the
visibility state machine. Windows keeps the proven evaluation with an
own-process check in place of the bubble class check; Linux reads the
EWMH active window state over X11; native Wayland and macOS report false.
tiennm99