Bit 3 is set here:
c5b2f55981/hw/input/ps2.c (L736)
Spurious mouse packets can be received without this bit set, for
example when double-clicking and keeping the mouse button depressed
instead of releasing it the second time (i.e. mousedown > mouseup >
mousedown). We should not process such packets.
This makes interaction with our buttons much smoother!
Fixes#5881.