The code incorrectly tried to ensure that the firstVisible tab was a
valid index, even though there might not be any visible tab left after
removing the last visible tab.
The same logic didn't exist of the lastVisible tab, so we tripped the
assert in qBound, as max (being -1) ended up smaller than min (0).
Fix this by removing the wrong correcting of firstVisible to be always
valid. Make sure we emit currentChanged with -1 when no visible tab is
left after removing the current tab.
Add a test.
Fixes: QTBUG-104003
Pick-to: 6.3 6.2
Change-Id: I27e6438a02d0a0f1ac4d0e0160cee4f33b3f3766
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>
ac80cee846