The three sidebar/coupling steps add, insert and remove tabs straight from
the framework thread while the message worker mutates the same list under
TabsListLock. CurrentTabCouplingStep's Insert(0, ...) is the worst of them:
it shifts every index, so DropOldestTempTab can remove the wrong tab between
its index lookup and RemoveAt.
Locks sit around the individual mutations, never around a Draw call, so no
step holds the lock across rendering.
JonKazama-Hellion