I see your point but that would take a very fast user to change tabs so quickly to experience this problem. At most it may cause some tests maybe to fail if they run things in parallel.

The flash is transferred via session indeed and the handler makes no difference about the originating request. If you would try to make a difference it would complicate things and, like yourself, I don't see it necessary for the moment. I am possibly the only user of this handler at the moment and I see no issues.