transition: region defaults to Region.ALL, not None
transition: region defaults to Region.ALL, not None

`None` means no region (e.g. the result of `Region.intersect` for
non-overlapping regions).

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester