The sidestep thing I first noticed in XCOM, but I agree it breaks in such close environments when applying both to shooter and target. It’s elegant, but hard to represent to the player with visuals like that (much more detailed than the gameplay granularity). It is a hard problem to deal with.
Corner to corner LOS (LOS checked between the 4 corners of the target and the 4 corners of the shooter) should work given their examples of edge cases, but I’m sure there will be still issues with that solution (it’s close, but not equal to, to half sidestep, instead of full sidestep).