Had an edge case come up last night while playing TTS.

Mortar Trooper firing at a Saber Tank.

When checking the firing arcs the very edge of the back end of the saber tank was in the front arc of the mortar trooper.
However, the section of the tank that was in the front arc was completely LOS blocked by terrain. The only part of the model that was visible to the emplacement trooper unit was the very front edge of the forks of the tank (so opposite side, completely within side arc of the mortar)

So, is it a legal shot or not?

Firing arcs and LOS are entirely separate from each other. You check them independently l. So yes that’s a legal shot.