Just popping this here for future generations of searches.

When running Torchlight II under Linux with the Unearthed Arcana mod installed, one of the enemies that spawn from broken Urns in Act 2, the Desiccated Warrior cannot be targeted by any skill or attack (incl. pets) until they, or you, move so that you are within range of their attack.

This wasn't an issue with the mod, or vanilla TL2, under Windows. It isn't an issue with vanilla TL2, or with other mods installed, in Linux.

It's a rather large mod, and if/when I have time I might try and unpack it and see if I can find the cause in GUTS but for now, if you get struck with this issue then removing UA from mods in Linux seems to be the only fix.

n.b. also tangentially related, targeting with prismatic bolt seems 'laggy' with UA installed as well.