Just tested trim command on 2 poly lines and R to L drag select can no longer be used in the trim command when selecting objects to trim. It works fine in V7 and V8 lost it somehow. I checked my Mouse settings and I’m set to Combo select.
Hello! Is it possible for you to post an example 3dm? I tried to replicate, but couldn’t duplicate the behavior you described.
Also, what service release are you currently on for Rhino 8?