When Trimming a shape path, how do I prevent the Offset from looping back around to the other end of an open path? It's behaving like a wormhole portal - out one end and back in the beginning... I just want it to leave.
You could always simply limit the range of your input sliders by tying Offset and the start/ end together with some simple linear() expression, i.e. if the offset exceeds a certain range, the end or start compensates. Pseudo-code to be applied to the end or start property:
Figuring out the exact values would take some experimentation, though. Unfortunately there is no simple way to derive the exact path length in pixels (unless one were to re-create the entire Bézier formula in the expression) to facilitate those calculations.