Hello, I searched for answers to this question and this thread claims the issue has been solved in a past update. I've tried using both groups and stacked groups, but I still get overlaps for both. Regular groups on left, stacked groups on right:
As you can see, some groups overlap each other or non-grouped islands in both cases. My mesh contains mostly imperfectly mirrored geometry. A workaround I found for both groups and stacked groups overlapping is by changing the outline from frontier to box, but this results in wasted UV space within the box outlines:
Edit: Is there an option to offset mirrored UVs outside of the 0-1 space by exactly one unit? In the Transform tab, I add a 1 to the value in the Tu box, but the UVs don't line up perfectly when viewed in 3ds Max.