Sorry to bump an old thread, but I feel it's warranted - running 1.5.19.37, I still encounter this exact same issue - completely random primary screen assignment. Was wondering if you ever solved it Eunomiac? It's pretty annoying, especially since the big screen don't even have a keyboard attached. I've even tried, in the last while loop of init.lua, setting my GPU address and manually binding it and calling setPrimary() before the shell is loaded - but still occasionally the shell starts up on the second GPU/screen.
The behavior I have noticed, is that it works most of the time - but occa