Ok, I played around a bit with your script and I think I know what the problem is now. It seems that the size of the panel is updated too late. Probably the event is called even after PROC_LATE-functions, but I think this would only explain a delay of one frame. I'll look into it and try to fix it or I'll put the workaround into the manual of the next LBGUI version (which I'm already working on ).

Here's the workaround: Put this line
LBG_manage_progressbar(my_progressbar);
into the event after you set the progress. Then it should work.