Looks like @elextr nailed it. Window isn't "full screen" in the OS X sense, but it is "maximized" in the Windows sense (ALT/OPTION+green button), which is apparently close enough to full screen to default to tabs. Setting the "merged windows" OS X setting to manual has resolved the issue. This was a fresh install on a new machine, previous machine hadn't updated to 1.30, so I just assumed it was a 1.30 thing. I'll close this out, as it's not a Geany issue. I may see about pushing a bug upstream at some point (GTK?).