Amiga also had explicit close buttons very early, Mac-style (and also had full pre-emptive multitasking in its very earliest days, which happened in 1984). I've seen pre-release screenshots of revision 24.24 of Workbench that had them (for reference, v1.0 of WB was approximately revision 30, 24.24 was in the era of the Velvet prototype where the system couldn't fully bootstrap itself)