guess Mastodon really doesn't play well with screens set up in portrait (I finally put a video card in here that supports multiple monitors AND got space on my desk for a second screen)
Last time I tried this, X ballsed the multiple monitors up, but this time, I got XFCE4 to do the right thing and have a properly-rotated display
Now to see if xscreensaver works (and hope to dog things stay the same when the screens are powered back up)