I plugged in a second monitor, smaller and to the right of my big primary monitor, and the desktop just nicely expanded. So far so good.
The panels however travelled to the new smaller monitor, which is not my primary monitor.
I can see how that might make sense to want in some cases, maybe. But I think the typical use case when you plug in a new monitor is that you just want more display area, not that all your stuff automatically moves. Principle of least surprise no ? It’s also much more likely you plugged in a smaller screen than a bigger screen than the first monitor you had.
Just spent an hour trying to figure out how the hell to move those panels back, painful. No amount of Ctrl/Shift/Alt/left/middle click drag would make it work (never mind that I had to remove a bunch of stuff just to have clickable panel area – still don’t get why for example the applications panel applet doesn’t give up the space around the logo to be panel area, looks like such an obvious area to release to me…). It was not locked_down in GConf, there was nothing in the right click menu to move it (except for top/bottom/left/right, all confined to monitor 2).
Hunting around I saw Lennart had the reverse problem in Fedora, and I tried all of the remedies in there. The only one that worked however is to dive into gconf-editor, go apps>panel>toplevels, and change monitor from 0 to 1 in bottom_panel and top_panel. Not sure why my primary is 1 and secondary is 0, but there you go…
I might be bothered to do something about this if I could figure out how the magic works when you plug in a second screen, but Xinerama is such a minefield of different preferences between different users, and I promised myself to not go too deep yak shaving so I get somewhere…