I have a widescreen TV, but for some reason, when I plug my laptop to it (with native resolution 1366x768), the automatic and recommended resolution it sets is 1024x768. I mean, it's fine, I imagine this is the TV's modus operandi, it shomehow stretches a 4:3 image to look natural on its widescreen.
The problem is when I unplug it, my wallpaper becomes kinda zoomed in and blurry. I imagine this has something to do with it adapting to the 4:3 aspect ratio and having to revert back. Upon reapplying the wallpaper, it fixes itself. But it's a minor annoyance because I sometimes do this several times a day. Gnome and KDE don't have this problem.
I think, perhaps, you could implement some kind of script that simply reapplies the wallpaper upon unplugging the HDMI.