Bug#475320: xserver-xorg: Strange screen size issue
This same bug has been reported on the Ubuntu and Red Hat bug tracking systems. The work-around for me was to turn the main display off by looking to see what my system was calling the laptop display. In my case, it was LVDS, and the command xrandr --output LVDS --off turned off the laptop display so and "unconstrained" my external monitor.
The explanation, from people far brighter than I, had to do with the display manager and how it handles "rectangles." Apparently, it overlays the two "rectangles," one for your laptop monitor and one for your external monitor (and one for TV OUT") and whichever one is smallest somehow determines what you see. In my case, I had a 1024x768 (max) laptop monitor and a 1440x900 (default) external widescreen monitor. Only the 1024x768 area would show up on my external monitor, exactly as you describe. As soon as I hit xrandr --output LVDS --off, the panels would spring to full-size, the screensaver would spring to full-size, and all would be well.
I hope this helps.
--David
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