Problem: Foreground resolution was 800x600. Background res was 1600x1200. As a result, the panels at bottom and right were beyond reach, so I couldn't use the menus. This was in root.
Solution is below, at end. Explanation follows here.
Settings said screen resolution was already correct (1600x1200). Except for the desktop pattern, everything was twice as big as if the res was wrong by that much (800x600 instead of 1600x1200). It happened in only one account (root), and only in the Gnome desktop, not in KDE's.
The desktop pattern is a horizontal gradient instead of a wallpaper, and it looked as it should. A photo wallpaper which is 1600x1200 and which I use in another account displayed as it does in the other account, at 1600x1200. I could see it edge to edge. But for everything else on the desktop, I saw only the top left quarter of the screen (as if 800x600) and at login the cursor, which was big, started in the lower right of the physical screen, that being the center of 1600x1200. If I opened an icon into a window, the window positioned relative to 1600x1200, so it wound up in the lower right (and some could easily have wound up beyond the monitor's edge). Since my panels are at bottom and right, I couldn't see them and I couldn't add any, so I couldn't access menus.
The gedit window, including document text and the toolbar, was twice as large as normal. Already maximized, it appeared its right edge was missing a few pixels, as if they were hidden behind an invisible right panel. Yet clicking where the right panel should have been did not result in panel-related action: left-clicking didn't go to another workspace and right-clicking didn't let me add a panel.
In root, /usr/bin/system-config-display (a link to /usr/bin/consolehelper), which yielded Display settings > Settings > Resolution, was already set at 1600x1200. The icon showed something akin to what was on my desktop surrounded by lots of black area, so I think the icon represented 800x600 as centered on 1600x1200.
If I logged in as one of the other 2 users on the laptop (my nonroot user account and a guest account), res was what it should be (1600x1200) in reality and according to settings from menus (where it is also set as the default for the computer only), despite recent temporary changes to 800x600 for a project. The all-accounts login screen was of normal size, as if for 1600x1200, so the problem began right after logging into root, starting with the Fedora splash screen being twice as large as normally. After other account logins, the Fedora splash screen was sized normally. When I logged out of root, the login screen started in its normal size and position and then, as soon as I started typing into it, quickly dropped to the lower half of the screen, moved to the side, and repeated. Thus, the top half of the physical screen showed a graphical hash of nearly horizontal mostly-black-white lines, the top right quarter of the login screen went to the bottom left of the physical screen, the top left quarter of the login screen went to the bottom right of the physical screen, and the bottom half of the login screen was unseen. I could type. If, right after being in root, I logged into either other account, or even into root with KDE, the result was the desktop and icons did the same thing: a hash for the top half, the two upper quarters moved to the bottom and switched in position, and the bottom half of the desktop unseen, but that can be remedied by dragging the cursor to the lower right, and once that's done if I try to unremedy it by dragging the cursor the other way I can't unremedy it (fortunately) (the unremedy is impossible because the cursor refuses to enter the hashed area or the lower left quadrant regardless of size). If I was in a nonroot account and logged into the other nonroot account, what I saw was normal.
At one point, multiload-applet-2 quit unexpectedly; I don't know if that's relevant.
Logouts and Restart did not solve the problem. Neither did shutdown, wait, and power up.
I don't know how to adjust res via a console (and this didn't seem to be exactly a res problem). man screen is about something else and whereis screen resolution yielded nothing else.
Fedora Core 4; Gnome 2.10.0. Other Display settings (colors, Hardware, and Dual head being off) appeared correct. KDE version unknown but supplied with distro.
Solution: /root/.gconf/desktop/gnome/screen/default/0/%gconf.xml said only "800x600" in XML. I deleted that file and rebooted. (I hope I shouldn't have kept the file and only deleted the content, since it was an XML file and I may not be able to reconstruct it.) Perhaps the containing folder, /0/, which had nothing else in it, even hidden, could be deleted with the file, but I don't know when it was created, so I don't know if I can delete that. I found the file by searching for "800" in content of all nonbinary files and folders created or modified recently using KDE's find function in root. I'm glad I have two desktops for every account.