Sometime when I logged on to my Gnome Linux desktop I noticed that windows decoration disappeared i.e. minimize / maximaize / close buttons and other decoration gone. How do I fix this annoying problem under Fedora / Mint / Debian / Ubuntu Linux gnome desktop?

metacity is a minimal X window manager and default for most gnome installation. It is designed to integrate well with the GNOME desktop.

metacity lacks some features that may be expected by traditional UNIX or other technical users; these users may want to investigate other available window managers for use with GNOME or standalone.

This error (or behavior) means GTK2 window manager is not working properly or crashed. If you are not using compiz or emerald windows decorator, type the following command to fix this issue:
$ metacity --replace
This will replace a window manager which is running or crashed is replaced by metacity for current session.

If you are using compiz

Type the following command:
$ compiz --replace
Also make sure, the Windows Decoration plugin which provides a window border for your windows is turned on. The decoration plugin is essentially. Type the following command:
$ ccsm
You need to click on the “Effects” > Enable “Windows Decorations”. This will fix the windows unusable problem such as window borders, resizing windows and moving windows which are initially disabled by default. Click on the “Effects” > Enable “Windows Decorations”:

Finally, type the following command to set it as default Gnome window manager:
$ gconftool-2 --type string --set /desktop/gnome/session/required_components/windowmanager compiz

If you are using emerald windows decorator

Type the following command:
$ emerald --replace

If you are using both compiz+emerald windows decorator

Type the following command:
$ compiz --replace 2>/dev/null &
$ emerald --replace 2>/dev/null &