It competes with the Alt behavior in Photoshop, so I want the left Alt key to do nothing.
You can change it from Alt to Super and vice-versa. Go to System ➜ Preferences ➜ Windows and in the Windows Preferences Choose which "Movement Key" you wish to use.
A screenshot to illustrate is placed here:
Additionally, if you are using compiz, you can change which key bindings you wish to use in order to "Initiate Window Move" when the "Enable Move Window" plugin is in use.
Here is a screenshot to illustrate what I say in the above words:
You can also change the X key from command line (Terminal):
Gnome 2 (Ubuntu 11.10)
sudo apt-get install -y gconf2 gconftool-2 -t str --set /apps/metacity/general/mouse_button_modifier '<Super>'
Gnome 3 (Ubuntu 13.04)
sudo apt-get install -y libglib2.0-bin gsettings set org.gnome.desktop.wm.preferences mouse-button-modifier '<Super>'
Xfwm4 window manager = Xubuntu 16.04 = Xfce mouse menu;
- application = Window Manager Tweaks;
- tab = Accessibility;
- firstLine = Key used to grab and move windows: Super (make your choice);
/* LibreOffice uses Alt-drag to move columns and rows. Window manager Xfwm4 interprets an Alt-drag before LibreOffice if assigned. */