iTerm – How to Exit Fullscreen Mode

If you're serious about terminal work, I would suggest looking at tmux, especially as new builds of iTerm 2 come with it precompiled in (though I use the 'vanilla' version). In your case, if you were using tmux you could detach the current session from the window you're in, and then reattach in a new one (i.e., full screen), or just have multiple windows attached to the same session. There are some other advantages to using tmux, but I'm sure you'll discover them as you're using the program more.

Alternatively, have iTerm show the tab bar even if you only have one tab (make sure Appearance > Hide tab bar… is unchecked), and then you can 'pull' the tab out into its own window to continue using it.

You can use iTerm2's system-wide hotkey with the Hotkey Window profile to do this.

In iTerm2 preferences, click on the "Keys" tab and choose "Hotkey". Click "Create a Dedicated Hotkey Window…" and assign the hotkey you'd like to use.

Check the "Hotkey toggles a dedicated window with profile:" option and choose "Hotkey Window" in the popup menu below (should be selected by default).

With default settings, the Hotkey Profile window will stretch across the top of the screen, and the hotkey will drop the window down from the top, complete with animation.

You can customize the settings for the "Hotkey Window" profile under the "Profiles" tab. To make it look like a Quake drop-down terminal, you can use similar "Window" preferences: