Viewing a part of the screen in a window on Linux

You can achieve this using the xrandr extension. Read this wiki page and/or this article for more information.

Make sure that the virtual desktop is not greater than your laptop screen. With the --pos option of xrandr you can then mirror parts of your screen to the projector, e.g. for a 1600x900 laptop screen and a 1024x768 projector at position 100,100:

xrandr --fb 1600x900 --output VGA --mode 1024x768 --pos 100x100

For a dual-head presentation, you can also use the following tools (depending on the format of the presentation you uses):

• pdfpc

• pympress

• LibreOffice Impress (perhaps also OpenOffice Impress)

jofel's answer is very good. Just some other alternatives if you have another monitor and the hardware available:

• If your projector has a video passthrough (my Sharp PG-F212X-L has a VGA and DVI output) and you have the cabling for it you can hook up a third monitor on your desk.
• If you have a splitter for whatever video cabling you are using you can split one to the projector and one to a third monitor.
• You could set up VNC or something similar and if you own, say, an iPad (or an iPhone or an Android), use a VNC client to keep tabs on the second desktop area (you could even run a VNC client connected to localhost to show the second display in the client window on the first and interact with it without requiring another display device).

I know those aren't the cleanest solutions, and they all require extra setup and desk space (aside from the local VNC option), but just wanted to throw them out there as something else to think about.

squint was developed exactly for that purpose. It duplicates the content of a monitor into a window.

The window automatically pops up/down when the cursor enters/leaves the presentation screen. This allows you to switch easily between the two monitors.