Is it possible to make a screencast (for example of a streaming video) on a nonactive workspace?
I usually use simplescreenrecorder for screencasts and xmonad as window manager, but the solution might involve other software.
Maybe it is possible to make the graphics card send signals to an imaginary second screen and capture the signals there?
Notice: this answer is based on my understanding of traditional X11. It is possible that this has changed with the introduction of compositing, in which case the following is wrong, but I believe it's still correct.
No, it's not possible to record what's happening on an inactive workspace.
When a workspace is "nonactive" (not shown), the windows it contains are all unmapped. The applications aren't getting expose events and therefore they aren't actually drawing anything (If an application draws a tree in a forest that isn't displayed anywhere, does the tree really exist?).
For your use case, I recommend that you run the application inside an Xvnc server and record that. Example with auto-started session:
# Launch VNC server
vncserver -geometry 1024x768 :10
# Launch a viewer so you can see what you're doing in there
xvncviewer :10
Raw example (start Xvnc directly, no security on VNC connections):
# Start VNC server
Xvnc -geometry 1024x768 :10 &
# Launch a viewer so you can see what you're doing in there
xvncviewer :10 &
# Set your display to the VNC server
DISPLAY=:10
# Launch the desired application in there as well as the recorder
xterm & # whatever your application is
xmonad & # if you want a window manager
my-screen-recorder &
