Off-screen rendering is quite common. The most basic use is to display a window double-buffered to avoid flickering - usually done with animations or window managers.
To implement it you need to write programs using one or more graphic libraries, like Xlib, Cairo, Allegro or similar.
A graphic canvas is actually just a memory allocation. To make it show up on your monitor it has to be copied to the video memory of your video device (monitor).
The term off-screen window can also mean a window that is simply placed outside of the visible part of the display. This was quite common with old Mac systems, where windows where hidden simply by moving them out from the visible view.
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