So, the Windows solution is not applicable and the GTK solution does not work because it only captures a black screen.

OpenGL is primarily a drawing API and the contents of the main framebuffer are undefined unless its drawn to by OpenGL functions themself.

That OpenGL could be abused was due to the way graphics system did manage their on-screen windows framebuffers: After a window without predefined background colorbrush was created, its initial framebuffer content was simply everything that was on the screen right before the windows creation.

With compositing each window has its own off-screen framebuffer and the screens contents are composited only at the end.

If you used that method outlined above, which relies on uninitialized memory containing the desired content, on a compositing window system, the results will vary wildly, between solid clear color, over wildly distorted junk fragments, to data noise.

Also that the GTK example returns a black screen is for the same reason as its undefined behavior for OpenGL: With a compositing WM the contents of a windows DC, and be it the root DC, will not contain the image visible on the screen.

This not a good answer, and even if it were, for the wrong problem.

OP wants to use OpenGL to make screenshots, i.e.

OpenGL, but whats visible on the whole display.

No need to unproject some location and it also doesnt solve OPs problem.

And the last link you posted, it describes a technique that relies on undefined behavior that will break the instant its running on a composited window manager system.

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