glfwSetWindowPos demonstrates virtual screen origin is not as expected [X11]

According to this image the virtual screen origin should be the same as the origin of the primary monitor. In the case of a secondary vertical monitor with virtual y-height lower (physically higher) than the primary, y=0 maps to the top of the secondary monitor, not the primary.

Given a monitor setup such as this with the secondary monitor's max height 800px above the primary, calling glfwSetWindowPos(window, 0, 800) results in the window being placed at the top of the primary screen instead of 800px below the top of the primary screen as expected.

Observed on GLFW 3.4 and 3.5 using X11.