The first test is to determine if the pixel at location 
in the
framebuffer is currently owned by the GL (more precisely, by this
GL context).
If it is not,
the window system decides the fate the incoming fragment.
Possible results are that the fragment is discarded or
that some subset of the subsequent per-fragment operations are applied
to the fragment.
This test allows the window system to control the GL's behavior,
for instance,
when a GL window is obscured.