Holy cow, I wrote a book!
An anonymous commenter was curious about how
the GetRandomRgn function
arrived at its strange name,
what the purpose of the third parameter is,
and why it is inconsistent between Windows 95 and Windows NT.
The sense of word "random" here is
its formal probabilistic definition,
specifically sense 2:
perhaps with a bit of sense 4 sprinkled in:
"Not well organized."
(Commenter Gabe suggested that
a better name would have been GetSpecificRgn.)
Once upon a time, when men were men and Windows was 16-bit,
there was an internal function used to communicate
between the window manager and GDI in order to set up device contexts.
Internally, the region was called the Rao Region,
the programmer who invented it,
and the function that calculated the Rao Region was
rather uncreatively called
When porting to 32-bit Windows,
the Windows NT and Windows 95 teams
both found that they needed this same internal
communication between the window manager and GDI.
GDI already has a bunch of functions named
GetXxxRgn, so instead of writing a separate
marshaler for each one, they opted to write a single
GetRandomRgn function which takes an
integer which serves as a function code,
specifying which region the caller actually wants.
(I suspect the Windows 95 team followed the cue of the Windows NT
team, since Windows NT ran into the problem first.)
Since this was an internal function,
it didn't matter that the name was a bit cutesy,
nor did it matter what coordinate
system it used, as long as the window manager and GDI agreed on the
name and coordinate system.
The Windows 95 team still had a lot of 16-bit code that they
needed to be compatible with, so they chose to generate the Rao region
the same way
that the 16-bit ComputeRaoRgn function did it.
The Windows NT folks, on the other hand,
decided that it was more convenient for them
that this internal function use screen coordinates,
so that's what it returns on Windows NT.
GetRandomRgn isn't really a function that
was designed to be public.
It was just an internal helper function that outsiders discovered
and relied upon to the point that
that it became a compatibility constraint so strong that it
turned into a de facto documented function.
And all the weirdness you see behind it is the weirdness of
a function never intended for public consumption.
The introduction of the Desktop Window Manager in Windows Vista
changed the way the visible region was managed (since all windows
are logically visible even when occluded because their drawing
is redirected to an off-screen surface),
GetRandomRgn function has to keep track
of the "visible region" anyway, for compatibility.