[cairo] is self-copy supposed to work?

[Jonathan Morton]

>> I also suppose that a lot of people would expect self-copies to do
>> special magic (like XCopyArea), and I think trying to provide that is
>> both a lot of work and often impossible anyway.
>
> Why would entering an error state be better than just introducing a
> temporary surface internally?

It's not necessarily better, but it's consistent with, eg. OpenGL.
Where render-to-texture is supported, the spec explicity leaves
undefined results when the target texture is also bound as a source
texture.

There's a design decision to be made here.  Does Cairo only ever do
what the user explicitly requests, or does it attempt to divine what
the user means to do?  The latter is what *you* want, and many would
agree with you, but the exact boundaries of that divination need to be
specified carefully so as not to leave further bugs or undefined
loopholes.

 - Jonathan