Function that return visible to user region and performing action on demand.

[Oleksandr Gavenko]

When I playing with:

  http://debbugs.gnu.org/cgi/bugreport.cgi?bug=12747

I got questions:

  How can I determine if selected buffer shown in any frame or in selected
  frame?

  How can I get visible to user region of buffer?

  That practices was used for delayed buffer processing? More concrete is the
  case then my code need highlight or hide text but only in portion that will
  be visible to user on display (not strictly to visible part, a few more is
  acceptable). Is that delayed practice have sense?

Please advise me best practices or point to source code that implement
asked techniques.

-- 
Best regards!