The windows team is probably working on the third generation of the DWM but the screen shot capability is still in the dark ages, just take a look at the B8 blog:
A third-party tool should not be required to create a simple screen shot of a window in 2011.
WindowsSucks 
August 20, 2012 at 20:29 |
You can use Alt+Print Screen to screenshot a single window.
August 22, 2012 at 22:37 |
…and when you do that you get broken corners like the screenshot.
August 26, 2012 at 12:36 |
That’s because of backward compatibility. Print Screen is meant to capture exactly what you see.
September 14, 2016 at 01:35 |
The only problem I see here is that some lunatic seems to have cut out the middle of the screenshot and added some obnoxiously-colored borders?
September 16, 2016 at 20:48 |
Maybe you need glasses then. It is a well known problem that Alt+PrnScr does not handle rounded windows nor alpha transparency correctly.