Screenshot of a single window comes up blank I followed the instructions here: https://support.apple.com/en-us/HT201361


  How to take a screenshot of a window Press Shift-Command-4. The
  pointer changes to a crosshair. Press the Space bar. The pointer
  changes to a camera. Move the camera over a window to highlight it.
  Click your mouse or trackpad. To cancel, press the Esc (Escape) key
  before you click. Find the screenshot as a .png file on your desktop.


But afterwards, I get a blank image (transparent everywhere). 

Has anyone seen this issue, and if so, what am I doing wrong? I am on Sierra 10.12.6
This happens when a window has contents that are rendered outside the windowserver. Mostly 2d/3d, video and modelling software might do this. Some applications do it on purpose, like the Apple DVD player.