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