https://www.itg.ias.edu/content/keyboard-shortcuts-capture-screen-shot-mac-os-x wrote:
Specific application window:
1) To capture a specific application window, press and hold Command-Shift-4 then tap on the Spacebar. The cursor will change to a camera, and you can move it around the screen. As you move the cursor over an application window, the window will be highlighted. The entire window does not need to be visible for you to capture it. When you have the cursor over a window you want to capture, just click the mouse button and the screen shot will be saved as a PNG file on your desktop.
2) To copy a specific application window, press and hold Command-Control-Shift-4 then tap on the Spacebar. The cursor will change to a camera, which you can move around the screen. As you move the cursor over an application window, the window will be highlighted. The entire window does not need to be visible for you to capture it. When you have the cursor over a window you want to capture, just click the mouse button and you can paste the screen shot into another application.
Before reading this, I was pressing all four keys at the same time, with mixed results.
