Full screen
Press ⇧⌘3 (requires Replace macOS Screenshot Keys) or choose menu bar › Screenshot › Full Screen. OneNotch captures the screen the pointer is on.
Window
Press ⇧⌘9 or choose menu bar › Screenshot › Window. OneNotch captures the active window right away, without you selecting anything.
To capture a different window without bringing it to the front, start an area capture (⇧⌘4), press A, then click that window.
Keep window shadow (on by default, in Settings › Screenshot › Capture) keeps the shadow around the window in the image.
Tip
When you capture from the menu in the menu bar, OneNotch waits for the menu to close completely before capturing, so the menu doesn't end up in the image.