Hold down the Shift key (after dragging to highlight an area but before releasing the mouse button or trackpad): This locks in each side of the selection area made with the crosshairs save the bottom edge, letting you move your mouse up or down to position the bottom edge.

It's very handy if your initial selection area is off by a few pixels just hold down the spacebar to reposition it before releasing the mouse button to snap a screenshot. Press and hold the spacebar (after dragging to highlight an area but before releasing the mouse button or trackpad): This locks in the shape and size of the selection area but lets you reposition it on the screen. A screenshot captured by this method features a white border around the window with a bit of a drop shadow. Click on your desired window to take a screenshot of it. Press and release the spacebar: The crosshair turns into a little camera icon, which you can move over any open window. You have a number of other options after hitting Cmd-Shift-4:

Release the mouse button or trackpad to take the shot. Use this keyboard combo to turn your cursor into a crosshair, which you can drag to select a portion of your screen to capture. This keyboard shortcut captures a screenshot of your entire screen.

How to take screenshot on mac mojave