You can toggle between moving the bottom edge and right edge by keeping the mouse button or touchpad engaged and pressing the Shift key.

Without releasing the mouse button, release the Shift key and hit it again to reposition the right edge of your selection area. 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 space bar to reposition it before releasing the mouse button to snap a screenshot.

Press and hold the space bar (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 space bar: 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 Shift-Command-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 screenshot in mac os