Preventing the mouse from leaving the window can be particularly useful when running applications or games that use the mouse as the primary input device.

A new option, namely "Grab the mouse input" added  into the "Input" panel of NoMachine menu inside the session can be used for confining the mouse cursor to the window.  The grabbing of the pointer will start when clicking in the client window. 

When the mouse pointer is grabbed, pressing the Ctrl+Alt key combination and clicking the left mouse button can release the pointer and give back the input to the local system.