From the Image Context Menu, then choose Mode > Roam.
From the Image Toolbar, click the 
button.
Roam Mode
Roam Mode is a command mode of the Image Window in which the mouse pointer (or Mouse Cursor) is free to roam about and for which there are no connections with specific commands. In Roam Mode, clicking the left mouse button inside the Image Window does not take any specific action as it does in other command modes.
Make sure the Image Window has focus.
Choose one of the following menu commands:
From the Image window's pull-down menu, View > Mode > Roam
From the Image Context Menu, then choose Mode > Roam.
From the Image Toolbar, click the button.
Roam Mode is automatically disabled when you enter any other command mode, such as Magnify Mode, Cursor Mode, or Line Profile Mode.