Starting from v9.1, when the scaling mode ('Scale to window') is enabled, the NoMachine client always keeps the aspect ratio of the remote desktop while resizing the session window. Keeping the aspect ratio means that the remote desktop displayed inside the session window is scaled by maintaining proportions between width and height. Content is not stretched or distorted.
 
Note that Window Managers on Linux are free to ignore a request to resize the window to match the session aspect ratio. In such condition the user has to manually resize it until the window has the appropriate size to show correctly the session.
 
To not maintain aspect ratio of the remote desktop at runtime, use the free resize mode by pressing Control key while resizing the window.
 
To disable it completely, launch the player with the --no-aspect-ratio command line option.