Note: This article only applies to Windows and Mac computers running VNC Viewer.
VNC Viewer for MacOS
You need to disable "Displays have separate Spaces" in System Preferences, Mission Control.
You also need to set UseLegacyFullScreenMode to True in VNC Viewer's global Preferences, via VNC Viewer -> Preferences in the menu bar and NOT via the toolbar while connected to the VNC Server.
VNC Viewer can be configured to fill all your local screens with the remote desktop.
To do so, first connect to VNC Server:
Next, click the Properties button on the VNC Viewer toolbar:
Select the Expert tab:
UseAllMonitors into the Filter, and set the Value to True:
Click OK. Click the Enter full screen button on the VNC Viewer toolbar:
VNC Viewer will now fill all your screens. To exit full screen mode, click the Exit full screen toolbar button again.