Tor Browser doesn't prevent maximizing windows
Tor Browser only shows a "helpful" message after the damage has been done. It should prevent maximizing the window in the first place (at least show a "are you sure" confirmation dialog).
Maximizing can accidentally happen with window managers, when a window is moved to the screen edge or the title bar is accidentally double-clicked.
In a virtual machine, or using certain app panels, the screen resolution can be fairly unique.