middle_click_ungrab
The option documentation is properly formatted yet
Description: "Ungrab Input on Middle Mouse Button" Default: 1 Example: 0 Type: boolean
If you want to use the middle mouse button in the emulated Amiga, you can disable this option. F12+G can be used to grab/ungrab.
Source: https://github.com/FrodeSolheim/fs-uae/blob/master/docs/options/middle_click_ungrab