keyboard_input_grab
The option documentation is properly formatted yet
Description: "Grab keyboard when input is grabbed" Default: 1 Example: 0 Since: 2.3.10 Type: boolean
Set this option to 0 to prevent input grab from also grabbing the keyboard, only the mouse will then be grabbed. This only works when FS-UAE uses SDL2. By default, both mouse and keyboard input will be grabbed together.
Source: https://github.com/FrodeSolheim/fs-uae/blob/master/docs/options/keyboard_input_grab