Feature Scaling - Raspberry Pi OS
Screen Resolution
When running FlightGear, using the full screen option, 1024 X 768 resolution works well. Possibly because this is FlightGear's native resolution. In order to choose this resolution go to the Raspbian program menu 'Preferences' and run 'Screen Configuration'. Now right click on the screen that needs to be adjusted to 1024 X 768, on the Screen Layout Editor seen to the right. If using two screens, it might be best to set both screens to the same resolution. Also, it is best to have the two HDMI boxes touching so that the mouse cursor doesn't have a dead spot. Click on the green check to finish.
Memory split
The memory split option is found in the raspi-config program, under "7 Advanced Options."
sudo raspi-config
This option adjusts gpu_mem and thus allots the GPU memory. Starting with the Pi4, most of the GPU memory is controlled by the the Linux kernal and there is a disadvantage when gpu_mem is set too high. Never exceed 512MB, less is better. vcgencmd get_mem gpu will report GPU memory set by gpu_mem.
For more information see Memory options in config.txt