The FSX, FSX 2018, and FSX 2020 software was designed to attempt to present to any resolution a video card is capable of presenting. With this in mind, not all resolutions have been optimized.
This means that User Interface assets and other items presented in the software may not present correct in untested resolutions. This may also impact performance or cause crashing if a unique or unusual resolution is being attempted.
A 1920x1080 resolution is recommended for best use. If you encounter an issue please test again with the recommended resolution to confirm if the issue you are experiencing is caused by the resolution being used.