Problem
Your mouse keeps drifting out of the App Player window while using the Aim, Pan and Shoot mode on BlueStacks Air.
Cause
BlueStacks Air needs accessibility permissions to provide Pan controls to play FPS games. If you do not grant accessibility permission, you will face issues in these games.
Solution
Grant Accessibility permission to BlueStacks as shown below.
Click on 
Apple menu on your Mac ➔ Open the Settings Menu ➔ Select the 
Privacy & Security tab from the sidebar ➔ Click Accessibility ➔ Enable permission for BlueStacks ➔ Restart BlueStacks
NOTE: If you don’t see BlueStacks listed, click the + button at the bottom, search for the app, and select it.
Thank you for choosing BlueStacks Air. We hope you enjoy using it. Happy Gaming!