Install the VirtualBox Guest Additions in the guest OS
In VirtualBox, open the guest window menu and go to View
Disable Auto-resize Guest Display if it is enabled
Set a fixed guest display resolution inside the guest OS
In the guest OS display settings, choose the native or supported resolution
Turn off scaling features in the guest OS display settings
Use Full-screen mode instead of resizing the window manually
If using Linux guests, restart the display manager after installing Guest Additions
If using Windows guests, reboot after installing Guest Additions
Make sure the VirtualBox window is large enough to fit the guest resolution
Disable HiDPI scaling in VirtualBox settings if it causes scrolling
Update VirtualBox to the latest version
Update the Guest Additions to match the VirtualBox version
Remove any custom screen scaling set by the host OS
Check that the guest OS is not set to a resolution larger than the VirtualBox window