The cause of this issue is not yet fully understood, but several factors could be contributing to it. One possible reason is a bug in the game's code that prevents the "Top" display mode from being recognized or selected. Another possibility is a compatibility issue with certain graphics cards or drivers.
Locate the file named and open it with Notepad. serious sam 3 cannot set display mode top
For NVIDIA / AMD users:
Windows tries to “optimize” older fullscreen games by overlaying a compatibility layer. This often breaks Serious Sam 3 . The cause of this issue is not yet