I can assure you it is no trojan.
But when I just uploaded the zip archive to virus total I was baffeled that 5 scanners think this is malicious software,
Here is the source, which is 90% the visual studio generated sample with some additions by me:
https://www.mediafire.com/file/ozl328ljhjjzkye/ZomboidClickToStartFix-src.zip/file
If you can read it you could compile it yourself....
If in doubt I suggest you use "Simple Runtime Window Editor" from github.
- then "Select running Application" and select Project Zomboid
- then on the tree select the window with the name "Project Zomboid"
- select the "Window Styles" tab
- click "WS_BORDER" two times
This is the same my program does....