Hi Aqrit,
I reviewed the thread, and tried the patch, but it made not change in fixing the issue.
I've done some more tests by having a clean installation of the GoG BG - The Original Saga and applied only the Widescreen mode, and issue is consistent.
I suspect cause is due to the latest changes in Windows 10 bundled DX legacy drivers, as I recall a couple of years (2-3) ago I had Widescreen mode working on Windows 10 without this issue.
J