The game doesn’t support Full HD, 2K, 4K resolutions, which was never much of a problem for the Steam release as it had been updated with an earlier version of the widescreen patch. However under 26H6 this is no longer the case, and “full screen” only covers 3/4 of the screen!
The solution can be found in form of a newer version of the patch which adds Full HD, 2K and 4K widecreeen support. It is available for the disc release, the GOG and the Steam release alike!