As a workaround, in the game's main menu go to Options, Graphics, and set the "Fullscreen resol.**" to both 0s. Set screen scaling to "Integer" (keep clicking the button until it says "Integer", if not already). Click Okay to save the changes, then exit and restart Lix.
This is not exactly what you want since most likely the scaling will leave wide black margins in 2 if not all 4 edges of the display, leading to the infamous "tiny Lix screen in middle" syndrome. You can set screen scaling to "Aspect" instead, but the scaling done by Lix in that mode may not look very good, depending on the native resolution of your display. The 0s tell Lix to stay with the default resolution of the system, which is typically the native/optimal resolution of your monitor.
What you are seeing is Lix using 640x480 for fullscreen-mode resolution, and the display driver handling it by scaling it unevenly in order to cover the entire display area of your widescreen monitor. Some graphics cards may have the ability to be configured to perform aspect scaling instead of stretch-to-fill in these situations. With that in mind, you can also try changing the values (mentioned in first paragraph) from 0s to other values that work better for your monitor.
It has been on the wishlist for Lix to not be internally hardcoded to operate in 640x480 mode. That may help, although maybe the game also needs an option (not uncommon in many games) where the player can explicitly indicate to the game whether their display is widescreen or not, and the game can then react accordingly.
