The problem is that in the newer Ubuntu versions the operating system doesn't draw the window if it has any amount of transparency. This doesn't happen always but often enough to be very annoying problem. The feature has worked before so it is probably caused by some regression in GTK or some other lower level component but unfortunately I don't have time to try to find out the cause and fix code which is not directly part of Rainlendar. I can consider making it still possible to change the transparency from the settings file but the transparency controls will be disabled in the next build as they should have been already.
But I don't understand that if a feature doesn't work for some users this is suppressed when there is an option to doesn't use it. Especially if it is a problem in a library alien to Rainlendar that will be adjusted in future versions of GTK.
When in the future this problem would be fixed in GTK, users of Linux still can't use transparency?
For these reasons I think you should reconsider your decision and keep this option:
1) It's an error in GTK library.
2) The transparency is configurable and it can be disabled in options.
3) It isn't a generic problem, it's a problem for a certain amount of persons.
You say "I don't have time to try to find out the cause and fix code...
" but you have time to disable this feature to Linux!?
This program has several shortcomings on Linux. This will only add one more.