When pressing F11, the Fullscreen option is changed. However, this doesn't get saved to disk, unless you open any options screen and close it (which saves the entire options.txt file).

This can be fixed by (with Yarn names) adding a call to GameOptions.write() in the onKey method in the Keyboard class.

A Mixin that does this fix is available at https://github.com/comp500/BorderlessMining/blob/1f4b93d340eb0a97b4d4ad4f72bc316077bedc2c/src/main/java/link/infra/borderlessmining/mixin/F11FixMixin.java.