The observer checkbox is disabled when in use by other ATC, and cannot be set or cleared. It needs to be enabled when in use, but only clearable.
A better solution would be to formalise observer (probably done as a way round the issue of someone taking ATC chair while connect, the classic DB lock problem). So it can be set or cleared at any time, whether in use or not.
Even better would be the option to combine this with the ability to access expert as observer. This would be great for those of us in the IFATC pipeline as we could watch the experts doing it from the controller POV.