We are aware of an issue with MacOS 10.14 and ConQuest where the maximise window button causes ConQuest to not behave correctly. Symptoms include black bars at the top and bottom of the display, and mouse/trackpad clicks are displaced from the position of the pointer. New ConQuest dialogs are also created as tabs to the current display rather than separate dialogs as they should be.
Currently there is no solution to this issue. We would instead recommend that if you wish to use ConQuest utilising all of your screen space that you manually resize the window by clicking on the edge of the ConQuest window and drag to the desired size.