If your scenery only covers part of your Window, like in the picture below, to use all the available window, you need to change the display's scale to 100% under Windows "Display Settings". However, your desktop icons will appear smaller and harder to see (a bummer...for old folks like me!). This is a limitation of the GLCanvas tool that I am using ... and there is no fix for this (except the above 'work-around'). I do apologize for the inconvenience!
