Indeed, thank you, Gerry. I have been running PCNC/Mach3 since 2006 and didn't even know that button existed. (I have only been off the Simple Run screen once in the past 3 or so years, and that was to reset M998 height...)
I have added the button to my tweaked version of the PCNC screenset. After experimentation, I see that it doesn't blank the toolpath window or anything, just stops interaction with it. I can still display the loaded gcode to make sure it's what I expect, then turn off the display updates.