Thanks to Norm Palardy for this link, showing how to override Xojo defaults:
With this guidance, you can create a file called Window.defaults (or DesktopWindow.defaults–or one of each) that contains the line
ImplicitInstance=false
As stated in the doc, using the Mac path here, stick the file(s) in ~/Documents/Xojo/Overrides, then restart Xojo.
This works great for me. It’s rare that I want an implicit instance. In my own apps such will only be a main window that is always open (I’ll even quit in its Close event.) With the default now being false, gone are the days when I have to remember to turn it off when adding a new window to the project–and sometimes forget.
Thanks again, Norm