Ahh, thx, now I see.
Maybe, the order in which we add controls matter. I mean, the order matters for sure, but in more than one way.
Suppose we simply swap the first two lines in your script, then the GUI looks identical.
The OK button with his coords and Default option is first, the non-default button "Calc" is added later, and gets a later position in the tab-order of controls.
With the Default button in second position, we see what you say, with the Default button in first position, we would not. (in one of my tests: run script - keep hitting enter - observation: no Calculator)
On the other hand - suppose we remove the default option - I can start the script, manually mouse over a button (Test1 click OK, Test2 click Calc)
I then find: having hit a button manually, sort of preselects the Button for later, meaning I can force a cycle of msgBoxes, or a cycle of calculators. And I can jump from one cycle to the next.
Hmmm, I have presently no explanation regarding priority of [ Last-used-button vs. Default-button ]
but I could recreate the issue. Thanks for pointing to it.