Sometimes the "any clickable area => make it a Windows control/button" works and sometimes it doesn't.

I talked with the programmer for the 16-bit Windows calculator app, calc.exe.

Any naive programmer with a first-reading of Charles Petzold's Programming Windows book would assume each button in the calculator app was an actual Windows button control.

Nope.

All those calculator buttons, back when Windows first shipped, used up too many resources.

So the buttons were drawn and the app did hit-testing to see if a button was mouse-clicked. see https://www.basicinputoutput.com/2017/08/windows-calculator-... for a pic of the 16-bit Windows calculator app.