Notepad.exe is just a wrapper around the edit control [1] that Windows ships as part of its core system. It doesn't even properly support undo as you can only undo the last action, which becomes the new last action, allowing you to only toggle between two states.

https://docs.microsoft.com/en-us/windows/win32/controls/edit...

It's a window that mostly uses an edit control, but, having code in Notepad, it's not just a wrapper around the edit control. Could you write Notepad in a day or two? Perhaps. But it's deceiving to say that it's just a wrapper around the edit control.

Unlikely. Notepad is just a wrapper around the Windows API's textbox/textarea/whatever it's called, so open sourcing it isn't much.

Apple open sourced TextEdit for exactly that reason, to show how easy it is to write text editing applications in Cocoa: https://developer.apple.com/library/archive/samplecode/TextE...

MSPaint please

Why? There is Paint.Net and it was open source for a long time.

Because Paint.Net isn’t MSPaint. It’s not exclusive. There’s lots of paint programs but only one MSPaint.

Why calc.exe?

I would very much welcome that on my Linux install.

Obviously not quite the same, but if you haven't seen it yet: https://jspaint.app/