> I'm genuinely surprised Microsoft's attitude towards "wndprocs don't have a context pointer"

They designed windows classes to be reusable, and assumed many developers going to reuse windows classes across windows.

Consider the following use case. Programmer creates a window class for a custom control, registers the class. Designs a dialog template with multiple of these custom controls in a single dialog. Then creates the dialog by calling DialogBoxW or similar.

These custom controls are created automatically multiple at once, hard to provide context pointers for each control.