"minimize to tray" function - clean programming

I think you didn't understand what I was asking for. I don't need to know how to use "tray icons". I know that myself, it's no problem and I didn't include it in my pseudo code above.

I want to know how to "hide" and "unhide" a window while still showing the animations for minimizing/restoring windows.

If you haven't disabled this visual feature in Windows, you will see a short (about 0.2 seconds) animation when minimizing/maximizing/restoring/resizing a window. This animation is missing when using "Variant 2" from above. If I use "Variant 4" I see the animation, but it is invalid code. Microsoft clearly says that you must not call "DefWindowProc" from DialogBox procudures or you will end up in an recursive infinite loop. "DefWindowProc" is only to be called from window procudures.

For some reason the code I posted works, but it is invalid code. I want to know how the correct code looks like.

The examples you posted only show how to create tray icons, not how to keep the animation.