Why is there no triangle?

Question

C++ / C#

I wrote a native window (WINAPI), got the OpenGl context (extended too), also got all the pointers to the OpenGl functions, there are no errors during compilation and linking. (Wrote for review purposes)
When using third-party lib like glfw+glad, everything works.
Code here

Answer 1

Incorrect handling of the WM_SIZE message:

The low-order word of lParam specifies the new width of the client area.
The high-order word of lParam specifies the new height of the client area.

case WM_SIZE:
        glViewport(0, 0, LOWORD(lParam), HIWORD(lParam));