Ok, so I have a rectangle drawn on my screen (via OpenGL), and I want to make it resizable — just like the standard resize box of all windows.
Is there a simple way, other than checking mannualy if mouse position is here or there and changing cursor again and again , to do it?

Handle the WM_NCHITTEST message and return HTSIZE when the cursor overlaps the rectangle.

The rectangle I mentioned represents a layer on wich my stuff will be drawn.
But I'm dealing with several layers. So, doesn't your suggestion implies in making HWND's of my layers?

I got it, but returning HTSIZE when mouse is over my area just makes it freeze.
This is what I get:

    case WM_NCHITTEST:
        if (onArea(LOWORD(lParam), HIWORD(lParam))
            return HTSIZE;
        return DefWindowProc(hWnd, msg, wParam, lParam);
 

Solved!
Instead of HTSIZE I return now HTBOTTOMRIGHT.