ChildWindowFromPoint retrieves the child of the provided window at a point. In this case it always returns 0 because HWND_DESKTOP is flag defined as 0, which is never a valid window handle and is not intended for use in place of a window handle. Forwarding of mousewheel events was added inadb08def61, and later modified to the current unworking state ine9645806f5. Sending mouse wheel events to the window under the cursor is a system preference and therefore should not be overridden by Blender, therefore the noop code has been removed.