When is a window visible yet not visible?

Today, a quick puzzler.

Consider the following code fragment:

 ShowWindow(hwnd, SW_SHOWNORMAL);
 assert(IsWindowVisible(hwnd));

We just showed the window, certainly it is visible, right? Yet the assertion can fire (even in the absence of multi-threading). Why?

Answer below – stop reading if you want to try to solve it yourself.

Take a look at the IsWindowVisible function.

If the specified window, its parent window, its parent’s parent window, and so forth, have the WS_VISIBLE style, the return value is nonzero. Otherwise, the return value is zero.

The WS_VISIBLE style indicates that this window is visible in its parent. But the parent itself might not be visible, in which case IsWindowVisible returns FALSE.

[Raymond is currently on vacation; this message was pre-recorded.]