::MoveWindow leaves blank area

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C++

Win32

MFC

GUI

I am quite desperate to resolve this very annoying issue :( . I am trying to display a child window on parent window.
The instantiation is fine and the window shows what it should.
At some point the window need to be resized. But for some reason, when I using MoveWindow function it leaves blank space on the top of the parent window. Here is the screenshot:<ahref>http://i32.fastpic.ru/big/2011/1016/39/9e4460be87c03499e8b183a130902d39.png[^]

Here is the code example:

HWND hwnd  // Comes from external function. Was defined as WS_CHILD previously
HWND hwndParent     = ::GetParent(hwnd);  
RECT parentRect     = {0,0,0,0};  
RECT childRect      = {0,0,0,0};  
::GetClientRect(hwndParent, &parentRect);  // Suppose it returns {0,0,600,300}  

BOOL ok = ::MoveWindow(hwnd, 0, 0, 600, 300, true);  
::GetClientRect(hwnd, &childRect);  // Will return {0,0,584,297}

Since the rect is smaller than asked not entire window rect will be used.
WHY ?????

What am I doing wrong? Did I forgot some flags with window initialization?!