Changing the window frame styles causes the window to disappear

Ok so I have this function that will let you switch whether a window can be resized or not.

Code:

void Window::SetResizable(bool resizable)
{
    DWORD dwStyles;
    
    if (resizable)
        dwStyles = WS_OVERLAPPEDWINDOW;
    
    else
        dwStyles = WS_OVERLAPPEDWINDOW & ~WS_THICKFRAME & ~WS_MAXIMIZEBOX;

    SetWindowLongPtr(GetHandle(), GWL_STYLE, dwStyles);
    
    // When changing frame styles with SetWindowLong()/SetWindowLongPtr()
    // be sure to call SetWindowPos() with SWP_FRAMECHANGED
    // otherwise the non client area of the window won't be recalculated
    // until the window is sized.
    UINT uFlags = SWP_NOZORDER | SWP_NOMOVE | SWP_NOSIZE | SWP_FRAMECHANGED;
    
    SetWindowPos(GetHandle(), // the window handle
                 HWND_TOP,    // the window to insert after
                 0,           // x
                 0,           // y
                 0,           // width
                 0,           // height
                 uFlags);     // flags
} 

Now this works great, with one minor flaw.
If the window was visible *before* I call this function,
the window disappears off the screen leaving behind artifacts on the screen.

I hacked up a fix, but I want to know why this happens.

Here is the fix:
Code:
bool wasVisible = IsVisible();

all that other code
....

if (wasVisible)
Show();