Certain WM_LBUTTONDOWN & WM_LBUTTONUP events should change focus to the
ComboBox (a child of ComboBoxEx), but instead the focus was set to the
Edit control.

Fixes a bug valgrind found.

    
This essentially reverts 2f69329a.
We really need the visible rect to be set atomically with the rest of
the window position info.

Also avoid dependency on winternl.h.