Windows不支持重叠子窗口、控件(overlapping controls)

同级别子窗口彼此覆盖时，Windows程序就会出现问题。尽管可以通过SetWindowPos()调整各个子窗口的z序，但是仍会存在问题。典型情况就是对话框中如果用一个子窗口覆盖整个客户区，对话框里的控件仍然会不时的露出来。设计程序时，要避免子窗口彼此重叠的问题。如果一定要重叠，解决方式是用重叠的“非子窗口”，Windows支持非子窗口重叠，但不支持overlapping controls.

Child window controls should not be overlapped in applications for the Windows operating system. When one control overlaps another control, or another child window, Windows may or may not draw the window or some of its elements correctly. This behavior is a consequence of the way that Windows is designed.

The following example illustrates the painting problems caused by the ambiguity of overlapping borders. Consider three edit controls, called A, B and C, which overlap each other, and an enclosing child window D:

     ____________________________________________
     | | A          | B          | C          | |
     | ---------------------------------------- |
     |                 D                        |
     --------------------------------------------

Assume that control B has the focus. If this set of controls is covered by another window, which is subsequently moved away, Windows will send a series of client and nonclient messages to each of the controls and to the enclosing child window. The result of these messages may appear as the illustration below, where the portion of window B's border that overlapped with part of window D's border is missing:

     | | A          | B          | C          | |
     | ---------------------------------------- |
     |                 D                        |
     --------------------------------------------

Repainting problems related to overlapping controls may vary depending on the control and version of Windows used. To avoid this problem, ensure that all child windows are not overlapping or that all windows are members of the same child-parent hierarchy. For example:

                    ________
                   | Parent |
                    --------
                   /        \ 
         _________            _________
        | Child A |          | Child B |
         ---------            ---------
         /        \     
     ________    ________
    | Child C|  | Child D|
     --------    --------

In this case, all of the child windows are control windows. Child A and Child B are overlapped sibling windows. Therefore, the all of the child windows may have painting problems. The way to correct this and still have overlapped control windows is to make Child B the parent of Child A (or vice versa).  

Another consequence of having overlapping controls is that the user of the application may be confused, because clicking the mouse in the common area may not activate the control that the user intended to activate.