Stupid question about SetParent

[Berserkr]

I apologize for my bad English.
Because when the child step on the button, it is "stained" from the button?

#include <WinAPi.au3>
#include <GUIConstantsEx.au3>


$hMain = GUICreate("Main", 800, 600)
$hButton = GUICtrlCreateButton("Click", 120, 120)
GUISetState(@SW_SHOW)


$hChild = GUICreate("Child", 400, 300)


While 1
    Switch GUIGetMsg()
        Case $GUI_EVENT_CLOSE
            ExitLoop
        Case $hButton
            _WinAPI_SetParent($hChild, $hMain)
            GUISetState(@SW_SHOW, $hChild)
    EndSwitch
WEnd