When a popup window is hidden (ShowWindow(hPopupWindow, SW_HIDE) call), the system sends an extra WM_MOUSEMOVE message to the popup's parent window. This message reports the current mouse buttons state (so the message is translated to DRAGGED, when the left mouse button is pressed when the Popup is hidden) and reports the last mouse position. The glass toolkit should filter out mouse move / drag messages which don't report changed mouse pointer coordinates.