WM_SHOWWINDOW Overview Group

The WM_SHOWWINDOW message is sent to a window when the window is about to be hidden or shown.

WM_SHOWWINDOW 
fShow = (BOOL) wParam; // show/hide flag 
fnStatus = (int) lParam; // status flag

Parameters

fShow: Value of wParam. Specifies whether a window is being shown. It is TRUE if the window is being shown or FALSE if the window is being hidden.
fnStatus: Value of lParam. Specifies the status of the window being shown. The fnStatus parameter is zero if the message is sent because of a call to the ShowWindow function; otherwise, fnStatus is one of the following values:

Value
Meaning

SW_OTHERUNZOOM
Window is being uncovered because a maximize window was restored or minimized.

SW_OTHERZOOM
Window is being covered by another window that has been maximized.

SW_PARENTCLOSING
Window’s owner window is being minimized.

SW_PARENTOPENING
Window’s owner window is being restored.

Return Values

If an application processes this message, it should return zero.

Default Action

The DefWindowProc function hides or shows the window, as specified by the message.

Remarks

If a window has the WS_VISIBLE style when it is created, the window receives this message after it is created, but before it is displayed. A window also receives this message when its visibility state is changed by the ShowWindow or ShowOwnedPopups function.

The WM_SHOWWINDOW message is not sent under the following circumstances:

When a top-level, overlapped window is created with the WS_MAXIMIZE or WS_MINIMIZE style.
When the SW_SHOWNORMAL flag is specified in the call to the ShowWindow function.