user32: Do not call SendMessage() to hide a window that is already hidden.

dlls/user32/tests/msg.c

@@ -4673,8 +4673,6 @@ static void test_messages(void)
     flush_events();
     ok_sequence(WmEmptySeq, "ShowWindow(SW_HIDE):overlapped", FALSE);
 
-    if (0)
-    {
     /* test ShowWindow(SW_HIDE) on a hidden window -  multi-threaded */
     hthread = CreateThread(NULL, 0, show_window_thread, hwnd, 0, &tid);
     ok(hthread != NULL, "CreateThread failed, error %d\n", GetLastError());
@@ -4682,7 +4680,6 @@ static void test_messages(void)
     CloseHandle(hthread);
     flush_events();
     ok_sequence(WmEmptySeq, "ShowWindow(SW_HIDE):overlapped", FALSE);
-    }
 
     ShowWindow(hwnd, SW_SHOW);
     flush_events();

dlls/user32/winpos.c

@@ -1219,6 +1219,9 @@ BOOL WINAPI ShowWindow( HWND hwnd, INT cmd )
     if ((full_handle = WIN_IsCurrentThread( hwnd )))
         return show_window( full_handle, cmd );
 
+    if ((cmd == SW_HIDE) && !(GetWindowLongW( hwnd, GWL_STYLE ) & WS_VISIBLE))
+        return FALSE;
+
     return SendMessageW( hwnd, WM_WINE_SHOWWINDOW, cmd, 0 );
 }