The False in there makes xmonad skip this code in XMonad/Main.hsc. It

might be a bug that your situation isn't handled properly.

-- set keyboard mapping
handle e@(MappingNotifyEvent {}) = do
    io $ refreshKeyboardMapping e
    when (ev_request e `elem` [mappingKeyboard, mappingModifier]) $ do
        setNumlockMask
        grabKeys