That setting is already set to "<Alt>". Alt does engage the 'mouse modifier' - for example, if you press it, you can no longer drag the window by clicking on its title bar. You can't drag the window at all.
That setting is already set to "<Alt>". Alt does engage the 'mouse modifier' - for example, if you press it, you can no longer drag the window by clicking on its title bar. You can't drag the window at all.