After signing out of an account, should the user be logged into at least one other account, the Minecraft Launcher defaults to taking users to an Accounts screen that has an option to log into a new one at the very top. With windowed mode, an issue arises as this screen doesn't have the option to scroll-meaning that if a user has too many accounts to appear on the window, they wouldn't be able to log into a new account without signing out of a few. The accounts simply just overflow outside the visible area. Maximizing the window helps, since the standard window size is a lot smaller than the full monitor size, but the "Add Account" option sometimes still goes off screen.

Of course, most people will never have this issue. Most people don't have enough accounts to cause an issue. I have quite a few I've used as decorative/character heads for servers, and I would prefer to stay logged into them all to be able to easily change their skins as opposed to logging out of them so I don't encounter this issue.