There is no such option and I don't know whether the developer considers such an option.

But you used 'message window', but you should realize that when you double click on a message, you don't get a message window, but a folder window. When you look to the 'view' submenu in the menu bar, you'll see that you can enable 'show message list' and that all other options only make sense for a folder and not for a single message.

Having said this, I think that there's not much chance that the developers are planning something like this, however you can add something to the wish list as this is not the place to for wishes:

You can insert wishes in the wish list, by logging in to:
https://www.ritlabs.com/bt/main_page.php
Choose 'The Bat! wishes' and add a new item.
First check whether someone has made the same wish, then you can
support that wish.
Wishes that are supported by more users are more likely to be granted
than wishes by one. And the system isn't smart enough to understand
that wishes might be the same.
And add just one wish per entry. Makes everything easier, both for
other users who want to add support and for the developers.