This I guess is a personal opinion, but the way “toggle unread/read” is implemented doesn’t work very well for me.
I’m used to get into a folder, select all messages and mark all of them as “read”.
You cannot do that with the new app because it toggles everything from one state to the other.
That forces you to select only the messages you haven’t read which is a lot more annoying in my opinion.
I cannot see a lot of cases in which you want to “toggle” a combination of read/unread messages to the oposite unread/read state anyway. So if you want to keep the unique button instead of two independent “mark read” and “mark unread”, one of the two should take preference when selecting a mixture of read/unread messages.
Using Firefox in Ubuntu.