Why can't we collapse the message panel anymore?

If I remember correctly the message panel on the left wasn't fully collapsable. But with the previous version you could click and drag it to make it smaller, than the minimal size now?

There is a "Show details" on the right which might take up some space, but I am guessing you have that collapsed..

In the very top left corner you also have File, Edit, View, Help. If you click on View you could "Zoom out" to make everything slightly smaller. Might be a decent workaround for the time being.

Perhaps, though I'm fairly confident that the panel with names was indeed collapsable if you clicked on the "Message" icon.

I traveled a fair amount this year and whenever I used my tablet as a second monitor I would make use of the feature to optimize space on the smaller screen.