Long time mRemoteNG user here and I'm very happy to hear about the continued development of mRemoteNG. I've tried several other RDP managers out there but keep coming back to mRemote due to its lean approach.
Have tried the new version and it seems to work great. I had to reset the view layout to get "Connections" panel to behave/display properly (to enable to pin/hide).
Since there are new developers, I figured I'd float some ideas for future versions. Not sure if these are possible to implement but here I go anyways.
1. Add option to move connected server tabs to the same area as Menu and Quick connect toolbars. This would allow for more RDP session screen real estate (especially in nested RDP sessions).
2. Option for a dark theme (or expand the current theming solution) so that the Menu toolbar and the Connections panel can be made dark.

Cheers,
Eddie