The overall experience of the map viewer beta is dramatically better than the current/previous map viewer (understanding that functionality is still being added)!
One thing we noticed is that when you're working with panes on the left and on the right that the map canvas becomes relatively limited.
A behavior similar to ArcGIS Pro and other applications where the panes auto-hide, with an option to pin them open, might be a nice solution to this - if implementation would be possible. There are times when having the pane open is nice and necessary, but in other cases it would be better to have the larger map after a selection or adjustment is made. Having these always auto-hiding or pinned could be annoying, so providing the option would be ideal.