At present, the best way to find the root bridge is via the terminal. To open the terminal for a device, go to the Network tab, hold the Shift key and click on the name of the Alta device. This brings up an in browser terminal.
~ # cat /var/run/ustpd.status
Root MAC: bc:b9:23:80:c7:32, Priority 0
Port 1: role=designated, speed=1000 Mbps, cost=20000, agg ports=0x0000000000000000
Port 2: role=designated, speed=100 Mbps, cost=200000, agg ports=0x0000000000000000
Port 3: role=designated, speed=100 Mbps, cost=200000, agg ports=0x0000000000000000
Port 4: role=designated, speed=100 Mbps, cost=200000, agg ports=0x0000000000000000
Port 5: role=designated, speed=1000 Mbps, cost=20000, agg ports=0x0000000000000000
Port 6: role=disabled, speed=4317224 Mbps, cost=200000000, agg ports=0x0000000000000000
Port 7: role=designated, speed=100 Mbps, cost=200000, agg ports=0x0000000000000000
Port 8: role=root, speed=1000 Mbps, cost=20000, agg ports=0x0000000000000000
~ # cat /var/run/ustpd.status | grep root
Port 8: role=root, speed=1000 Mbps, cost=20000, agg ports=0x0000000000000000