Wow, glad I asked. The topo based vlan mgmt is pretty cool!
Some observations:
I could not make the extend a vlan work by link until I first added the vlan to the 2nd device - try to extend the new vlan by adding by link failed. "Operation failed. Some allowed VLANs do not exist on the device."
But with both devices added to the current vlan, adding the link to the vlan adds tags to the ports involved.
Remove link from vlan does as expected, removing the link from vlan removes only the tags from the affected ports.
The multiple selected devices works to allow specific ports is also very useful, but the UI process seems a bit clunky - you could select devices and ports in one step. But it works.
Also allows for one of my use cases (yeah!) where I need to add tag to one side of a link - the other is added by authentication. In that case I just unselect the uneeded device.
BTW In this case, remove link from vlan removes the tag from the side that is tagged - it doesn't fail becasue its assymetric - which is again good for me.
The visuals sometimes don't match, when in doubt resynch.
Big thanks for that!