This is an easy one I think. There exists the key-command:
Channel & Track Visibility → Agents: Show Channels that are Connected to the First Selected Channel.
A great feature, easily accessible by key-command, yet hidden a bit deep for mouse work, or touch (just saying 
)
Paired with undo visibility it allows for fast and intuitive viewing of group channels.
But, speaking of group channels, just beneath the R and W buttons (mixconsole), there are two (2)! empty spaces.
What I’m suggesting is: Have a button like the picture below, which acts as a toggle, and does exactly as the keycommand does. (The toggle part is that when you press it a second time it undoes the visibility change.)
Bumping to add that an inverse Channel & Track Visibility → Agents: Hide Channels that are Connected to the First Selected Channel could also be very useful.
Interesting that no one is interested in a spill button for the mixconsole though. 
- New Visibility Agents Menu Option Request (with assignable key command)
Show/Hide Connected Channels
This command would allow non-exclusive spill/close of multiple selected groups (or buses).
Example: Select a group and issue the command. The channels that are connected to the group are hidden. Issue the command again, the channels are shown. All other channels remain visible throughout.
Option per group: Locking to zone also locks connected channels (with assignable key command)
This option would allow group faders to “bring over” their connected channels to their locked zone if ticked.
Example: If you want to have your group faders to the right like in an analog mixing desk, you just lock the group channels to the right zone. However, if you would like to see the channels comprising the group right next to the group fader when you lock it to the right zone, you would tick this option.
To Left Zone
This command would allow us to easily lock selected channels to the left zone.
To Right Zone
This command would allow us to easily lock selected channels to the right zone.
Edit:
Screenshots!
Select Groups
Give command… and
Selecting another group and issuing the command
Should not interfere with the others.
Lock to zone also locks connected channels: Off
Lock to zone also locks connected channels: On
Yes.
I’ve added it to the requests list.
