Similar to how git shows mulitple git repositories, debug could show multiple debug sessions in a new split view section which we would be first in the debug viewlet.
In case we choose to do this we would no longer show multiple sessions in the callstack view but only the focused one.
This new section would only show up if a user has multiple sessions running.

Pros: easy and clear to change active session (also if your debug toolbar is docked), consistent with git
Cons: eats up vertical space

This idea was proposed by @bpasero
fyi @roblourens @weinand @auchenberg