Thanks for getting in touch and for going into detail about this issue.
We agree this is not discoverable or very intuitive at present.
The reason we didn’t include a filter here was due to the nature of each tenant tag set having multiple tags. A single dropdown for all tag sets, each with their own tags quickly becomes overwhelming, and if we populated multiple dropdowns here for each tag set, we found that got out of control for customers working with tens/hundreds of tenant tag sets. So we made the decision to limit this from the in-line variable scoping filter, and instead leave all that responsibility to the variable dialog’s scoping page.
This does however, as you say, make it difficult for discoverability and monotonous for data entry, because it takes you out of the flow of editing the variables in-line (especially if you’re using keyboard shortcuts to quickly enter/edit variables).
We’ve created a GitHub issue here with our thoughts on how we can improve this, that you can track to be notified when a fix is available. If you have any thoughts on how you’d like to see this improved (other than the approaches we’ve mentioned in the issue), we’d welcome any comments on this issue.
In terms of a workaround for now, one shortcut that’s not obvious is the “Open editor” link at the bottom of that scoping popover. This goes straight to the scoping page of the variable editor dialog, so you can easily select your tags without having to switch to the “define scope” page first. Not ideal, but a little easier than opening the variable editor and having to switch to the scoping page.
Hope this helps.