Video explanation:
https://www.loom.com/share/970fea32e98b4a6db81d60e0060ee2fa
I like this idea. Maybe extending the current toolbar button within the preview to show Instances, could be changed into a drop-down button with these options:
- Show preview text; using variation from active panel (default)
- Show preview text; using variation from font panel
- Show selected glyph; all instances
- Show selected glyph; all masters
- Show selected glyph; between masters
Awesome!! I like it 
Show preview text; using variation from font panel
Show selected glyph; between masters
Can you clarify these? I don’t fully understand what they do by the name.
Also, when switching between masters (in the masters and layers panel), the Variation settings should remain as they were. So it will be easier to move between masters and make changes, but also see the preview of the extrapolated values. Without this, the designer will need to re-make the changes in the Variation panel on each master switch.
However I do think it’s important that we can do double click on a master and then it will override the current Variation values. (Right now it does this but with one click. Change to to double click so we can then do the above)
What do you think?
→ Show preview text; using variation from font panel
When the font panel is active you will now see the preview with that variation, but if a glyph panel is active, you will see that variation. With the option “Show preview text; using variation from font panel” you will always see the preview with the variation from the font panel, even when a glyph panel is active.
→ Show selected glyph; between masters
With this we will show the selected glyph with variations between all masters. For example if you have 6 masters:
Weight 100 Width 80
Weight 100 Width 120
Weight 400 Width 80
Weight 400 Width 120
Weight 800 Width 80
Weight 800 Width 120
You will then get these variations:
Weight 100 Width 100
Weight 400 Width 100
Weight 800 Width 100
Weight 250 Width 80
Weight 250 Width 120
Weight 600 Width 80
Weight 600 Width 120
This suggests another option like:
Show preview text; using current/frozen variation
I still don’t understand the difference between these two:
Show preview text; using variation from active panel (default)
Show preview text; using variation from font panel
I see, so this is just a nice way to make sure all masters work?
Yes! This is what I meant by the feature request, to show preview text using frozen variation (while still being able to edit the masters, and the preview would change accordingly). All the other suggestions you made are great too, but they aren’t related to the initial reason I opened this topic.
I also believe that all the new options you provided should have a “frozen variation” option too. No? Perhaps place another button 
next to the dropdown?
For now you can open two glyph panels, and move the interpolated one to a second monitor, if available.
That’s cool! I wasn’t aware of that.