I think Jan’s suggestion is great. I normally do these comparisons with proofs created in House Industries’ GlyphProofer, but comparing masters onscreen could save me a lot of time. “All Masters” and “All Instances” option could be added to the existing drop-down menu for instance previews to do this without adding any UI clutter. Each would use the preview area to display only all masters/instances for the current glyph.
What happens if you have several glyphs entered in the edit view and enable “Show All Masters”. Does it show the hole text for each master or each glyph in all masters next to each other:
e.g. if you have “Text” in edit view do you get “TextText” or “TTeexxtt”? Or only the current glyph?
I think it would make sense to limit a multiple master preview to only the current glyph. Previewing multiple glyphs would be useful, but probably only for short words and fonts with just two masters. Beyond that there just isn’t likely to be enough space to display the word repeatedly at a size that matters.
In my mind I imagined only the current (only one) glyph. You can fast switch Masters (cmd+number) or even see them side by side in edit view, so I feel more useful to have an All Instances preview only.
Please keep in mind layers[xx] must work properly and we can easily have 30 or 40 active instances to be displayed.