When currently the preview window has the option to select any instance, or all instances overlayed ("-"), or all instances of a single glyph, the one option I am missing is: Keep preview in sync with the master active in the work area.
From my understanding Glyphs generates a font for the preview window whenever something is changed. However, here this wouldn’t result in any additional overhead, since still only one single instance is previewed at any time. It’s merely a shortcut for saving me from having to switch the preview window’s weight whenever I change between masters.
Do you perhaps have the eye icons in the layers palette on the right sidebar open? If so, try closing them. When the eye is set open, that layer is displayed behind the glyph when other layers are selected, including in the Preview area. The “-” in the instance selection of the Preview area should then show the current layer. The layers palette is described in section 4.4 Layers of the Glyphs 2.3 Handbook. https://www.glyphsapp.com/get-started
Thanks! Oh wow, yes, that was exactly the case. Apparently then the feature exists, I just managed to unwittingly get my interface into a state that prevented it from working.