A jitter occurs when toggling between two masters (with Show All Instances enabled), where the preview panels (in the editor and the separate window) display the selected glyph at a different size/position. I’m guessing this has to do with differing x-height values because the jitter doesn’t happen when switching between masters with the same x-height.
What is the jitter? If the x-height is larger, should the lowercase letters not appear larger in the preview?
The jitter can be seen in the Preview Panel. With Show All Instances enabled, all the instances should be displayed together in one overall font size (with their differing x-heights) which is based on the height of the preview panel.
The issue is when switching to another master, something happens in the preview panel display that causes the instances to shift up and down.
The size and position of the instances in the preview should remain constant.
I see, the preview panel sizes the text according to the current master, but since it is showing all instances that is not necessary and sizing with respect to a global reference would be desirable.
Exactly. Currently, the global reference size is determined by the height of the preview panel which is also not ideal.
Ideally, in both the preview panels the user should be able to zoom in/out (as on the iPhone/iPad GlyphsViewer App), as well as, be able to set a specific size.
Related post: Feature request: Zoom in Preview Panel - #7 by gor.jious
The preview uses the ascender/descender of the current master. If it jumps like this, it is an indicator that the font might jump outside of Glyphs, too.
Ah, I see. The winAscent and winDescent values were the culprits.