Variable Font bug: "wiggling" top of "U"
Download and unzip https://www.dropbox.com/s/3b6k0izkm5ma3dy/200719-AdobeVFBug.zip?dl=1
Install the font from font/DaikonVF-mini.ttf
Run Illustrator 24.2.1
Type "AU" and choose the Daikon VF font
In the property bar, click the Variable Font button and move the weight slider between the extreme locations. Observe how the top of "U" "wiggles" (jumps up and down). This is shown in videos/DaikonVF-Illustrator-24.2.1,.mp4
The same problem occurs in InDesign 15.1.1
The same problem occurs in DTL OTmaster 7.9 and in FontView 2019 (an open source app built from https://github.com/googlefonts/fontview ) — both apps use older versions of the FreeType library.
The problem no longer occurs in FontView 2020 (which I’ve rebuilt with newest FreeType). I’m including both the 2019 and 2020 version of FontView in the ZIP.
The problem also does not occur in Chrome 83 (which I think uses FreeType for VF), nor it occurs in Safari 13.1.2. Videos are included in the "video" folder.
So it seems that it’s a bug in FreeType which was fixed by the FreeType developers. If Illustrator and InDesign use FreeType for VF, then Adobe should update FreeType to the newest version
Adam Twardoch commented
I’ve also confirmed this a bug in InDesign and filed at https://indesign.uservoice.com/forums/601180-adobe-indesign-bugs/suggestions/40945027-variable-font-top-of-u-wiggles-as-you-change-we