When a popup (doesn't matter which one, Reference Previews, Page-Previews, with or without an image) opens to the bottom (i.e. the little arrow is at the top), the distance from the top corner of the popup to the text is not the full expected 16px but much less. This is because the arrow is part of the 16px margin, not outside of it.
I did a git bisect and narrowed the issue down to this patch:
- T269336: Preview popups: Replace SVG with clip-path
Before the patch we had 16px all around: