A few remarks when using compact:
Avatar’s flair keeps the regular size, which looks weird, and they overlap posts’ content.
Suggested and Related buttons at the end of a topic overlap the topics list below.
User preferences have a font size setting.
I don’t think the compact view should alter the global font size at all. They should remain separate settings.
The topic map container doesn’t use 100% of the width. It should normally reach the right side.
Same with the footer buttons.
In the regular layout, .topic-meta-data, which contains the date, has a right padding that aligns the date with reply buttons:
This is not the case with the compact layout:
The absence of the horizontal line that separates posts in the regular layout makes the post list right side look weird. The dates and reply buttons are too close to each other. Each button and the date below look like they’re parts of the same post, but they actually belong to different posts.
