I've audited all special pages looking for issues. Here's what I found, not including causes or solutions. Some of these might be upstream bugs.

Monobook

• There's the issue reported in the task description, due to the icons getting a position: static that probably doesn't work with other rules. Might be a bug in the apex them itself.
• The event page header is affected by the same bug:
  
• Additionally, the "manage registration" menu displayed to participants shows weird truncation of its options. There's plenty of space on either side, so it's unclear why truncation is needed in the first place. Also wondering why all options are truncated to the same length; it looks odd here for the first item, where the ellipsis has the same length as the deleted "ion".
  
• In Special:EnableEventRegistration, the "required" marker for the event page field is misplaced:
  
• In Special:EventDetails, section icons are too dark and perhaps bigger than in vector:
  
• In Special:InvitationList, the lists look quite packed. We really should stop trying to overwrite list styles as we're doing in several places, and just leave it up to the skin.
  

Timeless

• In Special:AllEvents there's a colour issue with the tabs:
  
• In the "more details" dialog, the section spacing looks off, and it's definitely off for the "location" sub-sections:
  
• There also seems to be too much spacing between participants. Refer to my recommendation above that we just stop customizing lists.
  
• The font size of the registration footer is too small (10.83px) and not accessible:
  
• Section headers in Special:EventDetails are HUGE:
  
• In Special:InvitationList, accordion titles aren't bolded. Unsure if this is intentional.
  

For the SpecialAllEvents one, and potentially other similar issues with icons, they might go away if we switch from OOUI to Codex. Which is something we'll need to do anyway for T385347.