Hi all,
Just an update that
Inventory Insight appeared to be the offending addon.
I will follow up if I continue running into issues, but from the addon's comments page it was already causing some issues. Hopefully it can get updated soon especially since LibAsync's recent update just "broke" it.
Clarification edit: LibAsync was not ever broken. Version 2.2.0 was temporarily not compatible with Inventory Insight. I knew enough to put a copy of LibAsync 2.1.0 in IIfa's libs folder and direct IIfa.txt accordingly. IIfa is now updated in any case!
I would still be interested if someone has an answer as to what that blotchy overlay is and why other addons like TOM would have messed up anchors (hopefully that makes sense).