Application.TickGlobalCallbacks causing issues working in Unity 2020. What is this?

A popup window in Unity with "Application.TickGlobalCallbacks" frequently, but not consistently, appears while I'm working in Unity 2020.1.1f1. It has appeared after I placed game objects in the scene or just clicked on an asset from the project folders or selected a field to edit in the Inspector pane or applied a texture layer to the terrain. It doesn't appear every time, but frequent enough to be very annoying.

All actions have to wait until this window disappears. Most of the time, the popup window disappears within a few seconds, and I can continue working in Unity. Sometimes, however, the window will show an amount of time (e.g., "busy for 8:10"); usually, when that happens, the application never recovers (I've waited overnight one time and it still said it was busy the next morning).

I don't have this issue if I'm working in earlier versions of Unity (2019 and before).

I can usually just search on Google for issues I have, but this one has me stumped. I tried searching for "TickGlobalCallbacks" but the search returns 0 results.

Does anyone know what this is?