I was tinkering with the settings of a viewport: vector to hybrid then raster, then tried the line scale menu, and now no updating will happen for any model or document in LayOut.
To test it I created a new blank SU model with one cubic object, and two scenes, created a new document and inserted the model - that works - but copy and change the view from scene 1 to scene 2, and now no updating will ever occur. It’s not the viewport. It’s not the Layout file. It’s not the SketchUp model. It’s the app itself.
So, rebooting didn’t work, and reinstalling the app didn’t work either (I did an incremental upgrade in the process). I didn’t really expect it to, because - at least on a Mac - it just replaces the app itself and none of the slew of preference files, etc that are associated with it.
Next I took the entire folder… (user)/Library/Application Support/SketchUp 2026/LayOut and moved it out to a reserve location so that it would rebuild on relaunch, and that fixed the problem. To my surprise, it still listed my recent files, so that was still being kept somewhere, but it wasn’t a problem.