Re: Oh, I get it! You were in multi-window mode but maximized all of the windows! Something I didn't try. I tried both multi-window 'restored' mode and single-window mode, and both of them seem to work as expected.

Yes, that's correct, I have always used Multiple Windows mode with all windows maximized (because I didn't realize there was a One Window mode).  It wasn't until I had all windows minimized and resized the Item List window to be almost full size (per your suggestion) that resizing columns worked.