Mark, this continues to occur and I have previously submitted this issue but I have tested more since then. This happens with every browser I have tested.
When viewing Messages or Topics in any group, Infinite Scroll absolutely fails with any browser window length in excess of 1600 pixels. However, when viewing Expanded, it seems to work just fine. The width of the browser window does not seem to be a factor.
Anyone using an HD 1920x1080 monitor in Portrait mode would experience this, I believe. I use a 43" UHD 3840x2160 monitor and like my browser windows at 1800x1800 but I have to make an exception for GIO and stick with 1920x1080.