This might be it…seems I do need an ending slash here, unlike app.yml which doesn’t want that slash. I agree with the points made here:
https://meta.discourse.org/t/a-small-story-about-long-polling-base-url/25781/2?u=jomaxro
https://meta.discourse.org/t/a-small-story-about-long-polling-base-url/25781/3?u=jomaxro
Seems that a PR wasn’t created a 18 months ago to fix this 
I will report back when there is a new update to pull.
Edit: Unfortunately this did not fix it @sam - still no progress bar or console text on /admin/upgrade.
Edti 2: Looks like mini_profiler also doesn’t respect subfolder install- looks like that is Sam’s tool too.