What is the line limit on reports viewed through the reporting (non-saved search) interface?

IIRC it's something like a million lines out of the box, but if you have something that exceeds that, you can request that they bump up your limit. I'm not sure if they'd do so if you're not on a pretty high tier.

The UI seems to truncate well below 1 mil. The aggregate totals are correct but detail lines aren't rendered past some unknown limit.

Actually you're right. I think it truncates at 75-100k by default, but you can bump it up to a million or more? It's been a while since I last dealt with the scenario, so my memory is fuzzy.