This popup was recently introduced in PR-2174.
I don't know exactly why, but voyager prompting me that I'm about to lose my progress was sort of strange to me in the context of scrolling on lemmy.
I understand why someone wants such a feature, but IMO this should be turned off by default and available as an option.
One of the developers told me it's impossible to return to the last feed position. I don't know why, but, I'm not a programmer. I don't doubt it. It was possible with Reddit clients. Could be something Lemmy hasn't implemented, or hasn't implemented yet.