Replicate zoom level when scrolling to message position

[pklaschka]

As requested by Mike Acler on Twitter (https://twitter.com/agilek/status/1319029990634229761?s=20)

"I'd like to see original zoom level when clicking go to viewport position."

One challenge with this is that the same zoom level looks very different on different window sizes. What probably should get replicated as good as possible are the bounds of the visible viewport area (instead of, as it is now, just the center).