The branding is standard throughout the site and comes from the same code.
That said, accidentally leaving the process shouldn't actually ruin the process, so I'd be more interested in hearing in what way the process broke for you.
We have a system in XF2 which tracks your progress through an upgrade, so even if you close your browser window, and come back to it, it should generally be able to resume without an issue.
There's potentially an edge case exception to this when it involves column renames or columns being dropped, where the step could throw errors if we try to rename or drop them again (because that action already happened).