To reproduce, have a long job that keeps printing logs, and compare Blueocean behavior with the old UI. In general, the experience with the new UI is not great:

Both auto-scroll as new logs are printed, ok so fra. But:

• There are two vertical scrollbars in Blueocean for some reason
• Blueocean does not stop scrolling in all cases where the old UI would. JENKINS-41885 made it stop the auto-scroll-on-update on a mouse-wheel scroll, but grabbing the slider and scrolling up by that does not stop the auto-scroll, resulting in a poor user experience. Old UI would stop in that case. Same deal with PgUp/ Home key presses.
• It is not possible to make Blueocean resume auto-scrolling. Old UI resumes auto-scrolling when the view is moved back to the last displayed log.
• It even seems like Bluoecean stops updating, not just auto-scroling on the mouse-wheel scroll, requiring a page refresh to see new logs after the user has scrolled up.
• There's a misleading "complete log" link at the top which leads to a truncated log when the build is in progress (in my test the dynamic view is at log line 22000, "complete log" ends at 9700). The old UI defaults to a partial view with live updates and has a "full log" link to a full view also with updates, so this new behavior is surprising.