It's... an improvement. But it feels like they've replaced one design flaw for another. I can understand why they might choose the multiple panes pattern for a tablet/mobile experience, but there's no need for it on a laptop/desktop, where the dynamic sliding
behaviour is a distraction. They've also moved the list of pages to be part of this multiple panes pattern - they were perfectly fine where they were, on the right-hand side of the window! Another issue is that the panes can't be set to a custom width - too
bad if you want to see the full titles of sections and pages at-a-glance.
OneNote works (on Windows, at least) because of its notebooks-at-the-left, sections-along-the-top, pages-down-the-right layout. To me, it's one of the fundamental parts of what makes OneNote (on Windows) worth using. The refusal to replicate a consistent
experience for other OS' is baffling.