You must have forgotten the iframe hell that was the early aughts. There are some sites that could benefit from individually scrolling elements. If you have a navigation pane that doesn't hide, it would be hell to have all of the navigation links above the fold while all of the relevant content exists multiple scrolls/folds/pages below.
I think that many sites wouldn't use individual scrollable elements correctly. Much like sticky headers, you end up with an upside-down L-shaped (like this: |‾) area of static, individually scrollable content. Your viewport shrinks to 75%. That's annoying on desktops at best and pretty much unusable on mobile at worst. It's reminiscent of Toolbar Explorer[0].
If you can use individual elements nicely, go ahead! I believe that if the trend caught on, it would be worse for everyone.
It's true that the usable screen area shrinks. It also shrinks with sticky headers or similar... If that's a problem, maybe add some Javascript to make the headers optional?
Or just don't do "frames"/"overlays"/"sticky". That's still better than messing with the global scrollbar (like Twitter does for example) and totally wrecking user experience.
I don't know that problem. Browsers should try to zoom everything proportionally, and I think they have gotten quite good at that (using Firefox almost exclusively).
Another thing that often annoys me is how sticky headers break text search. When you search and the browser scrolls just enough to get the hit "on the page", but still covered by the sticky header. The problem is: not using proper frames. Sticky covers part of the other frame. Bad.
Can't reproduce a problem with Firefox and don't have a phone handy. Maybe you can post a screenshot? The sidebar zooming too is what I would expect. At some insane zoom level the sidebar goes away, which is probably due to CSS and could be intended.
I don't see a sidebar covering content. Of course the sidebar takes some space, but as long as it's not over the content that's a different issue IMO.
This should no longer be the case, at least for Safari, Edge, and Chrome. All of these "detach" fixed/stick position elements from the viewport once you start zooming.
I think that many sites wouldn't use individual scrollable elements correctly. Much like sticky headers, you end up with an upside-down L-shaped (like this: |‾) area of static, individually scrollable content. Your viewport shrinks to 75%. That's annoying on desktops at best and pretty much unusable on mobile at worst. It's reminiscent of Toolbar Explorer[0].
If you can use individual elements nicely, go ahead! I believe that if the trend caught on, it would be worse for everyone.
[0]: https://c1.staticflickr.com/3/2040/1924189728_668c4bc4e2.jpg