It would help if you linked to the thread so we could see what they had said:
You've reported both problems in one thread there, so it's difficult to know which is being discussed in most of the replies, except where there are screenshots.
The screenshots of the post editor (going too high up the screen, so you can't drag the splitter down again) show an issue I've seen myself. A screenshot would've helped here originally! It's not about scrolling things, but dragging the splitter to resize the post editor.
That problem happens because the height of the post editor seems to extend too high up sometimes, meaning the splitter is off the top of the page and can't be dragged. I've seen it myself a few times, but I'm not sure what triggers it exactly, so it's difficult to test if it happens on any other forum.
The post editor remembers how high it was, and at least should be limited by the current window size (which works OK here from a quick test today).
If you see the problem you can fix things by closing the editor, resizing the browser smaller (assuming the tablet lets you) and then opening the editor again and resizing it larger. That should let you see the splitter and drag it smaller, which is then remembered.
The only theme/extension we use that might change what happens is Header Submenus - theme-component - Discourse Meta which uses a small amount of space at the top of the screen for the extra menus.
Discourse used to have an issue where that space wasn't always accounted for, so maybe that's still the case now, but it's difficult to say without knowing a way to reproduce it, and I haven't seen the problem myself in months.
Edit: I think we also tweaked the CSS for it to try and fix that ourselves. But it also sounds like you saw the same problem on their forum eventually, which suggests it's not likely to be down to that theme or CSS:
However, I cannot scroll through the replies once I start to reply here. I can only see the bottom of the post I’m responding to. Now it’s making a liar out of me. It started scrolling but I don’t know what I did.