That's a really good question actually!
I do have an answer for you:
Go into your Project Settings, then Input Devices > Pointing (or just search "touch"), and enable "Emulate Touch from Mouse"
Now... by default the user can scroll downwards even if there's no vertical scroll bar....for some reason....
(it's an engine quirk)
And that is obviously undesirable.
If you have vertical content then you can ignore this (because the problem goes away), but to solve it: select your ScrollContainer node and in its properties, set: Vertical Enabled to be disabled.
And, also.... thanks!
You're welcome! (every single time!)
It is fun to answer questions here sometimes, and it's also really nice to answer questions like yours where you have all the details for your question, and you always give feedback so it's really nice to see your response and be satisfied in knowing if the answer(s) had actually helped in some way.
I don't know enough or have barely enough time to answer even half the answers that pass through here, but I really think of good questions and QA sites as valuable resources for future developers of all walks of life.
So the best of luck with your project and please feel very welcome to keep asking questions if you need! : )