I think this problem has been around for me for several versions and it's still happening after I updated to 12.31, so finally I decided to take the time to write a post about it today.
As you can see, the scroll bar jumps back up when I scroll down in this specific folder. I have tried various other folders containing different number of files and subfolders, it only seems to occur for this particular folder.
The same thing happens when the trackpad or a different mouse is used.
This does not happen with File Explorer or anywhere else on my computer.
If you turn on Preferences / Folders / Folder Display / Show generic icons for... = All folders, does that make a difference?
If it doesn't change things, I would try isolating the problem to narrow down what triggers it. For example, more the folder into an empty folder on its own, and then create some empty folders above it so you can scroll down to it and see if it still happens in the other location.
There aren't many things in Opus itself I can think of which would make the file list scroll when an item appears, but (as an example) something the folder triggers (like fetching its icon) could be calling into someone else's code that is sending a scroll message for some reason.
We've also seen some mouse drivers overflow the scroll amount parameter if you scroll too quickly. They end up scrolling backwards as a result, if you move the wheel too fast or too long. But I'd be surprised if it's that from your video, as it happens so quickly. Still, it's worth checking if the same thing happens when you scroll via other methods, like dragging the scrollbar and holding down on the cursor key.
Turning on Show generic icons for all folders did not solve the issue.
I tried moving this folder into an empty folder and creating empty folders above it, no jumping observed.
Back to within folder itself,
Scrolling slowly using the wheel: problem persists
Scrolling by dragging the scrollbar and holding down cursor key: no more jumping observed
Scrolling by holding down on the down arrow at the bottom of the scrollbar: problem persists
This jumping seems to happen only for the first time you open it in a tab, once the jumps have occurred, the tab showing this folder will scroll without a problem. But if you open a new tab showing this folder, the scrollbar will do the jumps.
I moved the contents within this folder into two different folders: one containing all the files and one containing all the subfolders. The one with the files scrolls without a problem, the one with the subfolders jumps back at one location along the scrollbar.
I tried scrolling slowly to see which folder appears at the bottom of the window when the jump back to the top occurs and removing this folder to see if any particular folder might be causing this. I removed multiple folders and the jump just seems to happen at the same location along the scrollbar regardless of the sorting order of the folders.
I might have misunderstood when you suggested clearing any labels set on the folder. I cleared the label set on the folder itself and it did not solve the problem but when I cleared the label and status of all the files and subfolders within the folder, the jumping back no longer occurs.
I'm really curious as to why labels could have such an effect on the scrollbar.
If you have a label that uses the "pin to top" option, that could be triggering it somehow, since it re-orders items.
Labels are processed when the item first becomes visible, which means the "pin to top" option doesn't work well in folders with enough files to scroll. This was an oversight when it was originally implemented, which we're going to correct fairly soon. I'm not sure why it would cause the list to scroll, though, unless the item that moved had focus (which could happen when using the arrow keys but wouldn't normally happen with the mousewheel, unless single-click/point-to-select mode was on, maybe).