When I first boot the system everything works fine. But then something happens (not sure what triggers it) and many programs start scrolling just by moving the mouse over the window. So far I see this happens in Nautilus, Document Reader, Archive Manager, Ubuntu Software Center, Meld, and System Monitor (what do they all have in common, GTK?). It does NOT happen in web browsers, thunderbird, programs using Mono, Java, etc, for some reason.
To be more specific of the issue: if the mouse starting position is over a document in Document Reader, for example, and then I move the mouse via touchpad, it'll scroll the document like an anchor, where the position of the mouse over the document doesn't change (yet it scrolls). I have already tried disabling "two finger scroll", "disable while typing", and the "tap to click" options, but they made no difference (after the glitchiness starts).
The other odd case is that in Nautilus, if the mouse starting position is over a file or directory, it sometimes selects it instead of scrolling. Single-clicking a directory will open the directory (so the initial touch acts like a clicks, and letting go acts as the second click).
The only thing I can do to temporarily fix the problem is restart the computer. There are no issues when using a mouse.
Update 2015/05/03: I see how to reproduce it now. It appears to be a glitchy two-finger scroll issue. If you scroll any window with two fingers a lot, eventually it'll start causing this problem. Try various actions with two fingers, like "clicking" with two fingers, or scrolling in various directions quickly, etc, and it will start happening. It's interesting that it only happens in certain programs and not in others, which makes me wonder whether it is really a trackpad issue or some framework's issue. I can confirm that this is happening on the 15.04 Live CD, so this happens out of the box. If anyone has any clues, please update me!
The only work-around for this issue is to disable two-finger scroll (if it's already glitchy you'll need to restart). Not sure if it matters, but I'm using the Dell XPS 13 2015 9343.
Update 2015/05/05: Apparently this bug can still happen even with two-finger scroll off, though it's more rare. Again, only certain programs have the issue, so it's hard to tell if it's a touchpad issue or something else.
Update 2015/05/12: I have filed a bug report here: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1454473