If you:

1. Pick up quest using "Move" command
2. Hold middle mouse key to slide screen a bit (It is important how much you move it)
3. Scroll mouse wheel (up or down)
   Basically how i see it. If you move picked icon away from original location scrolling speed increase drastically.

If you done it, you screwed. It would take AGES to return icons back at normal quest coords.

Here is GIF
https://i.gyazo.com/5153abd9c10f14289bde6a5fd5f2dbac.gif

I've done this to myself a few times. Funny thing is, the tooltip for the coordinates was added because I kept doing this and needed to figure out where I had ended up. It's a kinda weird edge case I need to write handling for but haven't really gotten around to due to it being a low priority annoyance and not all that hard to avoid in most cases.

PS: The easiest way I get back to where I was is to right-click cancel the move tool and then it'll snap back to the normal quest line bounds. Then you can retry moving it, hopefully without zooming off into the abyss this time.

Oh ok...
But maybe you can limit side scroll speed? Artificial limitation.