When using GTKv2, using the middle mouse button to apply some objects in the inventory will cause the inventory list to suddenly scroll to a different location. I have observed this on v1.70 and 1.71 on Linux, and as of recently, in Partmedia's r19668M Windows build. Right now, it seems to occur exclusively on wand-type items and spellbooks.
Screen capture of the bug, sent by submitter:
https://www.dropbox.com/s/rnjvwhhhm4hl6as/Desktop%2012.30.2014%20-%2023.55.42.04.mp4?dl=0
I've been able to somewhat reproduce this problem by applying consumable items (e.g. scrolls) with a large number of items in my inventory.
I can also say I've experienced this issue. I think there was a commit a while back that was supposed to have fixed this, but it only made it less prevalent.
This also happens on identifying items.
My experience is that it happens when there is a large number of items so that a button appears to show the next group. It seems to occur when, for example, trying to read a document at the bottom just above the click to show next group. I think it may also be true that the "click to show next group button" can sometimes show when there actually aren't more items to show (perhaps an off-by-one issue?).
I've never seen a "show more items" option in the main inventory pane, are you referring to the ground contents pane?
There is an option to remove the inventory scrollbar, and instead have the first and last item of the inventory list change inventory pages.