I've observed this effect as well, it's probably some kind of rounding error.

We fail to reproduce this with R59.

I did have this happen to me on daily basis in R59 though. I don't use mouse in ADOM at all and despite that the bug happened rather often. When the mouse was moved, the bug stopped but eventually started again. Might had something to do with me often using alt+tab to switch to different things on my PC while the game was running. I'd love to give a more detailed report on this, but after a while I kind of got used to it and I stopped noticing it. I'll try to pay attention when it happens next.

I'm playing in full screen mode, OS is Win 8.1, desktop resolution is 1440x900 (NotEye set to preserve it).

Happens for me in R59 as well. In fullscreen mode. My resolution is 1366x768.

OK, I observed this bug for a bit and here's what I could figure out:

http://i.imgur.com/1DsMrQH.png - admire my mad Paint skills. I wanted to grab a screenshot with NotEye, but it resulted in hiding the gauntlet cursor AND moving the yellow arrows cursor somewhere else for some reason, so I had to make this image instead.

So, whenever this bug happens, the game behaves as if it can't decide where yellow arrows should be, making them jump from one tile to adjacent and back in rapid, but not constant pace. However when you move your mouse (moving it several pixels also stops the jumping of the yellow arrows), it turns out the now visible gauntlet cursor is actually on a different tile, as pictured. When this bug happens and you try to move your mouse slowly, you'll see that the game incorrectly detects on where exactly gauntlet cursor is, the possible difference being anywhere from a few pixels to as much as the whole cursor size. So, it's possible to have one cursor pointing to one tile and the other cursor highlighting another tile. If you get the gauntlet to a position messing up the detection (either to where you started or a new one), the jumping yellow arrows part of the bug will reactivate.

You can get rid of this problem by moving the gauntlet cursor to the edge of the screen. If, as pictured, the game thinks the gauntlet cursor is higher than it really is, you need to move your mouse to the top edge of the screen. When the game thinks the gauntlet cursor is more to the left than it really is (bug can happen horizontally or vertically, never had it happen both at once yet though), you need to move your mouse to the left edge etc. This fixes the cursor position detection.

To make the bug appear again you need to alt+tab from and back to the game and then move your PC enough to make the screen scroll to adjust to PC's position.

I haven't been testing this extensively, but it seems I can reproduce this rather consistently after it appears for the first time.

Thanks Scooter Fox, this was very helpful. Fixed for r60.

(The problem was because NotEye: (1) auto-scrolled the map, (2) computed cursor location based on the mouse position and map position, (3) fixed the map location so that it does not get scrolled off the screen, (4) drawn the map. So, when auto-scrolling required a fix (which happened on large screens / when the PC was close to the border), cursor location was still based on unfixed scrolling. Now fixing also happens just after auto-scrolling, not just before drawing the map, which fixes the problem.)