Greetings,

I had a rather large update yesterday and after restarting my emacs
today (I haven't restarted since the update), I found an annoying
arrow-like button at the corner of the window. The only thing I can do
with it is to double click to close the window (I use a tiling WM so
it's useless for resizing). A snippet where you can see it is at [1]. I
can't find anything online on what this is, where it came from, or more
importantly, how to get rid of this eyesore. Any clues appreciated.

[1] - http://fuuzetsu.co.uk/misc/corner.png

-- 
Mateusz K.