The rotated triangle in the top left corner?

[meflou]

https://transfer.sh/18q5L4/screen-shot-2015-01-23-at-00.49.36.png

What does it serve for? and how can I remove or hide it?

[FichteFoll]

They allow to scroll the tab list if it gets too large.

Set “enable_tab_scrolling” to false.