Linux 64 bit, NVidia GTX1080.
Blender 2.8, rB3df809958b7, rB6d152c59971 (the former one seems to be a culprit commit)
Short description of error
Hover buttons space header with a mouse, scroll the wheel.
Note that I cant redo this on my system with 6d152c599712bf4024b29f86a6050daba2919397
Managed to redo - was caused by scroll bar drawing, finding the active action zone was causing a redraw.
Split this into find / refresh versions of this function, fixed by rB372754c164793e8e1b0d8a7f1f0dbe9fd432fdf3 - now merged.