Blender 2.80 (sub 45), 893fa598319e, 2019-02-24 22:02
How to reproduce:
Move mouse pointor over brush`s big icon, it reacts with highlight. Then, before brush-search window appear, press and hold MMB.
Then LMB to discard. Now scroll sticks. It releases after Esc. But at first I was forced to restart blender, because pointer stuck there.
Suggestion: brush search should not appear if MMB pressed, until it released. Should not appear if after release mouse pointer is not over brush-icon
Here is the video