regression: scrolling with mouse wheel in explorer will freeze the system after some seconds

r75580 gcc dbg
on VBox 4.3.40 no guest additions.
Just scroll a bit with the mouse-wheel in explorer file-browser (e.g. in system32)

The system will freeze. Nothing in log, no break into debugger.
I got it three times already, Pi_User5 one time.
This regression must have been introduced within the last 2days - ultra-fresh.

We suspect r75566, but I did not double-check the guilty rev yet.