Created attachment 4424 xfce4-diskperf-plugin-facilitate-bringing-up-the-config-dialog.patch Similar to bug #7251, place the event box above its childs.
I've adapted it in commit d663055 since the added line was a copypasto of systemload fix and didnt compile :) Please at least try your changes to see if they effectively fixes issues before blindly submitting patches ..
Thanks for adapting and applying. I always test the changes, so I must have uploaded a wrong file. My mistake. Many bugs and patches about the same issues recently, so I might have got confused ;-)