Anybody seen that too? OS X 10.11.6.
Yes, I see it: no bottom border when listbox.border = false (also setting it to false in the open event).
Please file a bug report.
I can’t (I cannot install Feedback).
It’s already been reported. I’m on my phone so I can’t find the number, but I reported it ages ago.
Here’s the case: feedback://showreport?report_id=38278