Question on Disclosure Navigation Menu

Hello!

I am trying to use the example for the Disclosure Navigation Menu on:
https://www.w3.org/WAI/ARIA/apg/patterns/disclosure/examples/disclosure-navigation/

I'm noticing the drop down menus appear to flash open and then close when
the page loads and I'm not sure what is causing this.

I noticed this behavior on the original example when I click on the link
w3.org
<https://www.w3.org/WAI/ARIA/apg/patterns/disclosure/examples/disclosure-navigation/>
and click on "Open in Code Pen
<https://www.w3.org/WAI/ARIA/apg/patterns/disclosure/examples/disclosure-navigation/>"
so I can confirm it's not my edits causing this behavior. I also tried on
different browsers such as chrome and edge with the same results.

Is this expected behavior or a bug of some sort?

Thank you,
Lailah

-- 

Lailah Hoque

Developer/Programmer Analyst

Enterprise Data and Business Intelligence

Office of Information Technology

Rice University

https://data.rice.edu/

(713) 348-6365 | lgh2@rice.edu