Thank you VERY much, Mr. dsoneuk!!!
Awhile back, my son set up an ad blocker on a dedicated PC. It's done a great job eliminating a lot of ads in the background before they are able to get to any of our devices. Unfortunately, it sometimes blocks sites it THINKS we don't want. For those, we have to manually add them to a white list. It appears the collapsible function needed to access a site the ad blocker had blocked. Once I added it to the white list, the problem went away. The w3school example and my web-site menu now are able to work as designed.
I wasted a lot of time trying to figure out what had gone wrong. Your suggestion pointed me right to it! (I'd never used the web development tools before.)
Thanks again!!!
Tony