x/pkgsite: border around examples after click

[prattmic]

What is the URL of the page with the issue?

https://pkg.go.dev/strings#TrimLeft (any page with examples)

What is your user agent?

Mozilla/5.0 (X11; CrOS x86_64 13421.102.0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/86.0.4240.199 Safari/537.36

Screenshot

What did you do?

1. Click "Example" to expand the example.
2. Click anywhere in the expanded view (the word "Code:", in the code/output box, or even the whitespace above the code box.)

What did you expect to see?

Click (2) does nothing.

What did you see instead?

Click (2) adds a black border around the example. Perhaps this is intentional? But it seems superfluous to me.

[prattmic]

Ooo, this one is fun: https://pkg.go.dev/golang.org/x/net/ipv4#example-PacketConn-ServingOneShotMulticastDNS. The border around this example extends up through some of the text above the example:

[jamalc]

Related to #42698

[julieqiu]

Closing as a dupe of #42698.