x/pkgsite: confusing scrolling (and header scrolls off the top)

[pjweinb]

What is the URL of the page with the issue?

https://pkg.go.dev/os?tab=doc

What is your user agent?

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/84.0.4147.105 Safari/537.36

Screenshot

What did you do?

search for the package os.
Click on Functions on the left. (the list of functions gets down to Getwd() so there's more of them to look at, on my landscape display.
The scrolling behavior of the list of functions is sensitive to exactly where the cursor is, without any hint to the user.
If the cursor is in the narrow functions column, the functions scroll. [good]
If the cursor is in the Overview column, that scrolls. [good]
If the cursor is to the left of the functions column, then the Overview column scrolls, which is weird.

Also and in all cases, the whole page scrolls first, losing the headers (I know they are replaced by a bar at the top, but now i've lost the ability to search for another package without scrolling back up to the top.)

What did you hope to see?

1. i expected to keep the "Search for a package" field at the top of the screen even when I scrolled, so I could search for another package without having to scroll back all the way to the top.
2. I expected that when my cursor is anywhere in the left side of the screen, the Functions would scroll, not the Overview.