Share drop-down separates from Share button when scrolling down using documentation theme, preventing sharing with more users than fit in one screen

Symptoms

When using the documentation theme, if the share button has been clicked, the drop-down scrolls together with the page. With the default theme this does not happen, the share drop-down stays together with the share button.

This wouldn't be too much of a problem by itself, but it becomes an issue when sharing the page with enough users that you would ordinarily need to scroll past the bottom of the share drop-down to add more.

Steps to Reproduce

1. Create or change a space to use documentation theme
2. Create or find a page that is long enough to scroll
3. Click on the Share button on that page
4. Scroll down
5. Observe the dialog follows the page and separates from the share button

Workaround

Do the page share in multiple batches, each including a sub-set of all users who need the page shared to them.