Universal `overflow: auto` rule on OpenStax hides adder about client HOT 2 CLOSED

OpenStax pages are also affected in a limited way by the font size issue captured in #4615.

from client.

We protect most of the UI from interference by the host page by using Shadow DOM, inside <hypothesis-*> host elements. The host elements are protected from most unwanted rules by having a custom element name, so as not to be affected by generic rules for <div> etc. However universal selector rules (* { ... }) can still affect them, as well as some combinators involving universal selectors (body > * etc.). The simplest solution in this instance would be to add a higher specificity selector that resets just overflow on the <hypothesis-*> host elements, and leaves a placeholder to reset any other rules that we find to be problematic in future. We could also do something more generic and add eg. hypothesis-adder, hypothesis-sidebar, hypothesis-notebook { all: revert }.

from client.