Highlight style controls can get hidden behind page header

The highlight style controls toolbar can get hidden behind headers on a page that have a Z index specified, such as https://data-apis.org/array-api/latest/purpose_and_scope.html.

I appreciate this bug might become invalid soon if the controls toolbar is moved to a different place in the UI.

**Steps to reproduce:**

1. Enable `styled_highlight_clusters` feature flag [1]
2. Activate client on https://data-apis.org/array-api/latest/purpose_and_scope.html

**Expected result:** Highlight styles toolbar is visible
**Actual result:** Highlight styles toolbar is hidden behind page header

[1] Hypothesis staff can add themselves to this feature cohort: https://hypothes.is/admin/features/cohorts/20