Adder not fully visible due to z-index miscalculation

[robertknight]

Originally reported in Slack

Steps to reproduce:

1. Go to https://immer.app/en/social-reading/
2. Activate the client
3. Select text

Result:

The adder appears but is not fully visible and does nothing when clicked:

Notes:

The <hypothesis-adder> element is given a z-index of 1. The document's main <article> however has a z-index of 2, so it gets rendered above the adder. Manually increasing the z-index of the <hypothesis-adder> makes it fully visible and interactive.

We have logic (Adder._findZIndex) to find the z-index of the content underneath the adder and choose a higher value. However it looks like it fails to take into account the z-index of ancestor elements of the points used to compute the index.

Related issues: #5622