The "Settings" button is encapsulated in a <gaia-header> element, which takes the entire horizontal space of the region that it occupies. This is much more than is needed, and the only advantage of using this element is the positioning that we get for free, incl. RTL support. To fix this, we can make it a single |<div><a></a></div>| block that we position manually.
https://github.com/fxos/customizer/commit/63a010b35b9cf6d86e039959651eb3b03c582dc6 I also landed some changes to MutationObserver here.