Moused-over results are currently styled gray in the CSS using :hover, which means that if your mouse happens to be over the popup when it opens, you'll see a gray result. Which is maybe surprising, annoying, or information-overloading. We should consider replacing the :hover rule with a mousemove handler in the XBL that sets an attribute on the mouse-moved result, and then style that attribute. That way a result turns gray only after you move the mouse.

We should also cancel the mouseover state if you start navigating with the keyboard and keep it canceled until you move the mouse again.

WIP that does what comment 0 says for the urlbar popup. Still need to do comment 1, and also figure out how to do comment 0 for tree-based autocomplete. Specifically wondering how to style a specific row. I haven't worked with trees in a while, but I think you have to implement a custom view that returns something for view.getRowProperties()...?

Oh yeah but this is autocomplete, so there's nsAutoCompleteController::GetRowProperties, which is populated according to the search results (e.g. by UnifiedComplete)...

This issue is also affecting the one-offs buttons on the latest Nightly 55.0a1 (Build ID: 20170525030225) on Windows 10, Ubuntu 16.04 and Mac OS X 10.12.