We're adding a 2px border and a new drop-shadow. We may need to review animations. We should also check that prefers-reduced-motion still behaves as expected, so we don't regress bug 1629303. To check with UX: will we be following system colours for the border, or will it be blue for all users?

To answer the question in comment 0, the border will be blue for all users.

This patch mainly does three things:

1. Changes the necessary CSS to meet the Proton spec.
2. Removes several calls to UrlbarInput.startLayoutBreakout(), because the Urlbar no longer eexpands when the panel is not open.
3. Introduces a [suppress-focus-border] attribute. This attribute hides the 2px focus ring around the Urlbar. This ensures that we don't flash the focus ring when the user clicks a Urlbar that will autoOpen, seeing as the open Urlbar no longer has a border. It also hides the focus border after the user presses Esc, as specified in Figma.

Dao points out at https://phabricator.services.mozilla.com/D105097#inline-590409 that prefers-contrast is not adequate when accounting for various Linux OS themes. We have to use the OS accent color on Linux if we want to maintain compatibility with OS themes. This follows not supporting the new color for the unfocused Urlbar on Linux (see D104898) and not supporting the grey result-highlight colour on Linux (see D105535). We should change focus-outline-color universally on Linux for the same reasons.

https://hg.mozilla.org/mozilla-central/rev/c42d19253920
https://hg.mozilla.org/mozilla-central/rev/f124983faac7

We verified this issue using Fx 88.0b5 on Windows 10 x64, macOS 10.13 and Ubuntu 18.04 LTS.