This is just a colour change in the default themes. The address bar will now be darker than the rest of the toolbar.

Duped from Jira:

We currently apply a subtle drop-shadow when the unfocused Urlbar is hovered. With the Proton redesign, the Urlbar appears flatter, without borders. Should we continue to apply this drop-shadow, or does that disrupt the flatter look?

I'll mark this blocked-ux because we can't land without an answer, but I'll continue working on this and post a patch for review. We can just drop the one drop-shadow rule if needed when we get an answer.

Another question duped from Jira:

We currently allow themes to set custom borders around the Urlbar. See the toolbar_field_border section at: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/theme. In the UX spec, the Urlbar has no border when unfocused. Are we dropping support for themes setting a border on the Urlbar, or will our themes just not set one?

Presumably, in the latter case, our themes would just set a transparent border so we have consistent spacing.

From UX:

The current shadow that appears on hover is very subtle. I propose we remove this drop shadow to maintain the flat appearance.

Unless it is an issue, we should support custom borders around the URL bar especially for people that currently have custom themes. Some themes have very similar colours between the URL bar and chrome and rely on the border style for visibility.

https://hg.mozilla.org/mozilla-central/rev/78f0c795c492

Hi,
Just to make sure I understood it all. With proton browser.proton.urlbar.enabled pref off I should still see the shadow that appears around the address bar on hover when the address bar is unfocused. With the proton on, I should see no shadow on hover. Am I right?

Yes, that's right.

Thanks, then I've verified this behavior on beta 87.0b2.