This aligns Mac's focus model with other platforms. Matches Chromium, but not
Safari.
Reasons why I think it's worth making this change:
* Consistency with all other platforms.
* Makes the :focus-visible implementation more useful.
* Fixes focus navigation after e.g. clicking a button.
* Shouldn't cause a lot more outlines to show up (at least not by default).
An example of the second point:
data:text/html,<button onclick="this.nextElementSibling.focus()">Click</button><button>Imagine I'm a dialog close button or something</button>
In non-macOS platforms, we won't show an outline for the button in that case,
which matches the developer expectations (links below). We don't show the
outline because the focus comes from an element that has been focused by mouse
(and thus didn't show an outline). But on macOS that doesn't work, because the
button is not focused.
For completeness, the actual heuristics for :focus-visible may change a bit as
a result of the discussions in:
* https://github.com/w3c/csswg-drafts/issues/5885
* https://github.com/web-platform-tests/wpt/pull/27806
But it's not clear to me how to best define this so it works on the macOS focus
model.
An example of the third point:
data:text/html,<input type=text><input type=submit><input type=text>
On Safari and Chrome (and Firefox on non-macOS platforms), clicking the button,
then pressing tab, goes to the input on the right. In Firefox on macOS it
doesn't because the button doesn't gain focus nor is selectable.
Differential Revision: https://phabricator.services.mozilla.com/D108808
An explanation of the Firefox Source Code Directory Structure and links to
project pages with documentation can be found at:
https://firefox-source-docs.mozilla.org/contributing/directory_structure.html
For information on how to build Firefox from the source code and create the patch see:
https://firefox-source-docs.mozilla.org/contributing/contribution_quickref.html
If you have a question about developing Firefox, and can't find the solution
on https://firefox-source-docs.mozilla.org/, you can try asking your question on Matrix at chat.mozilla.org in `Introduction` (https://chat.mozilla.org/#/room/#introduction:mozilla.org) channel.
Nightly development builds can be downloaded from:
https://archive.mozilla.org/pub/firefox/nightly/latest-mozilla-central/
- or -
https://www.mozilla.org/firefox/channel/desktop/#nightly
Keep in mind that nightly builds, which are used by Firefox developers for
testing, may be buggy.
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