Affected versions

• 89.0a1(2021-03-28), 88.0b4;

Affected platforms

• Ubuntu 20, WIndows 10;

Steps to reproduce

1. Launch Firefox, access about:addons
2. Press the TAB key a couple of times;

Expected result

• no visual issues;

Actual result

• dotted line selecting whole page, but only the right part and section picker area display it; rest of page hides the line;

Regression range

• will check and provide one asap;

Additional notes

• reproduces with proton prefs both on/off.
• attached screenshot to illustrate the issue.

Confirmed on Win10 as well with current Nightly.

Ran a bisection from 2019-07-01 to 2020-04-02 (as mozregression used from CLI failed to download the latest build, but managed to download this one which is affected by the issue), with the following results:
6:46.82 INFO: Got as far as we can go bisecting nightlies...
6:46.82 INFO: Last good revision: 518df4329a20f76b3bffd5f6201b007fdebc274f (2019-11-01)
6:46.83 INFO: First bad revision: 16aac90f6572f363d2b5c8205e13115ac818869d (2019-11-02)
6:46.83 INFO: Pushlog:
https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=518df4329a20f76b3bffd5f6201b007fdebc274f&tochange=16aac90f6572f363d2b5c8205e13115ac818869d

From the pushlog, I suspect that the regressor might be https://bugzilla.mozilla.org/show_bug.cgi?id=1525175 ([about:addons] Move header into HTML views).

Please note that since Nightly 69.0a1 until today, the selection area has been increased to the whole about:addons page from just the area beneath the page header comprising of the list of recommended add-ons. Please see the attached screenshots from more details.