When the Fennec implementation of browserActions got replaced with a new implementation for GeckoView/Fenix, many tests were removed.
There was a test that verified that after opening an extension panel, that the original tab is still viewed as the active tab, but that got removed here:
https://hg.mozilla.org/mozilla-central/rev/64fde5f3d49f#l15.2
(side note: the test is imperfect - it does not verify that the tab.url is as expected, even though it should)

I just ran a manual test on Fenix, and the behavior works. We should restore the unit test to make sure that this continues to work.

The test can be one shared by desktop browsers and mobile (put in toolkit/), by writing it as a mochitest that uses the AppTestDelegate.clickPageAction helper (from bug 1681360).

Moving to extensions component, since this bug consists of just adding a test in toolkit/components/extensions/test/mochitest/ (or mobile/android/components/extensions/test/mochitest/).