If about:addons is open when a background update happens, any hotfix or system add-on updates found show up. On refresh they go away. System add-on updates are never uninstallable or user-disableable so there isn't a danger of users accidentally removing something important.
I suspect the issue here is there is an install listener that isn't checking to see that an add-on is hidden. The fact that this happens to hotfix too makes me think this is probably an old bug and so was overlooked for system add-on updates too.
Comment on attachment 8871348 [details] Bug 1367800 - do not display hidden add-ons in about:addons during upgrades. https://reviewboard.mozilla.org/r/142826/#review147138 I think we should also check if the add-on is hidden before adding the item, so they don't briefly show up during the download/install and then mysteriously disappear. r=me with that.
Attachment #8871348 - Flags: review+
aswan points out that we do have browser tests for ensuring that hidden add-ons don't appear in the UI, so I will add some tests now (was going to defer it to a followup, when I thought we didn't have test coverage for this).
Attachment #8871348 - Flags: review?(aswan)
Created attachment 8874399 [details] System add-ons visible in the Add-ons Manager As I’ve said in the duplicate bug 1367559 comment 2, I’ve previously not had a screenshot of system add-ons in the Add-ons Manager. After a recent system add-ons update, I finally got a screenshot of system add-ons in the Add-ons Manager, so I’m posting it here, because bug 1367559 is a duplicate of this bug.
Summary: hotfix and system add-on updates show up in about:addons if open during update → system add-on updates show up in about:addons if open during update
Summary: system add-on updates show up in about:addons if open during update → hidden extension updates show up in about:addons if open during update
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