We should consider the case of when a user has two engine aliases registered, one of which begins with the other in its entirely. For example, they have Amazon registered as "a" and AliExpress registered as "ali". When the user types "a" we should show the Amazon tab-to-search result. But when they then type "l", we should replace the Amazon tab-to-search result with the AliExpress one. We want to be sure that we don't reanimate the tab-to-search result to avoid annoyance.

The description talks about engine aliases, but I think there was some confusion between the specified behaviour of tab-to-search results appearing for autofilled engine domains and bug 1647901, which proposes different behaviour. My patch only addresses the autofill case. For example, when the user has amazon.com and aliexpress.com registered as search engines and has both in their browsing history. If they type "a", we should show the tab-to-search engine for whichever domain is autofilled. As they continue typing and the other domain is autofilled, we show the other tab-to-search engine.

https://treeherder.mozilla.org/#/jobs?repo=try&revision=8f730c3b3717a5a141452d672eb64fdc9c18a6cb
https://treeherder.mozilla.org/#/jobs?repo=try&revision=f3753ac0c685bcdfc3ff38dcbb9c1253eb1fc225

https://hg.mozilla.org/mozilla-central/rev/aa9339a3e611

We can confirm the implementation of this enhancement. We verified it during our Nightly testing. The verification was done using Fx 83.0a1 (2020-10-13) on Windows 10 x64 and macOS 10.13.