HTML Error 0 for autocomplete input fields when disconnected to server

This is a duplicate of #1232416: Drupal alerts "An AJAX HTTP request terminated abnormally" during normal site operation, confusing site visitors/editors (for 8.x). It would have to be committed to 8.x, then 7.x, then 6.x. The chances aren't great. But the answer there is probably appropriate.

Reopen if you see fit.

I don't think this is a duplicate. The other issue is an "error" being alerted when there's absolutely no error of any kind. This issue is an "error" being alerted due to a network timeout / disconnection, which is in fact an error. We may want to fix both issues, and their solutions might be related, but I think it makes sense to keep both open.

This behavior exists in D8 too, so setting version accordingly. A good solution, if there is one, might be backportable to 7 and 6.

Same as the other issue, need #1419652: JavaScript logging and error reporting to do anything about this.

Although this one is marked postponed, we are triaging it as part of a Bug Smash triage session. I'll try to test this on 8.9 and 9.1 to see if it's still an issue.

I tested on 9.1 as follows:

1. Used the Umami profile and Firefox
2. Installed the masquerade module
3. Placed the masquerade block in a region
4. Typed into the username autocomplete field using the beginning letters from some users (e.g. "h" and "g")
5. Shut off the server
6. Using same the browser tab from step 4, typed in the same letters as before
7. In some cases, I got the throbber image immediately and, in some cases, I did get it after awhile
8. The browser cached the autocomplete options so they did show up
9. I chose a username and clicked the "Switch" button
10. The "Unable to connect" error was shown
11. Clicked the browser's back arrow
12. Then the browser did not have the usernames cached anymore and the throbber did not show up
13. Repeated the above steps 3 times and it was consistent
14. Checked one last time with the dev tools console and didn't see any errors

I didn't not see the HTML Error 0 error as a popup or in the console log.

Forgot to add the screenshots.

Throbber not showing up but browser cache shows name

Throbber showing up and browser cache shows name

Unable to connect error page

After using browser back button, throbber not showing up and browser cache does not show name

Nothing in the console log

Testing 8.9 showed the same behavior as 9.1. I didn't not see the HTML Error 0 error as a popup or in the console log.

I'm going to consult with the bugsmash team to see if I can close this or it needs to perhaps stay open for Drupal 7.

Since this is not reproducible in 8.9 or 9.1, @jibran said this can be moved back to 7 so switching it.

For completeness, I tested this on 7.8 in a similar way as follows:

1. Installed 7.8 standard profile and Firefox
2. Added two users: holly and grace
3. Installed the masquerade module
4. Placed the masquerade block in a region
5. Typed into the username autocomplete field using the beginning letters from some users (e.g. "h" and "g")
6. Shut off the server
7. Using same the browser tab from step 4, typed in the same letters as before
8. If I used the letters I used before, I didn't get the error popup as the browser cached the usernames
9. If I used a new letter, the error shows up
10. Typed in holly and clicked the "Go" button
11. The "405 Not Allowed" error was shown (note that I used ddev for this test while using quickstart for the 8.9 and 9.1 tests)
12. Clicked the browser's back arrow
13. When I did this the first time, I got an un-themed version of the page and was able to get the error to popup
14. When I tried all the steps again, at this step, I got an error page
15. Ran through this once with the console log and see errors there

Error when typing in a letter that I hadn't used before the server was shut down

Error when entering valid username and clicking Go button (expected)

After using browser's back button, error when typing in a letter that I hadn't used before the server was shut down

Error shown in console

After running through tests again and using browser's back button, no cached page and showed error (this is okay)