User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:85.0) Gecko/20100101 Firefox/85.0

Steps to reproduce:

• build 20201205093858
• load long URL, can be https://www.reddit.com/r/firefox/comments/k75oaz/weekly_discussion_for_nightly_builds_for_20201205/
• restore Firefox to "windowed" state, put some buttons or search bar in toolbar to make URL in address bar fade-out on right side
• double-click on www in address bar in attempt to select

Actual results:

• some word right of the www is selected

Expected results:

• www should be selected

Regression range: https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=6359a76a8df76826dd3b18bfcb6c1a0e011e2cd7&tochange=8ae44c88ef965eaedf349b1bb7f3f247740f0263

input.select();
input.setSelectionRange(0, Number.MAX_SAFE_INTEGER);

setSelectionRange() (and also other setters) has a Gecko-only behavior that autoscrolls to the end of the text, and now select() also behaves same.

This is intended to be a hotfix for the URL bar regression. Ideally there should be a spec or this behavior should be removed completely.

Previously select() scrolled back to the start position before the regression, but this fix does not restore that behavior.

Backed out for failure on browser_blockingCookies.js.

Backout link: https://hg.mozilla.org/integration/autoland/rev/62495078fd2913be744a941058c4c5ae04e6bedb

Push with failure: https://treeherder.mozilla.org/jobs?repo=autoland&selectedTaskRun=AapKrGqxTZGwtRC01tYlQQ.0&revision=29bbd5887d66c786dae5f559a5674af56291fa0e&searchStr=linux%2C18.04%2Cx64%2Copt%2Cmochitests%2Ctest-linux1804-64%2Fopt-mochitest-browser-chrome-e10s%2Cbc7

Failure log: https://treeherder.mozilla.org/logviewer?job_id=323897907&repo=autoland&lineNumber=4387

It doesn't seems to be related to my patch, but a reverting commit does fix it (only on Linux and macOS). Hmm. 🤔

https://hg.mozilla.org/mozilla-central/rev/a8ef444371de