This bug is a followup from the investigation into bug 1753874. Even with the intervention there, with my main profile (but no others so far), on Tik-tok, I cannot like or comment on comments -- likes on comments don't survive page reloads and replies to comments produce a "failed to comment" notification. I haven't been able to find any pref changes that affected it and troubleshooting mode doesn't help.

Amazingly, I have been able to test this successfully with mozregression. We just missed the 1 year window for autoland so the range is kind of big but I believe it can be narrowed further to just the Apr 10, 2021 commits.

I'm currently trying to narrow this window.

Manual builds narrowed this down to Bug 1703274 - Enable Proton Urlbar. NI to mak as the author. I don't have a way to reproduce this beyond using my profile but I assume there are others with the same issue. I can run experiments if that helps.

I'm very confused, the changes in Bug 1703274 have no effect on content. At a maximum I could expect the urlbar to misbehave, but that wouldn't affect web pages in any way.
Are you 100% sure of that range, like if you update to the changeset just before that bug (52a2586bc322ece5ebb877f5ac67697c54037e6a) it works every time, while 135c5397086dac5623bc45d92bd6e7709806d89d is broken every time. May the issue be intermittent?

I agree that this is really weird but it's what I'm seeing. I should mention that my manual bisection landed me on a "Enable Proton Urlbar" checkin that was backed out -- changeset bd396182f84a57886364464e5d56c8e48d32357c .

I went back and created local builds w/o --enable-debug (all tested local builds have had optimizations turned on). This is potentially important because there is some reason to believe bug 1753874 might have a timing component to it. But I've now tried builds with very different performance (w/ asserts vs. w/o) and come up with the same results so that's looking less likely.

On tik-tok, the test is to try liking a comment and refreshing the page. I did this five times with each of these new local build and all five worked without the patch and all five failed with it. It was also 100% reproducible in the earlier attempts.

I also tried the checkin that backed out the changeset -- 1cfcf8190e7c000c193a916c5ab51d399e553adc . It restores the Tik-Tok functionality (works 5/5 times).

The only browser.proton config setting I have is browser.proton.toolbar.version which sets itself to 3 as expected.

Using the backed out changeset 1cfcf8190e7c000c193a916c5ab51d399e553adc again (so, w/o the proton urlbar change), the default prefs of browser.proton.enabled=true and browser.proton.urlbar.enabled=false show up. Changing to browser.proton.urlbar.enabled=true and restarting the browser then breaks the Tik-tok functionality. So bug 1703274 was picked only because the proton urlbar was made default-on by it.

Proton Urlbar will make outerHeight - innerHeight larger. Will window size, find bar, bookmark toolbar, menu bar, or undocked DevTools affect the result?

The new urlbar did indeed make the toolbar slightly taller (maybe also introduce some rounding?), introduce a shadow around the urlbar and remove a couple things like the page-actions button, that potentially could also include add-ons!
Masatoshi's idea to try with other toolbars visible, or maybe enlarge the navbar height in pixel steps through the borwse inspector sounds interesting to try...
Otherwise I could just suggest to start from the build with browser.proton.urlbar.enabled=false, and manually flip parts of the code/css until the culprit is found, but that's quite boring and slow.

Sounds like this probably is related to the failure. I'd forgotten that earlier, bug 1753874 found an issue with (docked) DevTools as well.

First, a new mozregression window, with the old profile and browser.proton.enabled = browser.proton.urlbar.enabled = true, suggests that Tik-Tok did work with the urlbar until around March 26-27 2021.

So I tried the central build from Mar 26 2021 -- before the new window:

• No changes: Tik-Tok works fine.
• W/ DevTools open:
  • Undocked: works
  • Docked: broken
    • After closing devtools: works again
• Windowed browser : works
• w/o drawTabsInTitlebar : broken
• w/ find toolbar open: works
• w/ Windows taskbar set to autohide: works

Mar 28 2021 build is after the new window:

• No changes: Tik-Tok is broken
• W/ DevTools open: broken
• Windowed browser: works (!)
  • Then maximized: bad

I tried the windowed browser with nightly and got the same results as Mar 28: it works when windowed.

It's also still unexplained why my profile is bad here but others are not.

The severity field is not set for this bug.
:adw, could you have a look please?

For more information, please visit auto_nag documentation.