I've just refreshed my Nightly 68 installation, turned off the bookmarks toolbar as I always did and a black divider line appeared just under the location bar.

It only appears when the Firefox window is full screen, and disappears if I resize the window. It also disappears if I open a second full screen Firefox window.

I've read on http://forums.mozillazine.org/viewtopic.php?f=38&t=2141729 that it might be an addon related issue. I tried disabling the only two addons I have (Facebook Container and LastPass) and restarting Firefox. It didn't help.

However, restarting Firefox in Safe Mode does remove the black line. I'm not sure what the difference is.

I'm using the default theme and I don't have any plugins installed except OpenH264 and Widevine decoders.

I suspect this is a theme issue... though it's confusing that this doesn't reproduce in safe mode - it might even be a graphics thing. Dão, I don't suppose you have any ideas here?

Now that you've mention themes, I tried switching to the built-in Light theme and there was no glitch on it. I could still see the glitch after switching back to the Default theme.

Then I switched to the Light theme, updated and restarted Nightly, and switched back to the Default theme. I don't see the glitch any more. I'm still not sure why it happened in the first place.

I spoke too soon. A second restart brought back the glitch. I can still see it on the default theme.

I can see the line when maximized with default theme if HiDPI settings(layout.css.devPixelsPerPx = 1.25).
And the line is same as Windows accent color.

Regression window:
https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=b72c02e3426199c098f542aba3ff0ef197e347d4&tochange=a5ba6ffec2a7b6431404bf1d87a67b90a6fe8a35

(In reply to Alice0775 White from comment #4)

I can see the line when maximized with default theme if HiDPI settings(layout.css.devPixelsPerPx = 1.25).
And the line is same as Windows accent color.

Regression window:
https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=b72c02e3426199c098f542aba3ff0ef197e347d4&tochange=a5ba6ffec2a7b6431404bf1d87a67b90a6fe8a35

Kris, can you take a look please?

(In reply to :Gijs (he/him) from comment #5)

(In reply to Alice0775 White from comment #4)

I can see the line when maximized with default theme if HiDPI settings(layout.css.devPixelsPerPx = 1.25).
And the line is same as Windows accent color.

Regression window:
https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=b72c02e3426199c098f542aba3ff0ef197e347d4&tochange=a5ba6ffec2a7b6431404bf1d87a67b90a6fe8a35

Kris, can you take a look please?

I can't see how my changes in that push could be related. The only effect they should have outside of Android and tests (aside from fixing the broken link in customize mode) is to fix the early blank window code. I suppose that could conceivably have caused this, but if so, it's really a bug elsewhere.

Bug 1544682 moves the normal, expected toolbar border from #navigator-toolbox::after to #navigator-toolbox. I don't think it fixes this bug but maybe it makes it easier to investigate.

(In reply to Dão Gottwald [::dao] from comment #8)

Bug 1544682 moves the normal, expected toolbar border from #navigator-toolbox::after to #navigator-toolbox. I don't think it fixes this bug but maybe it makes it easier to investigate.

Hmm, right now with that bug fixed I can't reproduce this anymore. Anyone else?

Nevermind, I can still reproduce with 125% scaling after restarting Firefox.

I can still reproduce it. And yes, I've been using %125 scaling as well.

there also appears to be some graphical corruption around the back button...

...at least that was where mozregression has led me to.

(In reply to [:philipp] from comment #13)

...at least that was where mozregression has led me to.

That's only because the early blank window code was briefly broken after migrating the default theme away from a persona.

Bug 1541798 also can't be the cause of this problem because it landed 14 days ago, while this bug was filed 21 days ago.

(I tried to reproduce the issue anyway, with various combinations of the steps described in this bug, but I haven't been able to do so in my Windows 10 VM.)

Hi Dao, do you think this will be fixed for 68?

I have no clue what's going on here and at this point I doubt it's a theme issue.

It seems like we need a bit of clarity on steps to reproduce and a better regression window for progress here.

(In reply to Jeff Muizelaar [:jrmuizel] from comment #18)

It seems like we need a bit of clarity on steps to reproduce and a better regression window for progress here.

Brindusa, can your team try and look at this?

Hi Guys, I managed to reproduce this issue in our latest Nightly build 68.0a1 (2019-05-16) after setting the layout.css.devPixelsPerPx = 1.25 and restarting the browser while in full screen you can see the dark line clearly.

I tried to do a mozregression of this issue but it needed a restart each time I changed the about:config otherwise the bug wouldnt reproduce for me and still couldnt find enough data to bisect. Regardless here is the last known good and the first bad builds I could find:

Last known Good:

https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=b72c02e3426199c098f542aba3ff0ef197e347d4&tochange=bdaf1b36c44275dd4f027b4a4c30afed86cdfe13

First known BAD:

https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=b72c02e3426199c098f542aba3ff0ef197e347d4&tochange=a5ba6ffec2a7b6431404bf1d87a67b90a6fe8a35

I hope this helps.

To reproduce on Windows 10:
Go to Start > Settings > System > Display. Set "Change the size of text, apps, and other items" to 125%.

To recap: This appears to have started with bug 1540856 landing. The theory was that this was due to that bug breaking the early blank paint, therefore exposing a preexisting bug that only manifests without the early blank paint. However in that case bug 1541798 should have covered up that bug again, which apparently it didn't. So at this stage it's still unclear how bug 1540856 triggered this. Can somebody check if (1) setting browser.startup.blankWindow = false or (2) opening a new Firefox window vs. starting a new instance of Firefox make a difference?

Yep, changing browser.startup.blankWindow to False removed the black line after restart. Opening a tab in a new window also does the trick.

So this is still confusing. This bug surfaced when bug 1540856 effectively disabled the early blank paint, and now this only manifests with the early blank paint enabled...

Unassigned, P2 and we shipped several releases with this bug, marking as wontfix for 70 and 71.

I can't reproduce this anymore on Nightly. browser.startup.blankWindow is still set to true. I never noticed when it was gone because it stopped being a distraction over time.