Thanks for reporting this. Confirming on https://de.wikipedia.org/w/index.php?search=&title=Spezial%3ASuche&go=Artikel on Firefox 56

I'm using FF ESR 52.4.0 64-bit with dewiki and enwiki and can ''not'' confirm that behavior. Maybe it is caused by some JavaScript (I have that disabled using the NoScript extension) or by some FF extension ?

The same happens on the Finnish Wikipedia.
https://de.wikipedia.org/wiki/Spezial:ISBN-Suche
https://fi.wikipedia.org/wiki/Toiminnot:Kirjal%C3%A4hteet

I have Firefox 58 beta.

Report upstream to Mozilla

I am able to consistently reproduce this at https://de.wikipedia.org/wiki/Spezial:ISBN-Suche in Firefox 57 on Mac, as well as in the Developer Edition. On reloads it only happens when I throttle the network speed to no more than "Good 3G".

I used ?safemode=1 to rule out any site-specific differences caused by site-wide styles or script. I then continued on mwdebug1001 with local hacks to slowly strip everything from the page to make it look more like Special:Blankpage, on which the issue does not happen.

After a while I narrowed it down to the use of autofocus on the input field.

Looking for upstream issues regarding FOUC, there are quite a few. Appears to be a regression in Firefox Quantum, reports date back several months ago to when earlier versions of Quantum started being released, and reports mention it consistently. (e.g. 1404468#c4)

• https://bugzilla.mozilla.org/show_bug.cgi?id=1419072 (FOUC on hex.pm)
• https://bugzilla.mozilla.org/show_bug.cgi?id=1407401 (FOUC on GitHub)
• https://bugzilla.mozilla.org/show_bug.cgi?id=1408090 (FOUC on GItHub, continued)
• https://bugzilla.mozilla.org/show_bug.cgi?id=1384276 (Don't render unstyled content when stylesheets are in flight)

Many link to this deeper issue:

• https://bugzilla.mozilla.org/show_bug.cgi?id=712130 (Autofocus should apply after the styles are applied)

I've confirmed that removing autofocus => true from these pages fixes the issue. It seems that as soon as Firefox discovers autofocus="" in the HTML stream, it starts rendering the page and highlighting the input field, and forgets about the blocking stylesheets.

Some of the issues mention a configurable first-paint delay in Firefox's config flags. I tried disabling those or setting a higher delay but that didn't make a difference for me E.g. even with a timeout of 3 seconds and no network delay, there is clearly a FOUC before the styles arrive on a page with autofocus.

I've reported what you seen in https://bugzilla.mozilla.org/show_bug.cgi?id=712130, great work @Krinkle !

I can also reproduce it with Firefox 57, even without throttling network speed (VDSL2).

Right, in case that wasn't clear, this happens easily, slowing down the connection just makes it happen consistently.

It also happens on the mobile version: https://fr.m.wikipedia.org/wiki/Wikip%C3%A9dia:Accueil_principal

I'm basically experiencing this on every Wikipedia page I load, is there anything we can do as a workaround?

TBH, the fault is in Firefox 57, and the same issue happens on a shitload of websites. So I don't think there is any fix or workaround to consider for MediaWiki, the issue has to be fixed in Firefox.

Just to mention the mobile version has this issue on all browsers actually, not only Firefox 57. Tried Chrome and Pale Moon. So maybe the mobile version has a related, but different, issue that would need to be investigated.

Indeed. If you see a rendering problem with the mobile site in a browser other than Firefox, please report it in a new task.

I can also reproduce it on the login page:
https://en.wikipedia.org/w/index.php?title=Special:UserLogin

Confirming that I could reproduce the problem in Firefox 59; Confirming that I cannot reproduce the problem anymore in Firefox 60.
Should this task be closed?