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New Member

Firefox extension causes flash of unstyled content

With the firefox extension enabled, some websites are being rendered before styles have been applied.


Firefox 105.0b9, LastPass for Firefox 4.101.0


To reproduce:

  • Open (also reproduced on several other websites, although not every one)
    • If this isn't sufficient to reproduce it, or to trigger it again, use Ctrl/Cmd+Shift+R to bypass cache and refresh

Expected behaviour:

  • Page loads normally, like it does with the LastPass extension disabled

Actual behaviour:

  • Page initially renders without any stylesheets, then reflows as the styles get applied
  • "Layout was forced before the page was fully loaded. If stylesheets are not yet loaded this may cause a flash of unstyled content." is logged to the dev console, with an origin of moz-extension://2d23e33b-fcbb-42d7-8327-40a3b7c4c9c4/web-client-content-script.js:1:325262
    • This points to `a = Object(l.useState) (visualViewport.height),` in the formatted minified code of web-client-content-script.js

This doesn't happen on Chrome, but Chrome does log "[Violation] Forced reflow while executing JavaScript took 31ms", so I think Chrome is just a bit more defensive against this.

This has definitely become more prevalent recently, although I'm not sure exactly when (the last extension update, in early August, seems plausible, though I would have guessed slightly more recent than that).

Active Contributor

Re: Firefox extension causes flash of unstyled content

I ran into that same problem today with the Firefox Extention. I had to log out of LastPass to get logged into the site.

New Member

Re: Firefox extension causes flash of unstyled content

We're a CMS developer and the LastPass extension is causing us a similar problem.

The prompt to save the password cuts half our editing window. It doesn't do it in every sites. We are not quite sure why.

An hidden images gets added to the DOM and that pushes a bunch of things out on the page. Disabling "Use improved save and fill" seems to suppress the behaviour.

Screenshot from 2022-11-28 15-48-28.jpg

Screenshot from 2022-11-28 15-53-18.jpg


New Member

Re: Firefox extension causes flash of unstyled content

I've been experiencing the "flash of unstyled content" on Firefox within the last year.


I research the problem now and then to see whether I can fix it. Today I found this post, so I tried disabling the LastPass extension. Sites load normally for me when I disable LastPass.


Firefox Version 108.0.1 (64-bit)