System Theme Detection doesn't work in Firefox

[kartonrad]

Description
In Firefox, changes to the System Theme do not reach the extension pages.

Even though:

• my System is set to light
• Website Theme is set to "Automatic"
• layout.css.prefers-color-scheme.content-override = 2 is set. (so it uses system theme)
• the themes i use don't have any properties or code that mess with color-sheme

This behaviour seems to be very related to issues the darkreader team had:
darkreader/darkreader#9582
darkreader/darkreader@29e9312

Their fix seems to be to collect the messages:
matchMedia('(prefers-color-scheme: dark)').addEventListener("change", ({matches}) => console.log(matches))
from all tabs, and then manage the resulting mayhem by filtering duplicate messages.

i can't say i've been able to fully parse their code into my brain, it's quite a lot.
the core mechanism of matching media queries seems to be the only way though, and it's unreliability must just be accounted for.

System Information

• OS: Windows 11
• Firefox update branch: Release (regular)
• Version 112.0.2 (64-bit)

Steps To Reproduce
Steps to reproduce the behaviour:

1. Open the extention tab, and execute matchMedia("(prefers-color-scheme: dark)").matches
2. Always returns true. Always.

Expected behaviour
It should return false in System Light Mode

[kartonrad]

Interestingly, the system theme doesn't reach the DevTool either. (i have it set to automatic - it is dark on every page)

For some pages ff just assumes dark, and never changes the theme - what a strange issue on their part
cus i have "light" specified everywhere that is remotely user facing

[kierandrewett]

Using the prefixed (-moz-system-dark-theme) media query is one way of checking if the OS theme is dark or not, however it only seems to be allowed for privileged pages like about:preferences or in the actual browser.

Not sure if it would be possible to use this in a WebExtension context though.

[kartonrad]

Doesn't work :(

The listener didnt fire going from dark to light to dark
And the direct query always returned false

[kierandrewett]

Did you run this from the browser toolbox?

[kartonrad]

On the bright side however that means my annoying-to-implement workaround wasn't invain

[kartonrad]

Did you run this from the browser toolbox?

I navigated to about:debugging, and opened the extension inspector

it's where the extension background script lives

[kartonrad]

my workaround has the disadvantage that it only works immediately if you are fcosing on a tab, that is not protected by firefox.
otherwise none of the contentScripts get the matchMedia change event - so you have to open a website, reload it, and focus the window
then it will update

but ig if you're not lurking on about:newtab, it kinda works

[kartonrad]

and if there was a better one, i figure the dark reader folks would've caught on

[kierandrewett]

my workaround has the disadvantage that it only works immediately if you are fcosing on a tab, that is not protected by firefox. otherwise none of the contentScripts get the matchMedia change event - so you have to open a website, reload it, and focus the window then it will update

but ig if you're not lurking on about:newtab, it kinda works

I mean, if you've just got the new tab page open, you're probably not really using the web browser anyway so I don't see this as too big of an issue tbh. It seems to be working fine for me on Linux.

[kartonrad]

My patch?
If so i'm glad!

It probably will have broken the "only on startup thing"

Something another global variable could fix
But i didnt want to make too many changes

[kierandrewett]

My patch? If so i'm glad!

It probably will have broken the "only on startup thing"

Something another global variable could fix But i didnt want to make too many changes

Yeah, your patch works really well. I don't see why it needs to send the window.location.href surely just the current color-scheme is sufficient?

[kartonrad]

Yeah just for logging

[easonwong-de]

and if there was a better one, i figure the dark reader folks would’ve caught on

Sad thing is, Dark Reader only needs to run on unprotected tabs. So, matchMedia("(prefers-color-scheme: dark)").matches always works for it.

Maybe we can refer to #31

[easonwong-de]

It seems like Firefox has been struggling to deal with system colour theme for a while. Like, in this repo, “system theme detection” issue has come out and been fixed for some times already.

[woj-tek]

It seems like Firefox has been struggling to deal with system colour theme for a while. Like, in this repo, “system theme detection” issue has come out and been fixed for some times already.

Hmm... in Firefox (on macOS) native "system theme" seems to work fine, same goes for page colours.

I just started using this theme and it seems great - it changed theme to the dark flavour last night but today it got stuck on dark one. For now I set fixed times (that maches my system) as a workaround and this seems to work...