"Opera Tracker Blocker Site Fixes" blocks first-party CSS and JS on our web apps, leaving pages unstyled

Erin104

Erin104

Title: "Opera Tracker Blocker Site Fixes" blocks first-party CSS and JS on our
web apps, leaving pages unstyled

Opera One 135.0.5973.133, Windows.

Summary
With tracker blocking enabled, our web apps load their HTML but receive no CSS
and no JavaScript. Pages render completely unstyled, and our Blazor app fails
with "An unhandled error has occurred" because its script never loads.

We isolated it to a single filter list: "Opera Tracker Blocker Site Fixes".
Unchecking that one list -- leaving ad blocking, EasyPrivacy, uBO: Privacy and
everything else enabled -- fixes it completely. Re-checking it breaks the site
again, every time.

These are ordinary first-party stylesheets and scripts on a business web
application. They are not trackers.

Reproduction

Visit https://portal.dev.micsservice.com/ with tracker blocking on.
-> Page renders unstyled.
opera://settings/adBlock -> Block trackers -> manage lists -> uncheck
"Opera Tracker Blocker Site Fixes". Reload with Ctrl+F5.
-> Page renders correctly.
The behaviour flips with that checkbox alone.

Blocked Requests (all first-party, same origin as the page)
/css/portal.css /css/bis-tokens.css
/app.css /BISWeb.styles.css
/_framework/blazor.web.js

What we ruled out first

Not an extension: Adblock Plus is installed and the site works fine with
only ABP enabled. The failure also occurs in a private window, where
extensions are off but the built-in blocker is not.
Not any public filter list: we tested the 10 asset URLs against 13 lists
(~240,000 rules) -- EasyList, EasyPrivacy, NoCoin, Fanboy Annoyance, Fanboy
Social, EasyList Germany, uBO Filters, uBO Badware risks, uBO Online
Malicious URL Blocklist, uBO Quick fixes, uBO Unbreak, uBO Resource abuse,
uBO Annoyances. No applicable rule matches. "micsservice" appears in none of
them, nor does our server IP.
Not the server: the stylesheets are served anonymously as HTTP 200 with
Content-Type: text/css, valid Let's Encrypt certificate, no CSP, no
X-Content-Type-Options. Nothing changed on our side after 2026-09-03; the
failure began 2026-09-11.

What we are asking
Which rule in that list matches these URLs, and whether it can be corrected.
We could not check it ourselves: unlike the other lists, the two "Site Fixes"
lists are loaded via opera.com and are not publicly downloadable, so the only
way we could identify it was by elimination.

leocg

leocg Moderator Volunteer @Erin104

@Erin104 See https://forums.opera.com/post/409354