1705326 - navigator.userAgent still returns custom UA after clearing browsingContext.customUserAgent if a reload happened while the custom UA was set

Status: NEW

(Core :: DOM: Core & HTML, defect, P3)

Description

[Nicolas Chevobbe [:nchevobbe]]

Steps to reproduce

1. On a non data url document
2. execute browsingContext.customUserAgent = "custom"
3. reload the page (navigator.userAgent does return "custom")
4. execute browsingContext.customUserAgent = "" to clear the custom user agent
5. Evaluate navigator.userAgent in the page

Expected results

I get the initial UA value

Actual results

I still get the custom user agent value

---

That's seemed to be caused by retrieving the UA from the User-Agent header (https://searchfox.org/mozilla-central/rev/0b90e582d2f592a30713bafc55bfeb0e39e1a1fa/dom/base/Navigator.cpp#2024-2031)

https://phabricator.services.mozilla.com/D112159 modifies docshell/test/browser/browser_ua_emulation.js to demonstrate the issue

Comment 1

[Nicolas Chevobbe [:nchevobbe]]

Attachment: WIP: Bug 1705326 - Modify browser_ua_emulation.js to highlight issue with reloading + clearing custom UA

Comment 2

[Henri Sivonen (:hsivonen)]

Farre, can you set the severity and priority on this one, please?