2054149 - about:privatebrowsing legacy (non-Nova) newtab promos are invisible: hidden attribute not cleared, defeated by global [hidden] !important rule

Status: RESOLVED FIXED

(Firefox :: Private Browsing, defect)

Description

[Emma Zühlcke [:emz]]

With browser.nova.enabled set to false (the current default), the legacy private-browsing newtab promos (Focus/Klar/VPN/Pin/cookie-banner) no longer render, either naturally or when forced via ASRouter devtools. The Nova promo layout (browser.nova.enabled=true) is unaffected.

Root cause

renderLegacyPromo in browser/components/privatebrowsing/content/aboutPrivateBrowsing.js reveals the promo by adding the .promo-visible class (display: flex) but never removes the hidden attribute on the .promo container. This relied on [hidden] being a low-priority presentation attribute that any author display rule overrides.

Bug 2051196 subsequently added a global :where([hidden]) { display: none !important } rule to the base stylesheets (global-shared.css / in-content/common-shared.css). The !important now outweighs .promo-visible, so the fully-populated legacy promo stays display: none !important. The Nova path is unaffected because renderNovaPromo explicitly sets container.hidden = false.

Confirmed in a private window: ASRouter returns a valid message (promoEnabled: true, valid action), renderPromo runs to completion and adds .promo-visible, but the container computes display: none; removing the hidden attribute restores display: flex.

Fix

Clear the hidden attribute in renderLegacyPromo when revealing the promo, mirroring renderNovaPromo.

Test gap

The existing legacy-path tests (test_default_promo, test_nova_disabled_uses_legacy_promo) pass with and without the regression because they only assert the static .promo container exists in the DOM, never that it is visible. Coverage should assert visibility (e.g. ContentTaskUtils.isVisible).

Comment 1

[Emma Zühlcke [:emz]]

Attachment: Bug 2054149 - Clear the hidden attribute when showing the legacy private browsing newtab promo. r=#desktop-theme-reviewers!

renderLegacyPromo revealed the promo by adding the .promo-visible class but
never removed the container's hidden attribute. This relied on [hidden] being a
low-priority presentation attribute that any author display rule overrode.

Bug 2051196 added a global :where([hidden]) { display: none !important } rule to
the base stylesheets, so [hidden] now wins over .promo-visible and the
fully-rendered legacy promo stays display: none. The Nova path was unaffected
because renderNovaPromo already sets container.hidden = false.

Clear the hidden attribute when revealing the legacy promo, mirroring the Nova
path, and extend the legacy-path tests to assert the promo is actually visible
rather than only present in the DOM.

Comment 2

[BugBot [:suhaib / :marco/ :calixte]]

Set release status flags based on info from the regressing bug 2051196

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/9b64fb055dc2
https://hg.mozilla.org/integration/autoland/rev/3399e6979323
Clear the hidden attribute when showing the legacy private browsing newtab promo. r=desktop-theme-reviewers,dao

Comment 4

[Sandor Molnar[:smolnar]]

https://hg.mozilla.org/mozilla-central/rev/3399e6979323