Bug 1941406: Remove the Nightly-only guard from about:config pref "layout.details.force-block-layout". r=layout-reviewers,emilio

This patch makes this pref unconditionally false, which enables the modern
behavior of letting HTML <details> elements be given any display value.

(Previously, this pref only had this false value on the Nightly channel,
whereas other release channels had true, which gave the old behavior of
forcing <details> to be a block.)

This patch also removes the now-unnecessary .ini annotation to explicitly
set the pref to be false for WPT tests in our own CI.

Differential Revision: https://phabricator.services.mozilla.com/D241502

Author: dholbert

modules/libpref/init/StaticPrefList.yaml

@@ -10856,7 +10856,7 @@
 # The modern behavior is 'false', i.e. not forcing it to be a block.
 - name: layout.details.force-block-layout
   type: bool
-  value: @IS_NOT_NIGHTLY_BUILD@
+  value: false
   mirror: always
 
 # Whether table cells can generate scroll boxes.

testing/web-platform/meta/html/rendering/the-details-element/__dir__.ini

@@ -1,2 +1,2 @@
-prefs: [layout.details.force-block-layout:false,layout.css.details-content.enabled:true]
+prefs: [layout.css.details-content.enabled:true]
 leak-threshold: [default:51200]