Bug 1626458 part 1: Remove pref for CSS Containment (layout.css.contain.enabled) r=AlaskanEmily

(Since we've been shipping with it default-enabled for a while now.) See https://bugzilla.mozilla.org/show_bug.cgi?id=1466008#c9 through #c13 for notes on the reftest.list change. Differential Revision: https://phabricator.services.mozilla.com/D71861
2020-04-23 05:01:07 +00:00 · 2020-04-23 05:01:07 +00:00 · 4adb473174
--- a/layout/reftests/bugs/1466008.html
+++ b/layout/reftests/bugs/1466008.html
@ -1,5 +1,5 @@
 <!doctype html>
 <div style="height: 100px"></div>
-<div style="will-change: contain;">
+<div style="will-change: backdrop-filter;">
  <div style="position: absolute; top: 0; left: 0; width: 100px; height: 100px; background: green"></div>
 </div>
--- a/layout/reftests/bugs/reftest.list
+++ b/layout/reftests/bugs/reftest.list
@ -2052,7 +2052,12 @@ fuzzy-if(Android,0-66,0-574) fuzzy-if(d2d,0-89,0-777) fuzzy-if(!Android&&!d2d,0-
 pref(layout.css.moz-document.url-prefix-hack.enabled,true) == 1446470.html 1035091-ref.html
 pref(layout.css.moz-document.url-prefix-hack.enabled,false) == 1446470-2.html 1035091-ref.html
 == 1456111-1.html about:blank
-test-pref(layout.css.contain.enabled,false) == 1466008.html 1466008-ref.html
+# Note: The following test depends on having a pref-controllable CSS property,
+# and the property needs to have a non-initial value that causes it to create
+# a containing block for fixed-position descendants. It doesn't matter whether
+# the pref is enabled by default; we just need to be able to pref it off here.
+# For now, we use the "backdrop-filter" CSS property (and its pref).
+test-pref(layout.css.backdrop-filter.enabled,false) == 1466008.html 1466008-ref.html
 fuzzy(0-1,0-625) == 1466638-1.html 1466638-1-ref.html
 == bug1472465-1.html bug1472465-1-ref.html
 == 1475971-1.html 1475971-1-ref.html
--- a/modules/libpref/init/StaticPrefList.yaml
+++ b/modules/libpref/init/StaticPrefList.yaml
@ -5454,12 +5454,6 @@
  mirror: always
  rust: true

-# Is support for CSS contain enabled?
- name: layout.css.contain.enabled
-  type: bool
-  value: true
-  mirror: always
-
 # Should stray control characters be rendered visibly?
 - name: layout.css.control-characters.visible
  type: RelaxedAtomicBool
--- a/servo/components/style/properties/longhands/box.mako.rs
+++ b/servo/components/style/properties/longhands/box.mako.rs
@ -623,9 +623,7 @@ ${helpers.predefined_type(
    engines="gecko",
    animation_value_type="none",
    flags="CREATES_STACKING_CONTEXT FIXPOS_CB",
-    gecko_pref="layout.css.contain.enabled",
    spec="https://drafts.csswg.org/css-contain/#contain-property",
-    enabled_in="chrome",
 )}

 // Non-standard