servo: Merge #16998 - The 'contain' property can affect whether a box is a fixed-pos containing block (from bzbarsky:fix-contain-flags); r=emilio

This makes will-change:contain work correctly.

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Source-Repo: https://github.com/servo/servo
Source-Revision: 96d6b30eff3c0449726b3ea49140b13c7dd7f1f8

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 11c8a2b95dd279f3eaa8cc3f5c2de1b45f9cb47c

servo/components/style/properties/longhand/box.mako.rs

@@ -2219,6 +2219,7 @@ ${helpers.single_keyword("transform-style",
 // like `content`(layout style paint) in gecko. We should implement `size` and `content`,
 // also update the glue once they are implemented in gecko.
 <%helpers:longhand name="contain" animation_value_type="none" products="gecko" need_clone="True"
+                   flags="FIXPOS_CB"
                    spec="https://drafts.csswg.org/css-contain/#contain-property">
     use std::fmt;
     use style_traits::ToCss;