Bug 1521631 - Implement overflow: overlay as an alias on auto, and enable on Nightly. r=boris

See related webcompat issues and discussion in
https://github.com/w3c/csswg-drafts/issues/8063

Differential Revision: https://phabricator.services.mozilla.com/D165058

layout/style/test/property_database.js

@@ -11648,15 +11648,27 @@ if (IsCSSPropertyPrefEnabled("layout.css.caption-side-non-standard.enabled")) {
 }
 
 {
-  const enabled = IsCSSPropertyPrefEnabled(
+  const mozHiddenUnscrollableEnabled = IsCSSPropertyPrefEnabled(
     "layout.css.overflow-moz-hidden-unscrollable.enabled"
   );
+  const overlayEnabled = IsCSSPropertyPrefEnabled(
+    "layout.css.overflow-overlay.enabled"
+  );
   for (let p of ["overflow", "overflow-x", "overflow-y"]) {
     let prop = gCSSProperties[p];
-    let values = enabled ? prop.other_values : prop.invalid_values;
-    values.push("-moz-hidden-unscrollable");
+    let mozHiddenUnscrollableValues = mozHiddenUnscrollableEnabled
+      ? prop.other_values
+      : prop.invalid_values;
+    let overlayValues = overlayEnabled
+      ? prop.other_values
+      : prop.invalid_values;
+    mozHiddenUnscrollableValues.push("-moz-hidden-unscrollable");
+    overlayValues.push("overlay");
     if (p == "overflow") {
-      values.push("-moz-hidden-unscrollable -moz-hidden-unscrollable");
+      mozHiddenUnscrollableValues.push(
+        "-moz-hidden-unscrollable -moz-hidden-unscrollable"
+      );
+      overlayValues.push("overlay overlay");
     }
   }
 }

modules/libpref/init/StaticPrefList.yaml

@@ -8566,6 +8566,13 @@
   mirror: always
   rust: true
 
+# Is support for CSS overflow: overlay enabled
+- name: layout.css.overflow-overlay.enabled
+  type: RelaxedAtomicBool
+  value: @IS_NIGHTLY_BUILD@
+  mirror: always
+  rust: true
+
 # Is support for overscroll-behavior enabled?
 - name: layout.css.overscroll-behavior.enabled
   type: bool

servo/components/style/values/specified/box.rs

@@ -2229,7 +2229,10 @@ impl Parse for Overflow {
             "clip" => Self::Clip,
             #[cfg(feature = "gecko")]
             "-moz-hidden-unscrollable" if static_prefs::pref!("layout.css.overflow-moz-hidden-unscrollable.enabled") => {
-               Overflow::Clip
+                Overflow::Clip
+            },
+            "overlay" if static_prefs::pref!("layout.css.overflow-overlay.enabled") => {
+                Overflow::Auto
             },
         })
     }