servo: Merge #17849 - style: Ensure we generate a ReconstructFrame hint when -moz-binding changes on a display:none root (from heycam:binding-display-none); r=emilio

From https://bugzilla.mozilla.org/show_bug.cgi?id=1375383.

Source-Repo: https://github.com/servo/servo
Source-Revision: 3eeb0e5e48fcb045100ed26b396ad85226be9af0

servo/components/style/gecko/restyle_damage.rs

@@ -6,7 +6,7 @@
 
 use gecko_bindings::bindings;
 use gecko_bindings::structs;
-use gecko_bindings::structs::{nsChangeHint, nsStyleContext};
+use gecko_bindings::structs::{nsChangeHint, nsStyleContext, nsStyleStructID};
 use matching::{StyleChange, StyleDifference};
 use properties::ComputedValues;
 use servo_arc::Arc;
@@ -62,6 +62,34 @@ impl GeckoRestyleDamage {
         StyleDifference::new(GeckoRestyleDamage(hint), change)
     }
 
+    /// Computes the `StyleDifference` between the two `ComputedValues` objects
+    /// for the case where the old and new style are both `display: none`.
+    ///
+    /// In general we don't need to generate damage for such elements, but we
+    /// do need to generate a frame reconstruction for `-moz-binding` changes,
+    /// so that we can start loading the new binding.
+    pub fn compute_undisplayed_style_difference(
+        old_style: &ComputedValues,
+        new_style: &ComputedValues,
+    ) -> StyleDifference {
+        let mut any_style_changed: bool = false;
+
+        // Just compute the Display struct's difference.
+        let display_struct_bit = 1 << (nsStyleStructID::eStyleStruct_Display as u32);
+        let hint = unsafe {
+            bindings::Gecko_CalcStyleDifference(old_style,
+                                                new_style,
+                                                display_struct_bit,
+                                                &mut any_style_changed)
+        };
+
+        // Only pay attention to a reconstruct change hint.
+        let damage = GeckoRestyleDamage(hint) & Self::reconstruct();
+
+        let change = if damage.is_empty() { StyleChange::Changed } else { StyleChange::Unchanged };
+        StyleDifference::new(damage, change)
+    }
+
     /// Returns true if this restyle damage contains all the damage of |other|.
     pub fn contains(self, other: Self) -> bool {
         self & other == other