Fill extra layers in background style struct before handing off to Gecko

components/style/properties/gecko.mako.rs

@@ -1178,6 +1178,24 @@ fn static_assert() {
 
         }
     }
+
+    pub fn fill_arrays(&mut self) {
+        use gecko_bindings::bindings::Gecko_FillAllBackgroundLists;
+        use std::cmp;
+        let mut max_len = 1;
+        % for member in "mRepeat mClip mOrigin mAttachment mPositionX mPositionY mImage".split():
+            max_len = cmp::max(max_len, self.gecko.mImage.${member}Count);
+        % endfor
+
+        // XXXManishearth Gecko does an optimization here where it only
+        // fills things in if any of the properties have been set
+
+        unsafe {
+            // While we could do this manually, we'd need to also manually
+            // run all the copy constructors, so we just delegate to gecko
+            Gecko_FillAllBackgroundLists(&mut self.gecko.mImage, max_len);
+        }
+    }
 </%self:impl_trait>
 
 <%self:impl_trait style_struct_name="List" skip_longhands="list-style-type" skip_additionals="*">

components/style/properties/properties.mako.rs

@@ -2034,6 +2034,10 @@ pub fn cascade(viewport_size: Size2D<Au>,
         }
     % endfor
 
+    % if product == "gecko":
+        style.mutate_background().fill_arrays();
+    % endif
+
     // The initial value of outline width may be changed at computed value time.
     if style.get_outline().clone_outline_style().none_or_hidden() &&
        style.get_outline().outline_has_nonzero_width() {