style: Add a '-moz-menulist-button' value to '-moz-appearance'.

The '-moz-menulist-button' value currently behavies identically to the
'menulist-button' value.  This is not implemented as an alias because later
patches in this patch series will change the behavior of our pre-existing
'menulist-button' value to more closely match what Chrome does.

Bug: 1428676
Reviewed-by: emilio

components/style/properties/longhands/box.mako.rs

@@ -515,7 +515,6 @@ ${helpers.predefined_type(
     products="gecko",
     alias="-webkit-appearance:layout.css.webkit-appearance.enabled",
     spec="Nonstandard (https://developer.mozilla.org/en-US/docs/Web/CSS/-moz-appearance)",
-    needs_context=False,
     animation_value_type="discrete",
 )}
 

components/style/values/specified/box.rs

@@ -20,11 +20,16 @@ use values::specified::{AllowQuirks, Number};
 use values::specified::length::{LengthOrPercentage, NonNegativeLength};
 
 #[cfg(feature = "gecko")]
-fn moz_display_values_enabled(context: &ParserContext) -> bool {
-    use gecko_bindings::structs;
+fn in_ua_or_chrome_sheet(context: &ParserContext) -> bool {
     use stylesheets::Origin;
     context.stylesheet_origin == Origin::UserAgent ||
-    context.chrome_rules_enabled() ||
+    context.chrome_rules_enabled()
+}
+
+#[cfg(feature = "gecko")]
+fn moz_display_values_enabled(context: &ParserContext) -> bool {
+    use gecko_bindings::structs;
+    in_ua_or_chrome_sheet(context) ||
     unsafe {
         structs::StaticPrefs_sVarCache_layout_css_xul_display_values_content_enabled
     }
@@ -33,9 +38,7 @@ fn moz_display_values_enabled(context: &ParserContext) -> bool {
 #[cfg(feature = "gecko")]
 fn moz_box_display_values_enabled(context: &ParserContext) -> bool {
     use gecko_bindings::structs;
-    use stylesheets::Origin;
-    context.stylesheet_origin == Origin::UserAgent ||
-    context.chrome_rules_enabled() ||
+    in_ua_or_chrome_sheet(context) ||
     unsafe {
         structs::StaticPrefs_sVarCache_layout_css_xul_box_display_values_content_enabled
     }
@@ -948,6 +951,9 @@ pub enum Appearance {
     Meterbar,
     /// The meter bar's meter indicator.
     Meterchunk,
+    /// The "arrowed" part of the dropdown button that open up a dropdown list.
+    #[parse(condition = "in_ua_or_chrome_sheet")]
+    MozMenulistButton,
     /// For HTML's <input type=number>
     NumberInput,
     /// A horizontal progress bar.