Handle fallback to default variable font in case of nonexistant generic

components/style/gecko_bindings/bindings.rs

@@ -1173,6 +1173,11 @@ extern "C" {
     pub fn Gecko_nsStyleFont_CopyLangFrom(aFont: *mut nsStyleFont,
                                           aSource: *const nsStyleFont);
 }
+extern "C" {
+    pub fn Gecko_nsStyleFont_FixupNoneGeneric(font: *mut nsStyleFont,
+                                              pres_context:
+                                                  RawGeckoPresContextBorrowed);
+}
 extern "C" {
     pub fn Gecko_GetBaseSize(lang: *mut nsIAtom) -> FontSizePrefs;
 }

components/style/properties/gecko.mako.rs

@@ -54,6 +54,7 @@ use gecko::values::convert_rgba_to_nscolor;
 use gecko::values::GeckoStyleCoordConvertible;
 use gecko::values::round_border_to_device_pixels;
 use logical_geometry::WritingMode;
+use media_queries::Device;
 use properties::animated_properties::TransitionProperty;
 use properties::longhands;
 use properties::{Importance, LonghandId};
@@ -1374,6 +1375,12 @@ fn static_assert() {
         }
     }
 
+    pub fn fixup_none_generic(&mut self, device: &Device) {
+        unsafe {
+            bindings::Gecko_nsStyleFont_FixupNoneGeneric(&mut self.gecko, &*device.pres_context)
+        }
+    }
+
     pub fn set_font_family(&mut self, v: longhands::font_family::computed_value::T) {
         use properties::longhands::font_family::computed_value::FontFamily;
         use gecko_bindings::structs::FontFamilyType;
@@ -1381,6 +1388,8 @@ fn static_assert() {
         let list = &mut self.gecko.mFont.fontlist;
         unsafe { Gecko_FontFamilyList_Clear(list); }
 
+        self.gecko.mGenericID = structs::kGenericFont_NONE;
+
         for family in &v.0 {
             match *family {
                 FontFamily::FamilyName(ref f) => {
@@ -1428,6 +1437,7 @@ fn static_assert() {
 
     pub fn copy_font_family_from(&mut self, other: &Self) {
         unsafe { Gecko_CopyFontFamilyFrom(&mut self.gecko.mFont, &other.gecko.mFont); }
+        self.gecko.mGenericID = other.gecko.mGenericID;
     }
 
     // FIXME(bholley): Gecko has two different sizes, one of which (mSize) is the

components/style/properties/properties.mako.rs

@@ -2609,7 +2609,11 @@ pub fn apply_declarations<'a, F, I>(device: &Device,
                                                  &mut cacheable,
                                                  &mut cascade_info,
                                                  error_reporter);
+                % if product == "gecko":
+                    context.style.mutate_font().fixup_none_generic(context.device);
+                % endif
             }
+
             if let Some(declaration) = font_size {
                 let discriminant = LonghandId::FontSize as usize;
                 (CASCADE_PROPERTY[discriminant])(declaration,