Auto merge of #18244 - kahsieh:master, r=Manishearth

stylo: Update CSS fallback font when lang changes

r=manishearth in BugZilla.

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix [Bug 1367860](https://bugzilla.mozilla.org/show_bug.cgi?id=1367860).

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components/style/properties/gecko.mako.rs

@@ -2169,9 +2169,10 @@ fn static_assert() {
         }
     }
 
-    pub fn fixup_system(&mut self) {
+    pub fn fixup_system(&mut self, default_font_type: structs::FontFamilyType) {
         self.gecko.mFont.systemFont = true;
         self.gecko.mGenericID = structs::kGenericFont_NONE;
+        self.gecko.mFont.fontlist.mDefaultFontType = default_font_type;
     }
 
     pub fn set_font_family(&mut self, v: longhands::font_family::computed_value::T) {
@@ -2223,7 +2224,16 @@ fn static_assert() {
         use gecko_bindings::structs::FontFamilyType;
         use gecko_string_cache::Atom;
 
-        ::properties::longhands::font_family::computed_value::T(
+        if self.gecko.mFont.fontlist.mFontlist.is_empty() {
+            let default = match self.gecko.mFont.fontlist.mDefaultFontType {
+                FontFamilyType::eFamily_serif => FontFamily::Generic(atom!("serif")),
+                FontFamilyType::eFamily_sans_serif => FontFamily::Generic(atom!("sans-serif")),
+                _ => panic!("Default generic must be serif or sans-serif"),
+            };
+            return longhands::font_family::computed_value::T(vec![default]);
+        }
+
+        longhands::font_family::computed_value::T(
             self.gecko.mFont.fontlist.mFontlist.iter().map(|gecko_font_family_name| {
                 match gecko_font_family_name.mType {
                     FontFamilyType::eFamily_serif => FontFamily::Generic(atom!("serif")),

components/style/properties/longhand/font.mako.rs

@@ -2491,11 +2491,13 @@ ${helpers.single_keyword("-moz-math-variant",
 
         use app_units::Au;
         use cssparser::{Parser, ToCss};
+        use gecko_bindings::structs::FontFamilyType;
         use properties::longhands;
         use std::fmt;
         use std::hash::{Hash, Hasher};
         use style_traits::ParseError;
         use values::computed::{ToComputedValue, Context};
+
         <%
             system_fonts = """caption icon menu message-box small-caption status-bar
                               -moz-window -moz-document -moz-workspace -moz-desktop
@@ -2584,6 +2586,7 @@ ${helpers.single_keyword("-moz-math-variant",
                     font_feature_settings: longhands::font_feature_settings::get_initial_value(),
                     font_variant_alternates: longhands::font_variant_alternates::get_initial_value(),
                     system_font: *self,
+                    default_font_type: system.fontlist.mDefaultFontType,
                 };
                 unsafe { bindings::Gecko_nsFont_Destroy(&mut system); }
                 ret
@@ -2615,6 +2618,7 @@ ${helpers.single_keyword("-moz-math-variant",
                 pub ${name}: longhands::${name}::computed_value::T,
             % endfor
             pub system_font: SystemFont,
+            pub default_font_type: FontFamilyType,
         }
 
         impl SystemFont {

components/style/properties/properties.mako.rs

@@ -3331,20 +3331,15 @@ where
                         }
                     }
 
-                    // In case of just the language changing, the parent could have had no generic,
-                    // which Gecko just does regular cascading with. Do the same.
-                    // This can only happen in the case where the language changed but the family did not
-                    if generic != structs::kGenericFont_NONE {
-                        let pres_context = context.builder.device.pres_context();
-                        let gecko_font = context.builder.mutate_font().gecko_mut();
-                        gecko_font.mGenericID = generic;
-                        unsafe {
-                            bindings::Gecko_nsStyleFont_PrefillDefaultForGeneric(
-                                gecko_font,
-                                pres_context,
-                                generic,
-                            );
-                        }
+                    let pres_context = context.builder.device.pres_context();
+                    let gecko_font = context.builder.mutate_font().gecko_mut();
+                    gecko_font.mGenericID = generic;
+                    unsafe {
+                        bindings::Gecko_nsStyleFont_PrefillDefaultForGeneric(
+                            gecko_font,
+                            pres_context,
+                            generic,
+                        );
                     }
                 }
             % endif
@@ -3371,7 +3366,9 @@ where
                         let device = context.builder.device;
                         if let PropertyDeclaration::FontFamily(ref val) = **declaration {
                             if val.get_system().is_some() {
-                                context.builder.mutate_font().fixup_system();
+                                let default = context.cached_system_font
+                                                     .as_ref().unwrap().default_font_type;
+                                context.builder.mutate_font().fixup_system(default);
                             } else {
                                 context.builder.mutate_font().fixup_none_generic(device);
                             }