Bug 1649676 - use fallback font if loading WR mac font descriptor fails. r=jfkthame

Differential Revision: https://phabricator.services.mozilla.com/D84625

gfx/webrender_bindings/src/bindings.rs

@@ -2139,7 +2139,18 @@ fn read_font_descriptor(bytes: &mut WrVecU8, index: u32) -> NativeFontHandle {
 fn read_font_descriptor(bytes: &mut WrVecU8, _index: u32) -> NativeFontHandle {
     let chars = bytes.flush_into_vec();
     let name = String::from_utf8(chars).unwrap();
-    let font = CGFont::from_name(&CFString::new(&*name)).unwrap();
+    let font = match CGFont::from_name(&CFString::new(&*name)) {
+        Ok(font) => font,
+        Err(_) => {
+            // If for some reason we failed to load a font descriptor, then our
+            // only options are to either abort or substitute a fallback font.
+            // It is preferable to use a fallback font instead so that rendering
+            // can at least still proceed in some fashion without erroring.
+            // Lucida Grande is the fallback font in Gecko, so use that here.
+            CGFont::from_name(&CFString::from_static_string("Lucida Grande"))
+                .expect("Failed reading font descriptor and could not load fallback font")
+        }
+    };
     NativeFontHandle(font)
 }