stylo: Animate font-size as NonNegativeLength

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

@@ -596,7 +596,7 @@ ${helpers.single_keyword_system("font-variant-caps",
     }
 </%helpers:longhand>
 
-<%helpers:longhand name="font-size" animation_value_type="ComputedValue"
+<%helpers:longhand name="font-size" animation_value_type="NonNegativeLength"
                    flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER"
                    allow_quirks="True" spec="https://drafts.csswg.org/css-fonts/#propdef-font-size">
     use app_units::Au;

components/style/values/computed/font.rs

@@ -7,10 +7,11 @@
 use app_units::Au;
 use std::fmt;
 use style_traits::ToCss;
+use values::animated::ToAnimatedValue;
 use values::computed::NonNegativeLength;
 use values::specified::font as specified;
 
-#[derive(Animate, ComputeSquaredDistance, ToAnimatedValue, ToAnimatedZero)]
+#[derive(Animate, ComputeSquaredDistance, ToAnimatedZero)]
 #[derive(Clone, Copy, Debug, PartialEq)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 #[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
@@ -74,3 +75,20 @@ impl ToCss for FontSize {
         self.size.to_css(dest)
     }
 }
+
+impl ToAnimatedValue for FontSize {
+    type AnimatedValue = NonNegativeLength;
+
+    #[inline]
+    fn to_animated_value(self) -> Self::AnimatedValue {
+        self.size
+    }
+
+    #[inline]
+    fn from_animated_value(animated: Self::AnimatedValue) -> Self {
+        FontSize {
+            size: animated.clamp(),
+            keyword_info: None,
+        }
+    }
+}

components/style/values/computed/length.rs

@@ -791,6 +791,16 @@ impl NonNegativeLength {
         self.0.px()
     }
 
+    #[inline]
+    /// Ensures it is positive
+    pub fn clamp(self) -> Self {
+        if (self.0).0 < 0. {
+            Self::zero()
+        } else {
+            self
+        }
+    }
+
     /// Scale this NonNegativeLength.
     /// We scale NonNegativeLength by zero if the factor is negative because it doesn't
     /// make sense to scale a negative factor on a non-negative length.