Allow cross interpolation between differing translate/scale functions

See ToPrimitive() in StyleAnimationValue.cpp

components/style/properties/helpers/animated_properties.mako.rs

@@ -1226,7 +1226,12 @@ impl Animate for ComputedTransformOperation {
                     fd_matrix.animate(&td_matrix, procedure)?,
                 ))
             },
-            // XXXManishearth handle crossover between translate and scale functions
+            (ref f, ref t) if f.is_translate() && t.is_translate() => {
+                f.to_translate_3d().animate(&t.to_translate_3d(), procedure)
+            }
+            (ref f, ref t) if f.is_scale() && t.is_scale() => {
+                f.to_scale_3d().animate(&t.to_scale_3d(), procedure)
+            }
             _ => Err(()),
         }
     }

components/style/values/computed/length.rs

@@ -706,6 +706,11 @@ impl CSSPixelLength {
     pub fn abs(self) -> Self {
         CSSPixelLength::new(self.0.abs())
     }
+
+    /// Zero value
+    pub fn zero() -> Self {
+        CSSPixelLength::new(0.)
+    }
 }
 
 impl ToCss for CSSPixelLength {

components/style/values/generics/transform.rs

@@ -249,7 +249,6 @@ pub struct Transform<T>(pub Vec<T>);
 
 impl<Angle, Number, Length, Integer, LengthOrNumber, LengthOrPercentage, LoPoNumber>
     TransformOperation<Angle, Number, Length, Integer, LengthOrNumber, LengthOrPercentage, LoPoNumber> {
-
     /// Check if it is any translate function
     pub fn is_translate(&self) -> bool {
         use self::TransformOperation::*;

components/style/values/specified/transform.rs

@@ -10,8 +10,8 @@ use selectors::parser::SelectorParseErrorKind;
 use style_traits::{ParseError, StyleParseErrorKind};
 use values::computed::{Context, LengthOrPercentage as ComputedLengthOrPercentage};
 use values::computed::{Percentage as ComputedPercentage, ToComputedValue};
-use values::generics::transform::{Matrix3D, Transform as GenericTransform};
 use values::computed::transform::TimingFunction as ComputedTimingFunction;
+use values::generics::transform::{Matrix3D, Transform as GenericTransform};
 use values::generics::transform::{StepPosition, TimingFunction as GenericTimingFunction, Matrix};
 use values::generics::transform::{TimingKeyword, TransformOrigin as GenericTransformOrigin};
 use values::generics::transform::TransformOperation as GenericTransformOperation;