Auto merge of #17613 - birtles:zero-length-lists, r=hiro

Handle zero length lists when interpolating

These are the Servo side changes for [Gecko bug 1377053](https://bugzilla.mozilla.org/show_bug.cgi?id=1377053).

These patches have been reviewed by @hiikezoe.

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

@@ -828,6 +828,10 @@ macro_rules! repeated_vec_impl {
         $(impl<T: RepeatableListAnimatable> Animatable for $ty {
             fn add_weighted(&self, other: &Self, self_portion: f64, other_portion: f64)
                 -> Result<Self, ()> {
+                // If the length of either list is zero, the least common multiple is undefined.
+                if cmp::min(self.len(), other.len()) < 1 {
+                    return Err(());
+                }
                 use num_integer::lcm;
                 let len = lcm(self.len(), other.len());
                 self.iter().cycle().zip(other.iter().cycle()).take(len).map(|(me, you)| {
@@ -842,6 +846,10 @@ macro_rules! repeated_vec_impl {
 
             #[inline]
             fn compute_squared_distance(&self, other: &Self) -> Result<f64, ()> {
+                // If the length of either list is zero, the least common multiple is undefined.
+                if cmp::min(self.len(), other.len()) < 1 {
+                    return Err(());
+                }
                 use num_integer::lcm;
                 let len = lcm(self.len(), other.len());
                 self.iter().cycle().zip(other.iter().cycle()).take(len).map(|(me, you)| {

ports/geckolib/glue.rs

@@ -545,7 +545,7 @@ pub extern "C" fn Servo_AnimationCompose(raw_value_map: RawServoAnimationValueMa
     };
     if let Ok(value) = from_value.interpolate(to_value, position) {
         value_map.insert(property, value);
-    } else if progress < 0.5 {
+    } else if position < 0.5 {
         value_map.insert(property, from_value.clone());
     } else {
         value_map.insert(property, to_value.clone());