Don't apply the rotation if it cannot be normalized.

According to the spec, the computed value of transform is as specified, but
with relative lengths converted into absolute lengths, so in Gecko, we do
nothing while computing the value of rotate3d(), and do normalization in
ProcessRotate3D(). If the direction cannot be normalized, we treat it as
an identity matrix.

However, in Servo, we do normalization in to_computed_value(), and looks
like we are trying to normalize any kind of direction vectors, so according
to the spec, let's move the normalization into Fragment::transform_matrix(),
and return an identity matrix if we cannot normalize its direction vector.

1 files changed, 11 insertions, 2 deletions

diff --git a/components/layout/fragment.rs b/components/layout/fragment.rs
index fd2d5698efd..2510b185d1c 100644
--- a/components/layout/fragment.rs
+++ b/components/layout/fragment.rs
@@ -2887,8 +2887,17 @@ impl Fragment {
         for operation in operations {
             let matrix = match *operation {
                 transform::ComputedOperation::Rotate(ax, ay, az, theta) => {
-                    let theta = 2.0f32 * f32::consts::PI - theta.radians();
-                    Transform3D::create_rotation(ax, ay, az, Radians::new(theta))
+                    // https://www.w3.org/TR/css-transforms-1/#funcdef-rotate3d
+                    // A direction vector that cannot be normalized, such as [0, 0, 0], will cause
+                    // the rotation to not be applied, so we use identity matrix in this case.
+                    let len = (ax * ax + ay * ay + az * az).sqrt();
+                    if len > 0. {
+                        let theta = 2.0f32 * f32::consts::PI - theta.radians();
+                        Transform3D::create_rotation(ax / len, ay / len, az / len,
+                                                     Radians::new(theta))
+                    } else {
+                        Transform3D::identity()
+                    }
                 }
                 transform::ComputedOperation::Perspective(d) => {
                     create_perspective_matrix(d)