Bugfix: Incorrect ordering for Affine::pre_rotate_about (#567)

Fixes #566

---------

Co-authored-by: Thomas Churchman <thomas@kepow.org>

Author: dannymcgee

CHANGELOG.md

@@ -31,6 +31,7 @@ This release has an [MSRV][] of 1.85.
 ## Fixed
 
 - Improved numerical robustness of winding numbers for paths. ([#537][] by [@jneem][])
+- `Affine::pre_rotate_about` calculation. ([#567][] by [@dannymcgee][])
 
 ## [0.13.0] (2025-11-27)
 
@@ -185,6 +186,7 @@ Note: A changelog was not kept for or before this release
 [@beholdnec]: https://github.com/beholdnec
 [@Bombaninha]: https://github.com/Bombaninha
 [@cmyr]: https://github.com/cmyr
+[@dannymcgee]: https://github.com/dannymcgee
 [@DJMcNab]: https://github.com/DJMcNab
 [@dominikh]: https://github.com/dominikh
 [@ErisianArchitect]: https://github.com/ErisianArchitect
@@ -291,6 +293,7 @@ Note: A changelog was not kept for or before this release
 [#549]: https://github.com/linebender/kurbo/pull/549
 [#559]: https://github.com/linebender/kurbo/pull/559
 [#561]: https://github.com/linebender/kurbo/pull/561
+[#567]: https://github.com/linebender/kurbo/pull/567
 
 [Unreleased]: https://github.com/linebender/kurbo/compare/v0.13.0...HEAD
 [0.13.0]: https://github.com/linebender/kurbo/releases/tag/v0.13.0

kurbo/src/affine.rs

@@ -189,7 +189,7 @@ impl Affine {
     #[inline]
     #[must_use]
     pub fn pre_rotate_about(self, th: f64, center: impl Into<Point>) -> Self {
-        Affine::rotate_about(th, center) * self
+        self * Affine::rotate_about(th, center)
     }
 
     /// A [scale] by `scale` followed by `self`.
@@ -720,4 +720,38 @@ mod tests {
             "The product of the singular values {s:?} ({prod}) should be equal to the absolute determinant {det}.",
         );
     }
+
+    #[test]
+    fn rotate_about_composition() {
+        let theta = core::f64::consts::FRAC_PI_2;
+        let center = Point::new(-1., 0.);
+        let translation = Vec2::new(0., 1.);
+        let probe = Point::ORIGIN;
+
+        let rotate_about = Affine::rotate_about(theta, center);
+        let translate = Affine::translate(translation);
+
+        // Establish baselines with raw matrix composition
+        // (also a sanity check to ensure the order of ops matters for this contrived test)
+        let rotate_then_translate = translate * rotate_about;
+        let translate_then_rotate = rotate_about * translate;
+        assert_near(rotate_then_translate * probe, Point::new(-1., 2.));
+        assert_near(translate_then_rotate * probe, Point::new(-2., 1.));
+
+        // Check .then_* semantics
+        affine_assert_near(
+            rotate_about.then_translate(translation),
+            rotate_then_translate,
+        );
+        affine_assert_near(
+            translate.then_rotate_about(theta, center),
+            translate_then_rotate,
+        );
+
+        // Check .pre_rotate_about semantics
+        affine_assert_near(
+            translate.pre_rotate_about(theta, center),
+            rotate_then_translate,
+        );
+    }
 }