From cc5b4a1f0f9dac7fc509191746ec048d6c8ffb1a Mon Sep 17 00:00:00 2001
From: Jason Brashear <115739170+webdevtodayjason@users.noreply.github.com>
Date: Mon, 12 Oct 2026 18:20:54 -0500
Subject: [PATCH] fix(color): mirror gradient midpoints when reversing

Move and mirror segment midpoints when reversing gradient stops so reversed gradients preserve color and opacity interpolation.

Validated locally on macOS with all six cargo xtask ci gates (1,277 tests passed, four existing tests ignored, 15 WASM compilation checks), plus independent regressions, undo/redo, native and IDML round trips, and PNG/PDF rendering.

Refs #638
---
 crates/color/src/gradient.rs | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/crates/color/src/gradient.rs b/crates/color/src/gradient.rs
index c5e30a9..4da4f2d 100644
--- a/crates/color/src/gradient.rs
+++ b/crates/color/src/gradient.rs
@@ -104,9 +104,12 @@ impl Gradient {
         out
     }
     pub fn reverse(&mut self) {
+        // A midpoint belongs to the segment after its stop, so it moves with that segment and mirrors.
+        let mids: Vec<_> = self.stops.iter().rev().skip(1).map(|s| 1.0 - s.midpoint).collect();
         self.stops.reverse();
-        for s in &mut self.stops {
+        for (s, m) in self.stops.iter_mut().zip(mids.into_iter().chain([0.5])) {
             s.offset = 1.0 - s.offset;
+            s.midpoint = m;
         }
     }
     pub fn sort(&mut self) {
@@ -219,8 +222,24 @@ mod tests {
     #[test]
     fn reverse_stops() {
         let mut g = Gradient::default();
+        g.stops[0].midpoint = 0.25;
         g.reverse();
         assert_eq!(g.stops[0].color.to_hex(), "#000000");
         assert_eq!(g.stops[0].offset, 0.0);
+        // The midpoint moves to its reversed segment and mirrors.
+        assert!((g.sample(0.75).0.to_rgb()[0] - 0.5).abs() < 1e-5);
+        assert_eq!(g.stops.iter().map(|s| s.midpoint).collect::<Vec<_>>(), [0.75, 0.5]);
+
+        // With several stops, the reversed gradient is the original mirrored.
+        let mut g = Gradient::default();
+        g.stops[0].midpoint = 0.3;
+        g.stops.insert(1, GradientStop { offset: 0.4, color: Color::rgb(1.0, 0.0, 0.0), opacity: 0.5, midpoint: 0.8 });
+        let original = g.clone();
+        g.reverse();
+        for t in [0.1, 0.3, 0.5, 0.7, 0.9] {
+            let ((c, o), (want_c, want_o)) = (g.sample(t), original.sample(1.0 - t));
+            assert!(c.to_rgb().iter().zip(want_c.to_rgb()).all(|(a, b)| (a - b).abs() < 1e-5), "colour at {t}");
+            assert!((o - want_o).abs() < 1e-5, "opacity at {t}");
+        }
     }
 }