fix: shadow is not applied with transform correctly (#1124)

Author: Brooooooklyn

__test__/regression.spec.ts

@@ -528,3 +528,32 @@ test('pass invalid args to setLineDash should not throw', (t) => {
     ])
   })
 })
+
+// https://github.com/Brooooooklyn/canvas/issues/1121
+test('shadow-offset-with-transform', async (t) => {
+  // Test for issue #1121 - shadow offset should be in device coordinates
+  const canvas = createCanvas(300, 300)
+  const ctx = canvas.getContext('2d')
+
+  // Fill with white background
+  ctx.fillStyle = 'white'
+  ctx.fillRect(0, 0, 300, 300)
+
+  // Apply transform - scale down by 0.5 and translate
+  ctx.transform(0.5, 0, 0, 0.5, 100, 100)
+
+  // Set shadow properties
+  ctx.shadowColor = 'rgba(0, 0, 0, 1)'
+  ctx.shadowBlur = 0
+  ctx.shadowOffsetX = 5
+  ctx.shadowOffsetY = 5
+
+  // Draw green rectangle
+  ctx.fillStyle = 'green'
+  ctx.rect(0, 0, 100, 100)
+  ctx.fill()
+
+  // The shadow should be offset by exactly 5px in both X and Y directions
+  // in device/screen coordinates, regardless of the transform applied
+  await snapshotImage(t, { canvas, ctx }, 'png', 0.3)
+})

__test__/snapshots/shadow-offset-with-transform.png

@@ -969,9 +969,21 @@ impl Context {
     shadow_offset_x: f32,
     shadow_offset_y: f32,
   ) -> result::Result<(), SkError> {
-    let mut shadow_transform = canvas.get_transform_matrix().clone();
-    shadow_transform.pre_translate(shadow_offset_x, shadow_offset_y);
-    canvas.set_transform(&shadow_transform);
+    // Following CanvasKit's approach: apply shadow offset in device coordinates
+    // by inverting the current transform, applying the offset, then re-applying the transform
+    let current_transform = canvas.get_transform_matrix().clone();
+
+    // Invert the current transform to get back to device coordinates
+    if let Some(inverted) = current_transform.invert() {
+      canvas.concat(&inverted);
+      // Apply shadow offset in device coordinates
+      canvas.concat(&Matrix::translated(shadow_offset_x, shadow_offset_y));
+      // Re-apply the original transform
+      canvas.concat(&current_transform);
+    } else {
+      // If the transform is not invertible, fall back to simple translation
+      canvas.concat(&Matrix::translated(shadow_offset_x, shadow_offset_y));
+    }
     Ok(())
   }