Auto merge of #18084 - hiikezoe:currentcolor-for-box-shadow-animation, r=xidorn

Currentcolor for box shadow animation

<!-- Please describe your changes on the following line: -->
https://bugzilla.mozilla.org/show_bug.cgi?id=1388220
---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors

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components/style/gecko_bindings/sugar/ns_css_shadow_item.rs

@@ -23,12 +23,7 @@ impl nsCSSShadowItem {
     #[inline]
     pub fn to_box_shadow(&self) -> BoxShadow {
         BoxShadow {
-            base: SimpleShadow {
+            base: self.extract_simple_shadow(),
-                color: Color::rgba(convert_nscolor_to_rgba(self.mColor)),
-                horizontal: Au(self.mXOffset),
-                vertical: Au(self.mYOffset),
-                blur: Au(self.mRadius).into(),
-            },
             spread: Au(self.mSpread),
             inset: self.mInset,
         }
@@ -53,16 +48,31 @@ impl nsCSSShadowItem {
         }
     }
 
-    /// Returns this item as a simple shadow.
     #[inline]
-    pub fn to_simple_shadow(&self) -> SimpleShadow {
+    fn extract_color(&self) -> Color {
-        debug_assert_eq!(self.mSpread, 0);
+        if self.mHasColor {
-        debug_assert_eq!(self.mInset, false);
+            Color::rgba(convert_nscolor_to_rgba(self.mColor))
+        } else {
+            Color::currentcolor()
+        }
+    }
+
+    /// Gets a simple shadow from this item.
+    #[inline]
+    fn extract_simple_shadow(&self) -> SimpleShadow {
         SimpleShadow {
-            color: Color::rgba(convert_nscolor_to_rgba(self.mColor)),
+            color: self.extract_color(),
             horizontal: Au(self.mXOffset),
             vertical: Au(self.mYOffset),
             blur: Au(self.mRadius).into(),
         }
     }
+
+    /// Returns this item as a simple shadow.
+    #[inline]
+    pub fn to_simple_shadow(&self) -> SimpleShadow {
+        debug_assert_eq!(self.mSpread, 0);
+        debug_assert_eq!(self.mInset, false);
+        self.extract_simple_shadow()
+    }
 }