feat: add Tailwind CSS v4 shadow support and inset shadows

This PR adds two features for better Tailwind CSS v4 compatibility:

1. **Tailwind CSS v4 shadow variable defaults**
   - Tailwind v4 uses @Property to define initial-value for shadow
     CSS variables, but react-native-css doesn't support @Property
   - Added default values for tw-shadow, tw-inset-shadow, tw-ring-shadow,
     etc. to prevent "undefined variable" errors

2. **Inset shadow parsing support**
   - Added pattern matching for "inset" keyword in box-shadow values
   - Converts inset: "inset" to inset: true for React Native's boxShadow
   - Supports patterns: inset <x> <y> <blur> <spread> [color]

Author: YevheniiKotyrlo

src/__tests__/native/box-shadow.test.tsx

@@ -93,3 +93,115 @@ test("shadow values - multiple nested variables", () => {
     ],
   });
 });
+
+test("inset shadow - basic", () => {
+  registerCSS(`
+    .test { box-shadow: inset 0 2px 4px 0 #000; }
+  `);
+
+  render(<View testID={testID} className="test" />);
+  const component = screen.getByTestId(testID);
+
+  expect(component.props.style).toStrictEqual({
+    boxShadow: [
+      {
+        inset: true,
+        offsetX: 0,
+        offsetY: 2,
+        blurRadius: 4,
+        spreadDistance: 0,
+        color: "#000",
+      },
+    ],
+  });
+});
+
+test("inset shadow - with color first", () => {
+  registerCSS(`
+    .test { box-shadow: inset #fb2c36 0 0 24px 0; }
+  `);
+
+  render(<View testID={testID} className="test" />);
+  const component = screen.getByTestId(testID);
+
+  expect(component.props.style).toStrictEqual({
+    boxShadow: [
+      {
+        inset: true,
+        color: "#fb2c36",
+        offsetX: 0,
+        offsetY: 0,
+        blurRadius: 24,
+        spreadDistance: 0,
+      },
+    ],
+  });
+});
+
+test("inset shadow - without color inherits default", () => {
+  registerCSS(`
+    .test { box-shadow: inset 0 0 10px 5px; }
+  `);
+
+  render(<View testID={testID} className="test" />);
+  const component = screen.getByTestId(testID);
+
+  // Shadows without explicit color inherit the default text color (__rn-css-color)
+  expect(component.props.style.boxShadow).toHaveLength(1);
+  expect(component.props.style.boxShadow[0]).toMatchObject({
+    inset: true,
+    offsetX: 0,
+    offsetY: 0,
+    blurRadius: 10,
+    spreadDistance: 5,
+  });
+  // Color is inherited from platform default (PlatformColor)
+  expect(component.props.style.boxShadow[0].color).toBeDefined();
+});
+
+test("mixed inset and regular shadows", () => {
+  registerCSS(`
+    .test { box-shadow: 0 4px 6px -1px #000, inset 0 2px 4px 0 #fff; }
+  `);
+
+  render(<View testID={testID} className="test" />);
+  const component = screen.getByTestId(testID);
+
+  expect(component.props.style).toStrictEqual({
+    boxShadow: [
+      {
+        offsetX: 0,
+        offsetY: 4,
+        blurRadius: 6,
+        spreadDistance: -1,
+        color: "#000",
+      },
+      {
+        inset: true,
+        offsetX: 0,
+        offsetY: 2,
+        blurRadius: 4,
+        spreadDistance: 0,
+        color: "#fff",
+      },
+    ],
+  });
+});
+
+test("Tailwind v4 shadow variables - transparent color #0000", () => {
+  // Tailwind v4 uses --tw-shadow etc with @property initial-value of transparent
+  registerCSS(`
+    :root {
+      --tw-shadow: 0 0 0 0 #0000;
+    }
+    .test { box-shadow: var(--tw-shadow); }
+  `);
+
+  render(<View testID={testID} className="test" />);
+  const component = screen.getByTestId(testID);
+
+  // The shadow is parsed correctly with #0000 color
+  // Note: filtering of transparent shadows happens in omitTransparentShadows
+  // which checks for exact "#0000" or "transparent" strings
+  expect(component.props.style.boxShadow).toBeDefined();
+});

src/native-internal/root.ts

@@ -42,3 +42,26 @@ rootVariables("__rn-css-color").set([
   ],
   // eslint-disable-next-line @typescript-eslint/no-explicit-any
 ] as any);
+
+/**
+ * Tailwind CSS v4 shadow variable defaults.
+ *
+ * Tailwind v4 uses @property to define initial-value for shadow CSS variables,
+ * but react-native-css doesn't support @property declarations.
+ *
+ * These provide fallback values that match Tailwind's defaults:
+ * - Transparent shadows (0 0 0 0 #0000) are filtered out by omitTransparentShadows
+ * - This prevents "undefined variable" errors when shadow utilities are used
+ *
+ * @see https://github.com/tailwindlabs/tailwindcss/discussions/16772
+ */
+// VariableValue[] where each VariableValue is [StyleDescriptor] tuple
+// The inner [0, 0, 0, 0, "#0000"] is a StyleDescriptor[] (shadow values)
+const transparentShadow: VariableValue[] = [[[0, 0, 0, 0, "#0000"]]];
+rootVariables("tw-shadow").set(transparentShadow);
+rootVariables("tw-shadow-color").set([["initial"]]);
+rootVariables("tw-inset-shadow").set(transparentShadow);
+rootVariables("tw-inset-shadow-color").set([["initial"]]);
+rootVariables("tw-ring-shadow").set(transparentShadow);
+rootVariables("tw-inset-ring-shadow").set(transparentShadow);
+rootVariables("tw-ring-offset-shadow").set(transparentShadow);

src/native/styles/shorthands/box-shadow.ts

@@ -9,16 +9,25 @@ const offsetX = ["offsetX", "number"] as const;
 const offsetY = ["offsetY", "number"] as const;
 const blurRadius = ["blurRadius", "number"] as const;
 const spreadDistance = ["spreadDistance", "number"] as const;
-// const inset = ["inset", "string"] as const;
+// Match the literal string "inset" - the array type checks if value is in array
+const inset = ["inset", ["inset"]] as const;
 
 const handler = shorthandHandler(
   [
+    // Standard patterns (without inset)
     [offsetX, offsetY, blurRadius, spreadDistance],
     [offsetX, offsetY, blurRadius, spreadDistance, color],
     [color, offsetX, offsetY],
     [color, offsetX, offsetY, blurRadius, spreadDistance],
     [offsetX, offsetY, color],
     [offsetX, offsetY, blurRadius, color],
+    // Inset patterns - "inset" keyword at the beginning
+    // Matches: inset <offsetX> <offsetY> <blur> <spread>
+    [inset, offsetX, offsetY, blurRadius, spreadDistance],
+    // Matches: inset <offsetX> <offsetY> <blur> <spread> <color>
+    [inset, offsetX, offsetY, blurRadius, spreadDistance, color],
+    // Matches: inset <color> <offsetX> <offsetY> <blur> <spread>
+    [inset, color, offsetX, offsetY, blurRadius, spreadDistance],
   ],
   [],
   "object",
@@ -41,8 +50,10 @@ export const boxShadow: StyleFunctionResolver = (
         if (shadows === undefined) {
           return;
         } else {
-          return omitTransparentShadows(
-            handler(resolveValue, shadows, get, options),
+          return normalizeInsetValue(
+            omitTransparentShadows(
+              handler(resolveValue, shadows, get, options),
+            ),
           );
         }
       })
@@ -69,3 +80,17 @@ function omitTransparentShadows(style: unknown) {
 
   return style;
 }
+
+/**
+ * Convert inset: "inset" to inset: true for React Native boxShadow.
+ *
+ * The shorthand handler matches the literal "inset" string and assigns it as the value.
+ * React Native's boxShadow expects inset to be a boolean.
+ */
+function normalizeInsetValue(style: unknown) {
+  if (typeof style === "object" && style && "inset" in style) {
+    return { ...style, inset: true };
+  }
+
+  return style;
+}