fix: resolve single box-shadow from runtime CSS variables

When a CSS variable holding a single box-shadow value is resolved at
runtime, the shorthand handler receives a flat array of tokens like
[0, 4, 6, -1, "#000"]. The handler incorrectly iterates each primitive
individually, none match any shadow pattern, and the result is an empty
array.

Detect flat vs nested arrays before entering the multi-shadow handler.
A flat array is a single shadow's tokens that should be passed directly
to the pattern handler. A nested array contains multiple shadows for
the existing flatMap logic.

Author: YevheniiKotyrlo

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

@@ -28,6 +28,96 @@ test("shadow values - single nested variables", () => {
   });
 });
 
+test("single shadow via runtime variable", () => {
+  registerCSS(
+    `.test {
+      --my-shadow: 0 4px 6px -1px #000;
+      box-shadow: var(--my-shadow);
+    }`,
+    { inlineVariables: false },
+  );
+
+  render(<View testID={testID} className="test" />);
+  const component = screen.getByTestId(testID);
+
+  expect(component.props.style.boxShadow).toStrictEqual([
+    {
+      offsetX: 0,
+      offsetY: 4,
+      blurRadius: 6,
+      spreadDistance: -1,
+      color: "#000",
+    },
+  ]);
+});
+
+test("single shadow via multi-definition variable (theme switching)", () => {
+  registerCSS(`
+    :root { --themed-shadow: 0 4px 6px -1px #000; }
+    .dark { --themed-shadow: 0 4px 6px -1px #fff; }
+    .test { box-shadow: var(--themed-shadow); }
+  `);
+
+  render(<View testID={testID} className="test" />);
+  const component = screen.getByTestId(testID);
+
+  expect(component.props.style.boxShadow).toStrictEqual([
+    {
+      offsetX: 0,
+      offsetY: 4,
+      blurRadius: 6,
+      spreadDistance: -1,
+      color: "#000",
+    },
+  ]);
+});
+
+test("transparent shadow via runtime variable is filtered", () => {
+  registerCSS(
+    `.test {
+      --my-shadow: 0 0 0 0 #0000;
+      box-shadow: var(--my-shadow);
+    }`,
+    { inlineVariables: false },
+  );
+
+  render(<View testID={testID} className="test" />);
+  const component = screen.getByTestId(testID);
+
+  expect(component.props.style.boxShadow).toStrictEqual([]);
+});
+
+test("multi-shadow via runtime variables (comma-separated)", () => {
+  registerCSS(
+    `.test {
+      --shadow-a: 0 4px 6px -1px #000;
+      --shadow-b: 0 1px 2px 0 #333;
+      box-shadow: var(--shadow-a), var(--shadow-b);
+    }`,
+    { inlineVariables: false },
+  );
+
+  render(<View testID={testID} className="test" />);
+  const component = screen.getByTestId(testID);
+
+  expect(component.props.style.boxShadow).toStrictEqual([
+    {
+      offsetX: 0,
+      offsetY: 4,
+      blurRadius: 6,
+      spreadDistance: -1,
+      color: "#000",
+    },
+    {
+      offsetX: 0,
+      offsetY: 1,
+      blurRadius: 2,
+      spreadDistance: 0,
+      color: "#333",
+    },
+  ]);
+});
+
 test("shadow values - multiple nested variables", () => {
   registerCSS(`
     :root {

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

@@ -34,20 +34,32 @@ export const boxShadow: StyleFunctionResolver = (
 
   if (!isStyleDescriptorArray(args)) {
     return args;
-  } else {
-    return args
-      .flatMap(flattenShadowDescriptor)
-      .map((shadows) => {
-        if (shadows === undefined) {
-          return;
-        } else {
-          return omitTransparentShadows(
-            handler(resolveValue, shadows, get, options),
-          );
-        }
-      })
-      .filter((v) => v !== undefined);
   }
+
+  // A flat array of primitives (e.g. [0, 4, 6, -1, "#000"]) is a single shadow
+  // resolved from a runtime variable. Pass it directly to the pattern handler.
+  // A nested array (e.g. [[0, 4, 6, -1, "#000"], [...]]) is multiple shadows.
+  if (args.length > 0 && !Array.isArray(args[0])) {
+    const result = handler(resolveValue, args, get, options);
+    if (result === undefined) {
+      return [];
+    }
+    const filtered = omitTransparentShadows(result);
+    return filtered !== undefined ? [filtered] : [];
+  }
+
+  return args
+    .flatMap(flattenShadowDescriptor)
+    .map((shadows) => {
+      if (shadows === undefined) {
+        return;
+      } else {
+        return omitTransparentShadows(
+          handler(resolveValue, shadows, get, options),
+        );
+      }
+    })
+    .filter((v) => v !== undefined);
 };
 
 function flattenShadowDescriptor(arg: StyleDescriptor): StyleDescriptor[] {