Fix radial-gradient position being dropped after an explicit size

In processBackgroundImage, the explicit-size branch shifts the next token
to look for a second size value and discards it when it is not a length or
percentage. When that token is 'at', the whole position clause is lost:
the position defaults back to center and the position values are then
re-parsed as a new size, silently overriding the declared one.

radial-gradient(circle 100px at 25% 75%, red, blue) previously parsed as
size {x: '25%', y: '75%'} with position {top: '50%', left: '50%'};
it now parses as size {x: 100, y: 100} with position {left: '25%', top: '75%'}.

The existing test for this syntax only used 'at center', which is
indistinguishable from the default position, so the bug was invisible.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: Titozzz

packages/react-native/Libraries/StyleSheet/__tests__/processBackgroundImage-itest.js

@@ -999,6 +999,22 @@ describe('processBackgroundImage', () => {
     expect(result[0].type).toEqual('radial-gradient');
   });
 
+  it('should handle radial gradient with explicit size and position', () => {
+    const input = 'radial-gradient(circle 100px at 25% 75%, red, blue)';
+    const result = processBackgroundImage(input);
+    expect(result[0].shape).toEqual('circle');
+    expect(result[0].size).toEqual({x: 100, y: 100});
+    expect(result[0].position).toEqual({left: '25%', top: '75%'});
+  });
+
+  it('should handle radial gradient with two explicit sizes and position', () => {
+    const input = 'radial-gradient(50px 100px at left bottom, red, blue)';
+    const result = processBackgroundImage(input);
+    expect(result[0].shape).toEqual('ellipse');
+    expect(result[0].size).toEqual({x: 50, y: 100});
+    expect(result[0].position).toEqual({left: '0%', top: '100%'});
+  });
+
   // 1. position syntax: [ left | center | right | top | bottom | <length-percentage> ]
   it('should handle radial gradient position length syntax', () => {
     const input = 'radial-gradient(circle at 20px, red, blue)';

packages/react-native/Libraries/StyleSheet/processBackgroundImage.js

@@ -348,6 +348,11 @@ function parseRadialGradientCSSString(
         size = {x: sizeX, y: sizeY};
       } else {
         hasExplicitSingleSize = true;
+        // The token after the size is not a second size value (e.g. 'at' or a
+        // shape keyword). Put it back so the loop can process it, otherwise
+        // the position would be silently dropped and its values re-parsed as
+        // a new size.
+        firstPartTokens.unshift(token);
       }
     } else if (tokenTrimmed === 'at') {
       let top: string | number;