fix: optimize zIndex (#289)

* fix: operations zIndex

* fix: operations zIndex

* feat: optimize code

Author: kiner-tang

src/Preview.tsx

@@ -324,7 +324,7 @@ const Preview: React.FC<PreviewProps> = props => {
         onFlipX={onFlipX}
         onFlipY={onFlipY}
         onClose={onClose}
-        zIndex={restProps.zIndex}
+        zIndex={restProps.zIndex !== undefined ? restProps.zIndex + 1 : undefined}
       />
     </>
   );

tests/basic.test.tsx

@@ -95,6 +95,6 @@ describe('Basic', () => {
       />,
     );
     const operationsElement = baseElement.querySelector('.rc-image-preview-operations-wrapper');
-    expect(operationsElement).toHaveStyle({ zIndex: 9999 });
+    expect(operationsElement).toHaveStyle({ zIndex: 10000 });
   });
 });