Add test for parent viewport ref

Author: zeusdeux

cypress/integration/useIntersectionObserver.test.js

@@ -81,5 +81,9 @@ describe('useIsInViewport', () => {
     cy.get('[data-testid="viewport"]').scrollTo(0, 250) // 75% or more of second child in viewport
     cy.get('[data-testid="first-child"]').should('contain', 'First child >= 75% in viewport')
     cy.get('[data-testid="second-child"]').should('contain', 'Second child >= 75% in viewport')
+    cy.window().then(window => {
+      expect(window.forwardedViewportRef).length(1)
+      expect(window.forwardedViewportRef[0].classList.contains('viewport')).to.be.true
+    })
   })
 })

examples/cra/src/viewportAnotherElement.js

@@ -67,7 +67,7 @@ export const RefForwardingElement = React.forwardRef(function RefForwardingEleme
       >
         {hidden ? 'Show boxes' : 'Hide boxes'}
       </button>
-      <div ref={finalParentRef} data-testid="viewport" className={container}>
+      <div ref={finalParentRef} data-testid="viewport" className={cx('viewport', container)}>
         <div
           ref={firstChildWrappedRef}
           data-testid="first-child"