test: Add test for fixed anchoring on scroll (#330)

Author: chuckcarpenter

test/cypress/integration/scroll.spec.js

@@ -0,0 +1,31 @@
+describe('Scroll', () => {
+  beforeEach(() => {
+    cy.visit('/examples/scroll/', {});
+  });
+
+  describe('tether stays near scrollbar', () => {
+    it('appears on scroll and stays in fixed position', () => {
+      let tetherFixed;
+      let tetherOffsetTop;
+
+      cy.get('.tether-target').and(el => {
+        // set the initial position coords
+        tetherFixed = el.find('.pointer');
+        // starting point is 9px from top of document
+        tetherOffsetTop = tetherFixed[0].getBoundingClientRect().top - 9;
+
+        expect(el).to.contain('.pointer');
+      });
+      cy.get('.pointer').should('have.css', 'opacity', '0');
+
+      cy.scrollTo(0, 1500);
+
+      cy.get('.pointer').should('have.css', 'opacity', '1');
+      cy.get('.pointer').and(el => {
+        const vpOffset = el[0].getBoundingClientRect();
+        // we measure from top minus scrolled coords
+        expect(vpOffset.top + window.pageYOffset).to.equal(tetherOffsetTop);
+      });
+    });
+  });
+});