feat: Add elementFillsContainer utility function

src/utilities.js

@@ -45,3 +45,15 @@ export function getElementsInContainerViewport(container, elementList) {
     );
   });
 }
+
+export function elementFillsContainer(container, element) {
+  const containerRect = getContainerRect(container);
+  const elementRect = element.getBoundingClientRect();
+
+  return (
+    elementRect.top <= containerRect.top &&
+    elementRect.bottom >= containerRect.bottom &&
+    elementRect.left <= containerRect.left &&
+    elementRect.right >= containerRect.right
+  );
+}

src/utilities.test.js

@@ -2,6 +2,7 @@ import {
   getScrollingElement,
   getTargetScrollTop,
   getElementsInContainerViewport,
+  elementFillsContainer,
 } from './utilities';
 
 describe('getScrolllingElement', () => {
@@ -118,3 +119,33 @@ describe('getElementsInContainerViewport', () => {
     expect(test2).toHaveLength(1);
   });
 });
+
+
+describe('elementFillsContainer', () => {
+  const SCREEN_WIDTH = 800;
+  function getElement(top, bottom, left = 0, right = SCREEN_WIDTH) {
+    const target = document.createElement('div');
+    target.getBoundingClientRect = () => ({
+      top, bottom, left, right,
+    });
+
+    return target;
+  }
+
+  test('checks element in body viewport', () => {
+    window.innerWidth = SCREEN_WIDTH;
+    window.innerHeight = 600;
+    expect(elementFillsContainer(document.body, getElement(-100, 700))).toBe(true);
+    expect(elementFillsContainer(document.body, getElement(0, 600))).toBe(true);
+    expect(elementFillsContainer(document.body, getElement(1, 601))).toBe(false);
+    expect(elementFillsContainer(document.body, getElement(-1, 599))).toBe(false);
+  });
+
+  test('checks element in non-body viewport', () => {
+    const container = getElement(200, 500);
+    expect(elementFillsContainer(container, getElement(100, 600))).toBe(true);
+    expect(elementFillsContainer(container, getElement(200, 500))).toBe(true);
+    expect(elementFillsContainer(container, getElement(201, 501))).toBe(false);
+    expect(elementFillsContainer(container, getElement(99, 499))).toBe(false);
+  });
+});