feat(DataTable): handle conditional shadow on pinned header

src/components/DataTable/DataTable.module.css

@@ -4,8 +4,6 @@
 
 /* Visible table caption */
 /* TODO-AH: make it so that we have the search bar and actions wrap together instead of separately */
-/* TODO-AH: implement showing of shadow below sticky headers and column */
-/* - https://stackoverflow.com/questions/38122751/positionsticky-adding-style-when-the-element-detaches-from-normal-flow */
 .data-table__caption-container {
   display: flex;
   align-items: flex-end;
@@ -163,7 +161,7 @@
 .data-table__header-row {
   border-bottom: calc(var(--eds-border-width-sm) * 1px) solid;
   position: sticky;
-  top: 0;
+  top: -1px;
 }
 
 .data-table__group-row {
@@ -182,6 +180,10 @@
     padding: calc(var(--eds-size-2) / 16 * 1rem)
       calc(var(--eds-size-3) / 16 * 1rem);
   }
+}
+
+.data-table--is-pinned {
+  box-shadow: var(--eds-box-shadow-sm);
 
 }
 

src/components/DataTable/DataTable.test.ts

@@ -2,6 +2,31 @@ import { generateSnapshots } from '@chanzuckerberg/story-utils';
 import * as stories from './DataTable.stories';
 import type { StoryFile } from '../../util/utility-types';
 
+// Stubbing out this class b/c it doesn't exist in non-browser contexts
+export class IntersectionObserver {
+  root = null;
+  rootMargin = '';
+  thresholds = [];
+
+  disconnect() {
+    return null;
+  }
+
+  observe() {
+    return null;
+  }
+
+  takeRecords() {
+    return [];
+  }
+
+  unobserve() {
+    return null;
+  }
+}
+window.IntersectionObserver = IntersectionObserver;
+global.IntersectionObserver = IntersectionObserver;
+
 describe('<DataTable />', () => {
   generateSnapshots(stories as StoryFile);
 });

src/components/DataTable/DataTable.tsx

@@ -1,6 +1,6 @@
 import { flexRender, type Table } from '@tanstack/react-table';
 import clsx from 'clsx';
-import React from 'react';
+import React, { useEffect } from 'react';
 
 import type { EDSBase, Size, Status, Align } from '../../util/variant-types';
 
@@ -411,6 +411,44 @@ export const DataTableTable = ({
     size && styles[`data-table--size-${size}`],
   );
 
+  // eslint-disable-next-line @chanzuckerberg/edu-react/use-effect-deps-presence
+  useEffect(() => {
+    /**
+     * Here, we set up a selector on the header of a table, and look for it to start
+     * clipping off the top edge of its visible area. Once it does the observer will trigger
+     * causing the state class to be added / removed (depending on the intersection value).
+     *
+     * We use the generated class names from the module for both the trigger and whats toggled
+     */
+    let observer: IntersectionObserver;
+    let el: Element | null;
+    function triggerEffect() {
+      el = document.querySelector(`.${styles['data-table__table']} thead`);
+      if (el && typeof window !== 'undefined') {
+        observer = new IntersectionObserver(
+          ([event]) => {
+            return event.target.classList.toggle(
+              styles['data-table--is-pinned'],
+              event.intersectionRatio < 1,
+            );
+          },
+
+          { threshold: [1] },
+        );
+
+        observer.observe(el);
+      }
+    }
+    triggerEffect();
+
+    // TODO: anything to de-allocate when using `observer`?
+    return () => {
+      if (el) {
+        observer.unobserve(el);
+      }
+    };
+  });
+
   return <table className={tableClassNames}>{children}</table>;
 };