[Unified Data Table] Fix elements with defined z-index overlapping grid in full screen mode (#168545)

## Summary

This PR fixes an issue where elements with defined z-index values
overlap the grid in full screen mode.

Before:
<img width="2007" alt="before"
src="49f5ad02-5c23-4e63-bf15-959ce1b822f0">

After:
<img width="2007" alt="after"
src="46d5ce0a-533a-4b7e-b9d0-be9ec75f2018">

Flaky test run:
- x100:
https://buildkite.com/elastic/kibana-flaky-test-suite-runner/builds/3453

Fixes #168331.

### Checklist

- [ ] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/packages/kbn-i18n/README.md)
- [ ]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials
- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios
- [x] Any UI touched in this PR is usable by keyboard only (learn more
about [keyboard accessibility](https://webaim.org/techniques/keyboard/))
- [ ] Any UI touched in this PR does not create any new axe failures
(run axe in browser:
[FF](https://addons.mozilla.org/en-US/firefox/addon/axe-devtools/),
[Chrome](https://chrome.google.com/webstore/detail/axe-web-accessibility-tes/lhdoppojpmngadmnindnejefpokejbdd?hl=en-US))
- [ ] If a plugin configuration key changed, check if it needs to be
allowlisted in the cloud and added to the [docker
list](https://github.com/elastic/kibana/blob/main/src/dev/build/tasks/os_packages/docker_generator/resources/base/bin/kibana-docker)
- [x] This renders correctly on smaller devices using a responsive
layout. (You can test this [in your
browser](https://www.browserstack.com/guide/responsive-testing-on-local-server))
- [x] This was checked for [cross-browser
compatibility](https://www.elastic.co/support/matrix#matrix_browsers)

### For maintainers

- [ ] This was checked for breaking API changes and was [labeled
appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

packages/kbn-unified-data-table/src/components/data_table.scss

@@ -161,3 +161,12 @@
     display: none;
   }
 }
+
+// Ensure full screen data grids are not covered by elements with a z-index
+.euiDataGrid__restrictBody *:not(
+.euiDataGrid--fullScreen,
+.euiDataGrid--fullScreen *,
+[data-euiportal='true'],
+[data-euiportal='true'] *) {
+  z-index: unset !important;
+}

test/functional/apps/discover/group2/_data_grid.ts

@@ -17,6 +17,8 @@ export default function ({ getService, getPageObjects }: FtrProviderContext) {
     const defaultSettings = { defaultIndex: 'logstash-*' };
     const testSubjects = getService('testSubjects');
     const security = getService('security');
+    const retry = getService('retry');
+    const browser = getService('browser');
 
     before(async function () {
       await security.testUser.setRoles(['kibana_admin', 'test_logstash_reader']);
@@ -47,6 +49,43 @@ export default function ({ getService, getPageObjects }: FtrProviderContext) {
       expect(await getTitles()).to.be('@timestamp Document');
     });
 
+    const isVisible = async (selector: string) => {
+      const element = await testSubjects.find(selector);
+      const { x, y, width, height } = await element.getPosition();
+      return browser.execute(
+        (innerSelector, innerX, innerY) => {
+          let currentElement = document.elementFromPoint(innerX, innerY);
+          while (currentElement) {
+            if (currentElement.matches(`[data-test-subj="${innerSelector}"]`)) {
+              return true;
+            }
+            currentElement = currentElement.parentElement;
+          }
+          return false;
+        },
+        selector,
+        x + width / 2,
+        y + height / 2
+      );
+    };
+
+    it('should hide elements beneath the table when in full screen mode regardless of their z-index', async () => {
+      await retry.try(async () => {
+        expect(await isVisible('unifiedHistogramQueryHits')).to.be(true);
+        expect(await isVisible('unifiedHistogramResizableButton')).to.be(true);
+      });
+      await testSubjects.click('dataGridFullScreenButton');
+      await retry.try(async () => {
+        expect(await isVisible('unifiedHistogramQueryHits')).to.be(false);
+        expect(await isVisible('unifiedHistogramResizableButton')).to.be(false);
+      });
+      await testSubjects.click('dataGridFullScreenButton');
+      await retry.try(async () => {
+        expect(await isVisible('unifiedHistogramQueryHits')).to.be(true);
+        expect(await isVisible('unifiedHistogramResizableButton')).to.be(true);
+      });
+    });
+
     it('should show the the grid toolbar', async () => {
       await testSubjects.existOrFail('dscGridToolbar');
     });