[Discover] [Unified Data Table] Conditionally use overscanRowCount …

…when Document column isn't visible (elastic#192332)

## Summary

Part of elastic#191249 included using the `overscanRowCount` property of EUI
data grid to render some additional data grid rows off screen, which
greatly reduces pop in when scrolling through the grid. While the
performance cost of this low in most cases, it has an impact in certain
situations such as when documents have many fields and the Document
column is visible (e.g. Discover's `many_fields` performance journey).
This is because the Document column can render _many_ DOM elements in
each of its cells, which EUI data grid struggles to handle.

This PR updates Unified Data Table to only use `overscanRowCount` when
the Document column isn't visible, so we'll still benefit from the
scrolling improvements when columns are selected, but won't take the
performance hit for the Document column.

Perf journey run:
https://buildkite.com/elastic/kibana-single-user-performance/builds/14355.

<img width="842" alt="Screenshot 2024-09-09 at 14 07 46"
src="https://github.com/user-attachments/assets/b074fd7a-ef8a-4917-ac5e-0675b0e60b7d">


### 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
- [ ] [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
- [ ] [Flaky Test
Runner](https://ci-stats.kibana.dev/trigger_flaky_test_runner/1) was
used on any tests changed
- [ ] 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)
- [ ] 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))
- [ ] 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.tsx

@@ -1136,7 +1136,9 @@ export const UnifiedDataTable = ({
               trailingControlColumns={trailingControlColumns}
               cellContext={cellContext}
               renderCellPopover={renderCustomPopover}
-              virtualizationOptions={VIRTUALIZATION_OPTIONS}
+              // Don't use row overscan when showing Document column since
+              // rendering so much DOM content in each cell impacts performance
+              virtualizationOptions={defaultColumns ? undefined : VIRTUALIZATION_OPTIONS}
             />
           )}
         </div>