[Security Solution] Exp flyout expandable widget rendering (#160625)

## Summary

This tiny PR ensures that widget contents are not rendered until
expanded (in the expandable flyout). This will
prevent unnecessary requests being sent when we open the flyout.

Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>

packages/kbn-expandable-flyout/src/index.tsx

@@ -27,6 +27,12 @@ export interface ExpandableFlyoutProps extends EuiFlyoutProps {
   handleOnFlyoutClosed?: () => void;
 }
 
+const flyoutStyles = css`
+  overflow-y: scroll;
+`;
+
+const flyoutInnerStyles = { height: '100%' };
+
 /**
  * Expandable flyout UI React component.
  * Displays 3 sections (right, left, preview) depending on the panels in the context.
@@ -65,9 +71,10 @@ export const ExpandableFlyout: React.FC<ExpandableFlyoutProps> = ({
     [mostRecentPreview, registeredPanels]
   );
 
-  // do not add the flyout to the dom if there aren't any panels to display
-  if (!left && !right && !preview.length) {
-    return <></>;
+  const hideFlyout = !left && !right && !preview.length;
+
+  if (hideFlyout) {
+    return null;
   }
 
   const flyoutWidth: string = leftSection && rightSection ? 'l' : 's';
@@ -77,9 +84,7 @@ export const ExpandableFlyout: React.FC<ExpandableFlyoutProps> = ({
 
   return (
     <EuiFlyout
-      css={css`
-        overflow-y: scroll;
-      `}
+      css={flyoutStyles}
       {...flyoutProps}
       size={flyoutWidth}
       ownFocus={false}
@@ -89,7 +94,7 @@ export const ExpandableFlyout: React.FC<ExpandableFlyoutProps> = ({
         direction={leftSection ? 'row' : 'column'}
         wrap={false}
         gutterSize="none"
-        style={{ height: '100%' }}
+        style={flyoutInnerStyles}
       >
         {leftSection && left ? (
           <LeftSection

x-pack/plugins/security_solution/cypress/e2e/investigations/alerts/expandable_flyout/alert_details_left_panel_correlations_tab.cy.ts

@@ -46,6 +46,7 @@ describe(
       waitForAlertsToPopulate();
       expandFirstAlertExpandableFlyout();
       expandDocumentDetailsExpandableFlyoutLeftSection();
+      createNewCaseFromExpandableFlyout();
       openInsightsTab();
       openCorrelationsTab();
     });
@@ -53,8 +54,6 @@ describe(
     it('should render correlations details correctly', () => {
       cy.log('link the alert to a new case');
 
-      createNewCaseFromExpandableFlyout();
-
       cy.get(DOCUMENT_DETAILS_FLYOUT_INSIGHTS_TAB).scrollIntoView();
 
       cy.log('should render the Insights header');

x-pack/plugins/security_solution/public/flyout/right/components/expandable_section.tsx

@@ -6,8 +6,8 @@
  */
 
 import { EuiAccordion, EuiFlexGroup, EuiSpacer, EuiTitle, useGeneratedHtmlId } from '@elastic/eui';
-import type { VFC } from 'react';
-import React from 'react';
+import React, { type VFC } from 'react';
+import { useAccordionState } from '../hooks/use_accordion_state';
 
 export const HEADER_TEST_ID = 'Header';
 export const CONTENT_TEST_ID = 'Content';
@@ -46,6 +46,8 @@ export const ExpandableSection: VFC<DescriptionSectionProps> = ({
 }) => {
   const accordionId = useGeneratedHtmlId({ prefix: 'accordion' });
 
+  const { renderContent, toggle, state } = useAccordionState(expanded);
+
   const headerDataTestSub = dataTestSub + HEADER_TEST_ID;
   const contentDataTestSub = dataTestSub + CONTENT_TEST_ID;
 
@@ -56,10 +58,10 @@ export const ExpandableSection: VFC<DescriptionSectionProps> = ({
   );
 
   return (
-    <EuiAccordion id={accordionId} buttonContent={header} initialIsOpen={expanded}>
+    <EuiAccordion forceState={state} onToggle={toggle} id={accordionId} buttonContent={header}>
       <EuiSpacer size="m" />
       <EuiFlexGroup gutterSize="none" direction="column" data-test-subj={contentDataTestSub}>
-        {children}
+        {renderContent && children}
       </EuiFlexGroup>
     </EuiAccordion>
   );

x-pack/plugins/security_solution/public/flyout/right/hooks/use_accordion_state.ts

@@ -0,0 +1,48 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+
+import { useReducer } from 'react';
+
+const CLOSED = 'closed' as const;
+const OPEN = 'open' as const;
+
+type ToggleReducerState = typeof CLOSED | typeof OPEN;
+const toggleReducer = (state: ToggleReducerState) => {
+  return state === CLOSED ? OPEN : CLOSED;
+};
+
+export interface UseAccordionStateValue {
+  /**
+   * Should children be rendered in the dom
+   */
+  renderContent: boolean;
+  /**
+   * Use this to control the accordion visual state
+   */
+  state: typeof CLOSED | typeof OPEN;
+
+  /**
+   * Handler function for cycling between the states
+   */
+  toggle: VoidFunction;
+}
+
+/**
+ * Tiny hook for controlled useAccordionState
+ * @param expandedInitially - is accordion expanded on first render
+ */
+export const useAccordionState = (expandedInitially: boolean): UseAccordionStateValue => {
+  const initialState = expandedInitially ? OPEN : CLOSED;
+  const [state, toggle] = useReducer(toggleReducer, initialState);
+  const renderContent = state === OPEN;
+
+  return {
+    renderContent,
+    state,
+    toggle,
+  };
+};