From 0ffbb3cdd0f10acc44cde5c9d70173c9e9b2a67b Mon Sep 17 00:00:00 2001
From: illionillion <yuto.0526.sano@gmail.com>
Date: Tue, 10 Mar 2026 00:29:05 +0900
Subject: [PATCH] feat(side-menu): animate nested menu expand/collapse

---
 src/components/SideMenu/MenuItems.tsx         | 97 ++++++++++++++++++-
 src/components/SideMenu/menuItemsA11y.ts      |  8 ++
 src/components/SideMenu/styled.elements.ts    | 16 ++-
 .../__tests__/menuItemsA11y.test.ts           | 17 ++++
 4 files changed, 133 insertions(+), 5 deletions(-)
 create mode 100644 src/components/SideMenu/menuItemsA11y.ts
 create mode 100644 src/components/__tests__/menuItemsA11y.test.ts

diff --git a/src/components/SideMenu/MenuItems.tsx b/src/components/SideMenu/MenuItems.tsx
index af9ab197a2..57c9cd9996 100644
--- a/src/components/SideMenu/MenuItems.tsx
+++ b/src/components/SideMenu/MenuItems.tsx
@@ -3,6 +3,7 @@ import * as React from 'react';
 
 import type { IMenuItem } from '../../services';
 
+import { getMenuItemsA11yProps } from './menuItemsA11y';
 import { MenuItem } from './MenuItem';
 import { MenuItemUl } from './styled.elements';
 
@@ -16,17 +17,107 @@ export interface MenuItemsProps {
   className?: string;
 }
 
+interface MenuItemsState {
+  animatedMaxHeight?: string;
+}
+
 @observer
-export class MenuItems extends React.Component<MenuItemsProps> {
+export class MenuItems extends React.Component<MenuItemsProps, MenuItemsState> {
+  private listRef = React.createRef<HTMLUListElement>();
+  private animationFrameId?: number;
+
+  constructor(props: MenuItemsProps) {
+    super(props);
+    const isRoot = !!props.root;
+    const expanded = props.expanded == null ? true : props.expanded;
+
+    this.state = {
+      animatedMaxHeight: isRoot ? undefined : expanded ? 'none' : '0px',
+    };
+  }
+
+  componentDidUpdate(prevProps: MenuItemsProps) {
+    if (this.props.root) {
+      return;
+    }
+
+    const expanded = this.props.expanded == null ? true : this.props.expanded;
+    const prevExpanded = prevProps.expanded == null ? true : prevProps.expanded;
+
+    if (expanded === prevExpanded) {
+      return;
+    }
+
+    if (expanded) {
+      this.expandWithAnimation();
+      return;
+    }
+
+    this.collapseWithAnimation();
+  }
+
+  componentWillUnmount() {
+    if (this.animationFrameId !== undefined) {
+      cancelAnimationFrame(this.animationFrameId);
+    }
+  }
+
+  private expandWithAnimation() {
+    const list = this.listRef.current;
+    if (!list) {
+      return;
+    }
+
+    const expandedHeight = `${list.scrollHeight}px`;
+    this.setState({ animatedMaxHeight: expandedHeight });
+  }
+
+  private collapseWithAnimation() {
+    const list = this.listRef.current;
+    if (!list) {
+      return;
+    }
+
+    if (this.animationFrameId !== undefined) {
+      cancelAnimationFrame(this.animationFrameId);
+    }
+
+    const expandedHeight = `${list.scrollHeight}px`;
+    this.setState({ animatedMaxHeight: expandedHeight }, () => {
+      this.animationFrameId = requestAnimationFrame(() => {
+        this.setState({ animatedMaxHeight: '0px' });
+      });
+    });
+  }
+
+  private onTransitionEnd = (event: React.TransitionEvent<HTMLUListElement>) => {
+    if (event.target !== event.currentTarget || event.propertyName !== 'max-height') {
+      return;
+    }
+
+    const expanded = this.props.expanded == null ? true : this.props.expanded;
+    if (expanded && this.state.animatedMaxHeight !== 'none') {
+      this.setState({ animatedMaxHeight: 'none' });
+    }
+  };
+
   render() {
     const { items, root, className } = this.props;
     const expanded = this.props.expanded == null ? true : this.props.expanded;
+    const isRoot = !!root;
+    const style = isRoot
+      ? this.props.style
+      : { ...this.props.style, maxHeight: this.state.animatedMaxHeight };
+
     return (
       <MenuItemUl
+        ref={this.listRef}
         className={className}
-        style={this.props.style}
+        style={style}
         $expanded={expanded}
-        {...(root ? { role: 'menu' } : {})}
+        $root={isRoot}
+        onTransitionEnd={isRoot ? undefined : this.onTransitionEnd}
+        {...getMenuItemsA11yProps(isRoot, expanded)}
       >
         {items.map((item, idx) => (
           <MenuItem key={idx} item={item} onActivate={this.props.onActivate} />
diff --git a/src/components/SideMenu/menuItemsA11y.ts b/src/components/SideMenu/menuItemsA11y.ts
new file mode 100644
index 0000000000..921498cd8f
--- /dev/null
+++ b/src/components/SideMenu/menuItemsA11y.ts
@@ -0,0 +1,8 @@
+import * as React from 'react';
+
+export function getMenuItemsA11yProps(
+  isRoot: boolean,
+  expanded: boolean,
+): React.HTMLAttributes<HTMLUListElement> {
+  return isRoot ? { role: 'menu' } : { 'aria-hidden': !expanded };
+}
diff --git a/src/components/SideMenu/styled.elements.ts b/src/components/SideMenu/styled.elements.ts
index 33de76ab3b..b797e4fe81 100644
--- a/src/components/SideMenu/styled.elements.ts
+++ b/src/components/SideMenu/styled.elements.ts
@@ -84,7 +84,7 @@ function menuItemActive(
   }
 }
 
-export const MenuItemUl = styled.ul<{ $expanded: boolean }>`
+export const MenuItemUl = styled.ul<{ $expanded: boolean; $root?: boolean }>`
   margin: 0;
   padding: 0;
 
@@ -96,7 +96,19 @@ export const MenuItemUl = styled.ul<{ $expanded: boolean }>`
     font-size: 0.929em;
   }
 
-  ${props => (props.$expanded ? '' : 'display: none;')};
+  ${props =>
+    props.$root
+      ? ''
+      : css`
+          overflow: hidden;
+          max-height: 0;
+          pointer-events: ${props.$expanded ? 'auto' : 'none'};
+          transition: max-height 0.2s ease-out;
+
+          @media (prefers-reduced-motion: reduce) {
+            transition: none;
+          }
+        `};
 `;
 
 export const MenuItemLi = styled.li<{ depth: number }>`
diff --git a/src/components/__tests__/menuItemsA11y.test.ts b/src/components/__tests__/menuItemsA11y.test.ts
new file mode 100644
index 0000000000..45a34b7a45
--- /dev/null
+++ b/src/components/__tests__/menuItemsA11y.test.ts
@@ -0,0 +1,17 @@
+/* tslint:disable:no-implicit-dependencies */
+
+import { getMenuItemsA11yProps } from '../SideMenu/menuItemsA11y';
+
+describe('getMenuItemsA11yProps', () => {
+  it('returns menu role for root list', () => {
+    expect(getMenuItemsA11yProps(true, false)).toEqual({ role: 'menu' });
+  });
+
+  it('hides collapsed nested list from assistive tech', () => {
+    expect(getMenuItemsA11yProps(false, false)).toEqual({ 'aria-hidden': true });
+  });
+
+  it('keeps expanded nested list visible to assistive tech', () => {
+    expect(getMenuItemsA11yProps(false, true)).toEqual({ 'aria-hidden': false });
+  });
+});