Enable horizontal scrolling with snap-to-default on mobile topic tree (#1034)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: thomasnordquist <7721625+thomasnordquist@users.noreply.github.com>

app/src/components/Tree/index.tsx

@@ -13,6 +13,9 @@ const MovingAverage = require('moving-average')
 const averagingTimeInterval = 10 * 1000
 const average = MovingAverage(averagingTimeInterval)
 
+// Mobile viewport breakpoint - matches CSS media queries in ContentView
+const MOBILE_BREAKPOINT = 768
+
 declare var window: any
 
 interface Props {
@@ -26,6 +29,7 @@ interface Props {
 
 interface State {
   lastUpdate: number
+  isMobile: boolean
 }
 
 function useArrowKeyEventHandler(actions: typeof treeActions) {
@@ -53,12 +57,16 @@ function useArrowKeyEventHandler(actions: typeof treeActions) {
 
 class TreeComponent extends React.PureComponent<Props, State> {
   private updateTimer?: any
+  private resizeTimer?: any
   private perf: number = 0
   private renderTime = 0
 
   constructor(props: any) {
     super(props)
-    this.state = { lastUpdate: 0 }
+    this.state = { 
+      lastUpdate: 0,
+      isMobile: typeof window !== 'undefined' && window.innerWidth <= MOBILE_BREAKPOINT,
+    }
   }
 
   private keyEventHandler = useArrowKeyEventHandler(this.props.actions)
@@ -66,6 +74,27 @@ class TreeComponent extends React.PureComponent<Props, State> {
     average.push(Date.now(), ms)
   }
 
+  private handleResize = () => {
+    // Debounce resize events - only update after user stops resizing
+    if (this.resizeTimer) {
+      clearTimeout(this.resizeTimer)
+    }
+
+    this.resizeTimer = setTimeout(() => {
+      const isMobile = typeof window !== 'undefined' && window.innerWidth <= MOBILE_BREAKPOINT
+      if (this.state.isMobile !== isMobile) {
+        this.setState({ isMobile })
+      }
+      this.resizeTimer = undefined
+    }, 150) // Wait 150ms after last resize event
+  }
+
+  public componentDidMount() {
+    if (typeof window !== 'undefined') {
+      window.addEventListener('resize', this.handleResize)
+    }
+  }
+
   public componentWillReceiveProps(nextProps: Props) {
     if (this.props.tree !== nextProps.tree) {
       if (this.props.tree) {
@@ -80,6 +109,18 @@ class TreeComponent extends React.PureComponent<Props, State> {
 
   public componentWillUnmount() {
     this.props.tree && this.props.tree.didUpdate.unsubscribe(this.throttledTreeUpdate)
+    if (typeof window !== 'undefined') {
+      window.removeEventListener('resize', this.handleResize)
+    }
+    // Clean up any pending timers to prevent memory leaks
+    if (this.resizeTimer) {
+      clearTimeout(this.resizeTimer)
+      this.resizeTimer = undefined
+    }
+    if (this.updateTimer) {
+      clearTimeout(this.updateTimer)
+      this.updateTimer = undefined
+    }
   }
 
   public throttledTreeUpdate = () => {
@@ -127,19 +168,25 @@ class TreeComponent extends React.PureComponent<Props, State> {
       return null
     }
 
+    const { isMobile } = this.state
+
     const style: React.CSSProperties = {
       lineHeight: '1.1',
       cursor: 'default',
       overflowY: 'scroll',
-      overflowX: 'hidden',
+      overflowX: isMobile ? 'auto' : 'hidden', // Enable horizontal scrolling on mobile
       height: '100%',
       width: '100%',
       outline: '24px black !important',
       paddingBottom: '16px', // avoid conflict with chart panel Resizer
+      // Scroll snap to default position on mobile
+      ...(isMobile && {
+        scrollSnapType: 'x mandatory',
+        WebkitOverflowScrolling: 'touch', // Smooth scrolling on iOS
+      }),
     }
 
-    return (
-      <div style={style} tabIndex={0} onKeyDown={this.keyEventHandler}>
+    const treeNode = (
       <TreeNode
         key={tree.hash()}
         isRoot={true}
@@ -151,6 +198,17 @@ class TreeComponent extends React.PureComponent<Props, State> {
         actions={this.props.actions}
         selectTopicAction={this.props.actions.selectTopic}
       />
+    )
+
+    return (
+      <div style={style} tabIndex={0} onKeyDown={this.keyEventHandler}>
+        {isMobile ? (
+          <div style={{ scrollSnapAlign: 'start', minWidth: '100%' }}>
+            {treeNode}
+          </div>
+        ) : (
+          treeNode
+        )}
       </div>
     )
   }