feat(v2): add sidecar crash detection, restart UI, and auto-scroll lock

Phase 3 polish: dispatcher listens for sidecar-exited events and marks
running sessions as error. AgentPane shows "Restart Sidecar" button on
error. Auto-scroll disables when user scrolls >50px from bottom with
"Scroll to bottom" button. Added agent_restart Tauri command.

v2/src-tauri/src/sidecar.rs

@@ -159,6 +159,12 @@ impl SidecarManager {
         self.send_message(&msg)
     }
 
+    pub fn restart(&self, app: &AppHandle) -> Result<(), String> {
+        log::info!("Restarting sidecar");
+        let _ = self.shutdown();
+        self.start(app)
+    }
+
     pub fn shutdown(&self) -> Result<(), String> {
         let mut child_lock = self.child.lock().unwrap();
         if let Some(ref mut child) = *child_lock {

v2/src/lib/adapters/agent-bridge.ts

@@ -25,6 +25,10 @@ export async function isAgentReady(): Promise<boolean> {
   return invoke<boolean>('agent_ready');
 }
 
+export async function restartAgent(): Promise<void> {
+  return invoke('agent_restart');
+}
+
 export interface SidecarMessage {
   type: string;
   sessionId?: string;

v2/src/lib/agent-dispatcher.ts

@@ -1,7 +1,7 @@
 // Agent Dispatcher — connects sidecar bridge events to agent store
 // Single listener that routes sidecar messages to the correct agent session
 
-import { onSidecarMessage, type SidecarMessage } from './adapters/agent-bridge';
+import { onSidecarMessage, onSidecarExited, type SidecarMessage } from './adapters/agent-bridge';
 import { adaptSDKMessage } from './adapters/sdk-messages';
 import type { InitContent, CostContent } from './adapters/sdk-messages';
 import {
@@ -10,14 +10,29 @@ import {
   setAgentModel,
   appendAgentMessages,
   updateAgentCost,
+  getAgentSessions,
 } from './stores/agents.svelte';
 
-let unlistenFn: (() => void) | null = null;
+let unlistenMsg: (() => void) | null = null;
+let unlistenExit: (() => void) | null = null;
+
+// Sidecar liveness — checked by UI components
+let sidecarAlive = true;
+export function isSidecarAlive(): boolean {
+  return sidecarAlive;
+}
+export function setSidecarAlive(alive: boolean): void {
+  sidecarAlive = alive;
+}
 
 export async function startAgentDispatcher(): Promise<void> {
-  if (unlistenFn) return;
+  if (unlistenMsg) return;
+
+  sidecarAlive = true;
+
+  unlistenMsg = await onSidecarMessage((msg: SidecarMessage) => {
+    sidecarAlive = true;
 
-  unlistenFn = await onSidecarMessage((msg: SidecarMessage) => {
     const sessionId = msg.sessionId;
     if (!sessionId) return;
 
@@ -39,10 +54,19 @@ export async function startAgentDispatcher(): Promise<void> {
         break;
 
       case 'agent_log':
-        // Debug logging — could route to a log panel later
         break;
     }
   });
+
+  unlistenExit = await onSidecarExited(() => {
+    sidecarAlive = false;
+    // Mark all running sessions as errored
+    for (const session of getAgentSessions()) {
+      if (session.status === 'running' || session.status === 'starting') {
+        updateAgentStatus(session.id, 'error', 'Sidecar crashed');
+      }
+    }
+  });
 }
 
 function handleAgentEvent(sessionId: string, event: Record<string, unknown>): void {
@@ -76,15 +100,18 @@ function handleAgentEvent(sessionId: string, event: Record<string, unknown>): vo
     }
   }
 
-  // Append all messages to the session history
   if (messages.length > 0) {
     appendAgentMessages(sessionId, messages);
   }
 }
 
 export function stopAgentDispatcher(): void {
-  if (unlistenFn) {
-    unlistenFn();
-    unlistenFn = null;
+  if (unlistenMsg) {
+    unlistenMsg();
+    unlistenMsg = null;
+  }
+  if (unlistenExit) {
+    unlistenExit();
+    unlistenExit = null;
   }
 }

v2/src/lib/components/Agent/AgentPane.svelte

@@ -1,12 +1,13 @@
 <script lang="ts">
   import { onMount, onDestroy } from 'svelte';
-  import { queryAgent, stopAgent, isAgentReady } from '../../adapters/agent-bridge';
+  import { queryAgent, stopAgent, isAgentReady, restartAgent } from '../../adapters/agent-bridge';
   import {
     getAgentSession,
     createAgentSession,
     removeAgentSession,
     type AgentSession,
   } from '../../stores/agents.svelte';
+  import { isSidecarAlive, setSidecarAlive } from '../../agent-dispatcher';
   import type {
     AgentMessage,
     TextContent,
@@ -30,6 +31,7 @@
   let inputPrompt = $state(initialPrompt);
   let scrollContainer: HTMLDivElement | undefined = $state();
   let autoScroll = $state(true);
+  let restarting = $state(false);
 
   onMount(async () => {
     if (initialPrompt) {
@@ -73,12 +75,31 @@
     stopAgent(sessionId).catch(() => {});
   }
 
+  async function handleRestart() {
+    restarting = true;
+    try {
+      await restartAgent();
+      setSidecarAlive(true);
+    } catch {
+      // Still dead
+    } finally {
+      restarting = false;
+    }
+  }
+
   function scrollToBottom() {
     if (autoScroll && scrollContainer) {
       scrollContainer.scrollTop = scrollContainer.scrollHeight;
     }
   }
 
+  function handleScroll() {
+    if (!scrollContainer) return;
+    const { scrollTop, scrollHeight, clientHeight } = scrollContainer;
+    // Lock auto-scroll if user scrolled up more than 50px from bottom
+    autoScroll = scrollHeight - scrollTop - clientHeight < 50;
+  }
+
   // Auto-scroll when new messages arrive
   $effect(() => {
     if (session?.messages.length) {
@@ -121,7 +142,7 @@
       </form>
     </div>
   {:else}
-    <div class="messages" bind:this={scrollContainer}>
+    <div class="messages" bind:this={scrollContainer} onscroll={handleScroll}>
       {#each session.messages as msg (msg.id)}
         <div class="message msg-{msg.type}">
           {#if msg.type === 'init'}
@@ -173,6 +194,9 @@
         <div class="running-indicator">
           <span class="pulse"></span>
           <span>Running...</span>
+          {#if !autoScroll}
+            <button class="scroll-btn" onclick={() => { autoScroll = true; scrollToBottom(); }}>Scroll to bottom</button>
+          {/if}
           <button class="stop-btn" onclick={handleStop}>Stop</button>
         </div>
       {:else if session.status === 'done'}
@@ -184,6 +208,11 @@
       {:else if session.status === 'error'}
         <div class="error-bar">
           <span>Error: {session.error ?? 'Unknown'}</span>
+          {#if session.error?.includes('Sidecar') || session.error?.includes('crashed')}
+            <button class="restart-btn" onclick={handleRestart} disabled={restarting}>
+              {restarting ? 'Restarting...' : 'Restart Sidecar'}
+            </button>
+          {/if}
         </div>
       {/if}
     </div>
@@ -407,11 +436,38 @@
 
   .stop-btn:hover { opacity: 0.9; }
 
+  .scroll-btn {
+    background: var(--bg-surface);
+    border: 1px solid var(--border);
+    color: var(--text-secondary);
+    border-radius: 3px;
+    padding: 2px 8px;
+    font-size: 10px;
+    cursor: pointer;
+  }
+
+  .scroll-btn:hover { color: var(--text-primary); }
+
+  .restart-btn {
+    margin-left: auto;
+    background: var(--ctp-peach);
+    color: var(--ctp-crust);
+    border: none;
+    border-radius: 3px;
+    padding: 2px 10px;
+    font-size: 11px;
+    cursor: pointer;
+  }
+
+  .restart-btn:hover { opacity: 0.9; }
+  .restart-btn:disabled { opacity: 0.4; cursor: not-allowed; }
+
   .done-bar, .error-bar {
     display: flex;
     gap: 12px;
     font-size: 11px;
     font-family: var(--font-mono);
+    align-items: center;
   }
 
   .done-bar { color: var(--ctp-green); }