fix: keep a deleted session's data in its components until they unmount (#12062)

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/lib/components/sessions/SessionWrapper.svelte

@@ -106,8 +106,10 @@
 		setContext<AIChatManager>('aiChatManager', runtime.manager)
 	}
 
-	// Reactive session reference (mutations to summary/target propagate via the $state proxy)
-	const session = $derived(sessionState.sessions.find((s) => s.id === sessionId))
+	// Reactive session reference (mutations to summary/target propagate via the $state proxy).
+	// Falls back to the mounted session once it is deleted: the layout's header reads this
+	// through PageHeaderContent, outside the page's tree, once more before the wrapper unmounts.
+	const session = $derived(sessionState.sessions.find((s) => s.id === sessionId) ?? initialSession)
 
 	// Only a session that has never sent opens as the starter block. Read off the record, not
 	// the transcript: a cold session's history is still loading from storage when it mounts,

frontend/src/routes/(root)/(logged)/sessions/+page.svelte

@@ -94,11 +94,7 @@
 		tabsToReload,
 		viewerTabsToReload
 	} from '$lib/components/sessions/previewReload'
-	import {
-		isPreviewListPath,
-		barePath,
-		agentPagePath
-	} from '$lib/components/sessions/previewPaths'
+	import { isPreviewListPath, barePath, agentPagePath } from '$lib/components/sessions/previewPaths'
 	import { listWorkspaceAgents } from '$lib/components/sessions/workspaceAgents'
 	import {
 		leafKeyFor,
@@ -268,6 +264,15 @@
 			.map((r) => sessionState.sessions.find((s) => s.id === r.sessionId))
 			.filter((s): s is NonNullable<typeof s> => s != null)
 	)
+	// The preview hosts take their runtime from the item rather than a lookup by id: a
+	// removed session's hosts are torn down after its runtime has left the map, and their
+	// teardowns (an editor unregistering from the runtime) still read the prop.
+	const warmHosts = $derived(
+		listRuntimes().flatMap((runtime) => {
+			const session = sessionState.sessions.find((s) => s.id === runtime.sessionId)
+			return session ? [{ session, runtime }] : []
+		})
+	)
 
 	// Chat columns stay mounted (stacked, visibility-toggled) so switching back
 	// keeps scroll position, MRU-capped like preview tabs: every mounted column
@@ -1272,9 +1277,8 @@
 								<div class="relative flex-1 min-h-0">
 									{#if previewTabHostComponent.current}
 										{@const TabHost = previewTabHostComponent.current}
-										{#each warmSessions as s (s.id)}
-											{@const rt = getRuntime(s.id)}
-											{@const tabs = rt?.previewTabs}
+										{#each warmHosts as { session: s, runtime: rt } (s.id)}
+											{@const tabs = rt.previewTabs}
 											{#each tabs?.tabs ?? [] as tab (tab.id)}
 												<!-- tabHosts is an imperative ref-bag (only tabHosts[key]?.reload() in
 										     reloadTabs); it is intentionally a plain object so component