diff --git a/packages/ui/src/App.tsx b/packages/ui/src/App.tsx
index 62caf483..cee2ab06 100644
--- a/packages/ui/src/App.tsx
+++ b/packages/ui/src/App.tsx
@@ -496,17 +496,24 @@ const App: Component = () => {
                   const isActiveInstance = () => activeInstanceId() === instance.id
                   const isVisible = () => isActiveInstance() && !showFolderSelection()
                     return (
-                      <div class="flex-1 min-h-0 overflow-hidden" style={{ display: isVisible() ? "flex" : "none" }}>
-                        <InstanceMetadataProvider instance={instance}>
-                          <InstanceShell
-                            instance={instance}
-                            escapeInDebounce={escapeInDebounce()}
-                            paletteCommands={paletteCommands}
-                            onCloseSession={(sessionId) => handleCloseSession(instance.id, sessionId)}
-                            onNewSession={() => handleNewSession(instance.id)}
-                            handleSidebarAgentChange={(sessionId, agent) => handleSidebarAgentChange(instance.id, sessionId, agent)}
-                            handleSidebarModelChange={(sessionId, model) => handleSidebarModelChange(instance.id, sessionId, model)}
-                            onExecuteCommand={executeCommand}
+                       <div
+                         class="flex-1 min-h-0 overflow-hidden"
+                         style={{ display: isVisible() ? "flex" : "none" }}
+                         data-instance-id={instance.id}
+                         data-instance-active={isActiveInstance() ? "true" : "false"}
+                         data-instance-visible={isVisible() ? "true" : "false"}
+                       >
+                         <InstanceMetadataProvider instance={instance}>
+                           <InstanceShell
+                             instance={instance}
+                             isActiveInstance={isActiveInstance()}
+                             escapeInDebounce={escapeInDebounce()}
+                             paletteCommands={paletteCommands}
+                             onCloseSession={(sessionId) => handleCloseSession(instance.id, sessionId)}
+                             onNewSession={() => handleNewSession(instance.id)}
+                             handleSidebarAgentChange={(sessionId, agent) => handleSidebarAgentChange(instance.id, sessionId, agent)}
+                             handleSidebarModelChange={(sessionId, model) => handleSidebarModelChange(instance.id, sessionId, model)}
+                             onExecuteCommand={executeCommand}
                             tabBarOffset={isPhoneLayout() && mobileFullscreenMode() ? 0 : instanceTabBarHeight()}
                             mobileFullscreenMode={isPhoneLayout() && mobileFullscreenMode()}
                             onEnterMobileFullscreen={() => void enterMobileFullscreen()}
diff --git a/packages/ui/src/components/instance/instance-shell2.tsx b/packages/ui/src/components/instance/instance-shell2.tsx
index 464a2acb..84e55578 100644
--- a/packages/ui/src/components/instance/instance-shell2.tsx
+++ b/packages/ui/src/components/instance/instance-shell2.tsx
@@ -62,6 +62,9 @@ const log = getLogger("session")
 
 interface InstanceShellProps {
   instance: Instance
+  // Provided by App-level instance tabs; lets us pause heavy rendering
+  // work for inactive instances while keeping them mounted for fast switching.
+  isActiveInstance?: boolean
   escapeInDebounce: boolean
   paletteCommands: Accessor<Command[]>
   onCloseSession: (sessionId: string) => Promise<void> | void
@@ -800,12 +803,14 @@ const InstanceShell2: Component<InstanceShellProps> = (props) => {
               >
                 <For each={cachedSessionIds()}>
                   {(sessionId) => {
-                    const isActive = () => activeSessionIdForInstance() === sessionId
+                    const isActive = () => Boolean(props.isActiveInstance) && activeSessionIdForInstance() === sessionId
                     return (
                       <div
                         class="session-cache-pane flex flex-col flex-1 min-h-0"
                         style={{ display: isActive() ? "flex" : "none" }}
                         data-session-id={sessionId}
+                        data-instance-id={props.instance.id}
+                        data-session-active={isActive() ? "true" : "false"}
                         aria-hidden={!isActive()}
                       >
                         <SessionView
@@ -841,7 +846,10 @@ const InstanceShell2: Component<InstanceShellProps> = (props) => {
 
   return (
     <>
-      <div class="instance-shell2 flex flex-col flex-1 min-h-0">
+      <div
+        class="instance-shell2 flex flex-col flex-1 min-h-0"
+        data-instance-id={props.instance.id}
+      >
         <Show when={hasSessions()} fallback={<InstanceWelcomeView instance={props.instance} />}>
           {sessionLayout}
         </Show>
diff --git a/packages/ui/src/components/markdown.tsx b/packages/ui/src/components/markdown.tsx
index 39638e1b..9e4d170a 100644
--- a/packages/ui/src/components/markdown.tsx
+++ b/packages/ui/src/components/markdown.tsx
@@ -202,5 +202,15 @@ export function Markdown(props: MarkdownProps) {
 
   const proseClass = () => "markdown-body"
 
-  return <div ref={containerRef} class={proseClass()} innerHTML={html()} />
+  return (
+    <div
+      ref={containerRef}
+      class={proseClass()}
+      data-view="markdown"
+      data-part-id={resolved().partId}
+      data-markdown-theme={resolved().themeKey}
+      data-markdown-highlight={resolved().highlightEnabled ? "true" : "false"}
+      innerHTML={html()}
+    />
+  )
 }
diff --git a/packages/ui/src/components/message-block-list.tsx b/packages/ui/src/components/message-block-list.tsx
index a0e89134..de5b8890 100644
--- a/packages/ui/src/components/message-block-list.tsx
+++ b/packages/ui/src/components/message-block-list.tsx
@@ -8,7 +8,7 @@ export function getMessageAnchorId(messageId: string) {
   return `message-anchor-${messageId}`
 }
 
-const VIRTUAL_ITEM_MARGIN_PX = 300
+const VIRTUAL_ITEM_MARGIN_PX = 800
 
 interface MessageBlockListProps {
   instanceId: string
diff --git a/packages/ui/src/components/message-item.tsx b/packages/ui/src/components/message-item.tsx
index a6df84b8..98e754dd 100644
--- a/packages/ui/src/components/message-item.tsx
+++ b/packages/ui/src/components/message-item.tsx
@@ -381,7 +381,15 @@ export default function MessageItem(props: MessageItemProps) {
 
 
   return (
-    <div class={containerClass()}>
+    <div
+      class={containerClass()}
+      data-view="message-item"
+      data-instance-id={props.instanceId}
+      data-session-id={props.sessionId}
+      data-message-id={props.record.id}
+      data-message-role={isUser() ? "user" : "assistant"}
+      data-message-status={props.record.status}
+    >
       <header class={`message-item-header ${isUser() ? "pb-0.5" : "pb-0"}`}>
         <div class="message-item-header-row message-item-header-row--top" ref={(el) => (topRowEl = el)}>
           <div class="message-header-left">
diff --git a/packages/ui/src/components/message-part.tsx b/packages/ui/src/components/message-part.tsx
index 7178eb93..038b3dfb 100644
--- a/packages/ui/src/components/message-part.tsx
+++ b/packages/ui/src/components/message-part.tsx
@@ -131,7 +131,12 @@ export default function MessagePart(props: MessagePartProps) {
     <Switch>
       <Match when={partType() === "text"}>
         <Show when={!shouldHideTextPart() && partHasRenderableText(props.part)}>
-          <div class={canRenderMarkdown() ? markdownContainerClass() : textContainerClass()} data-role={textContainerRole()}>
+          <div
+            class={canRenderMarkdown() ? markdownContainerClass() : textContainerClass()}
+            data-role={textContainerRole()}
+            data-part-type="text"
+            data-part-id={typeof (props.part as any)?.id === "string" ? (props.part as any).id : undefined}
+          >
             <Show when={canRenderMarkdown()} fallback={<span class="text-primary">{plainTextContent()}</span>}>
               <Markdown
                 part={createTextPartForMarkdown()}
diff --git a/packages/ui/src/components/message-section.tsx b/packages/ui/src/components/message-section.tsx
index 8ebff4f1..c310c1f5 100644
--- a/packages/ui/src/components/message-section.tsx
+++ b/packages/ui/src/components/message-section.tsx
@@ -913,13 +913,30 @@ export default function MessageSection(props: MessageSectionProps) {
   })
 
   return (
-    <div class="message-stream-container">
+    <div
+      class="message-stream-container"
+      data-instance-id={props.instanceId}
+      data-session-id={props.sessionId}
+      data-stream-active={isActive() ? "true" : "false"}
+    >
       <div
         class={`message-layout${hasTimelineSegments() ? " message-layout--with-timeline" : ""}`}
         data-scroll-buttons={scrollButtonsCount()}
       >
-        <div class="message-stream-shell" ref={setShellElement}>
-          <div class="message-stream" ref={setContainerRef} onScroll={handleScroll} onMouseUp={handleStreamMouseUp}>
+        <div
+          class="message-stream-shell"
+          ref={setShellElement}
+          data-instance-id={props.instanceId}
+          data-session-id={props.sessionId}
+        >
+          <div
+            class="message-stream"
+            ref={setContainerRef}
+            onScroll={handleScroll}
+            onMouseUp={handleStreamMouseUp}
+            data-instance-id={props.instanceId}
+            data-session-id={props.sessionId}
+          >
             <div ref={setTopSentinel} aria-hidden="true" style={{ height: "1px" }} />
             <Show when={!props.loading && messageIds().length === 0}>
               <div class="empty-state">
diff --git a/packages/ui/src/components/message-timeline.tsx b/packages/ui/src/components/message-timeline.tsx
index 61d42793..20e5328f 100644
--- a/packages/ui/src/components/message-timeline.tsx
+++ b/packages/ui/src/components/message-timeline.tsx
@@ -411,7 +411,14 @@ const MessageTimeline: Component<MessageTimelineProps> = (props) => {
   })
  
   return (
-    <div class="message-timeline" role="navigation" aria-label={t("messageTimeline.ariaLabel")}>
+    <div
+      class="message-timeline"
+      role="navigation"
+      aria-label={t("messageTimeline.ariaLabel")}
+      data-view="timeline"
+      data-instance-id={props.instanceId}
+      data-session-id={props.sessionId}
+    >
       <For each={props.segments}>
         {(segment) => {
           onCleanup(() => buttonRefs.delete(segment.id))
diff --git a/packages/ui/src/components/tool-call.tsx b/packages/ui/src/components/tool-call.tsx
index a1125e73..e855ee71 100644
--- a/packages/ui/src/components/tool-call.tsx
+++ b/packages/ui/src/components/tool-call.tsx
@@ -976,6 +976,12 @@ export default function ToolCall(props: ToolCallProps) {
         setToolCallRootEl(element || undefined)
       }}
       class={`tool-call ${combinedStatusClass()}`}
+      data-part-type="tool"
+      data-tool-name={toolName()}
+      data-instance-id={props.instanceId}
+      data-session-id={props.sessionId}
+      data-message-id={props.messageId}
+      data-part-id={toolCallIdentifier()}
     >
       <div class="tool-call-header">
         <button
diff --git a/packages/ui/src/components/virtual-item.tsx b/packages/ui/src/components/virtual-item.tsx
index 27c65d47..52e3fba5 100644
--- a/packages/ui/src/components/virtual-item.tsx
+++ b/packages/ui/src/components/virtual-item.tsx
@@ -62,6 +62,42 @@ function shouldRenderEntry(entry: IntersectionObserverEntry) {
   return true
 }
 
+function getViewportRect(): { top: number; bottom: number } {
+  if (typeof window === "undefined") {
+    return { top: 0, bottom: 0 }
+  }
+  return { top: 0, bottom: window.innerHeight }
+}
+
+function isRenderableRoot(root: ObserverRoot): boolean {
+  if (!root) return true
+  if (root instanceof Document) return true
+  if (typeof window === "undefined") return false
+
+  const element = root as Element
+  const style = window.getComputedStyle(element as Element)
+  if (style.display === "none" || style.visibility === "hidden") {
+    return false
+  }
+  const rect = (element as Element).getBoundingClientRect()
+  return rect.width > 0 && rect.height > 0
+}
+
+function shouldRenderByRects(params: {
+  wrapperRect: DOMRect
+  rootRect: { top: number; bottom: number }
+  margin: number
+}): boolean {
+  const { wrapperRect, rootRect, margin } = params
+  const distanceAbove = rootRect.top - wrapperRect.bottom
+  const distanceBelow = wrapperRect.top - rootRect.bottom
+  const threshold = margin + VISIBILITY_BUFFER_PX
+  if (distanceAbove > threshold || distanceBelow > threshold) {
+    return false
+  }
+  return true
+}
+
 function subscribeToSharedObserver(
   target: Element,
   root: ObserverRoot,
@@ -120,7 +156,10 @@ interface VirtualItemProps {
 export default function VirtualItem(props: VirtualItemProps) {
   const resolved = resolveChildren(() => props.children)
   const cachedHeight = sizeCache.get(props.cacheKey)
-  const [isIntersecting, setIsIntersecting] = createSignal(true)
+  // Default to hidden until we can determine visibility.
+  // This avoids keeping heavy DOM alive when IntersectionObserver
+  // doesn't fire (common for hidden/zero-sized scroll roots).
+  const [isIntersecting, setIsIntersecting] = createSignal(false)
   const [measuredHeight, setMeasuredHeight] = createSignal(cachedHeight ?? 0)
   const [hasMeasured, setHasMeasured] = createSignal(cachedHeight !== undefined)
   let hasReportedMeasurement = Boolean(cachedHeight && cachedHeight > 0)
@@ -148,12 +187,12 @@ export default function VirtualItem(props: VirtualItemProps) {
     })
   }
   const virtualizationEnabled = () => (props.virtualizationEnabled ? props.virtualizationEnabled() : true)
+  const measurementsSuspended = () => Boolean(props.suspendMeasurements?.())
   const shouldHideContent = createMemo(() => {
     if (props.forceVisible?.()) return false
     if (!virtualizationEnabled()) return false
     return !isIntersecting()
   })
-  const measurementsSuspended = () => Boolean(props.suspendMeasurements?.())
  
    let wrapperRef: HTMLDivElement | undefined
  
@@ -229,14 +268,44 @@ export default function VirtualItem(props: VirtualItemProps) {
   function refreshIntersectionObserver(targetRoot: Element | Document | null) {
     cleanupIntersectionObserver()
     if (!wrapperRef) {
-      setIsIntersecting(true)
+      setIsIntersecting(false)
       return
     }
     if (typeof IntersectionObserver === "undefined") {
       setIsIntersecting(true)
       return
     }
+
     const margin = props.threshold ?? DEFAULT_MARGIN_PX
+
+    // If the scroll root is hidden / 0x0, IntersectionObserver can report
+    // `isIntersecting` in unexpected ways (often "true" with null rootBounds),
+    // which keeps heavy DOM alive in background tabs.
+    //
+    // In that state, force-hide and skip attaching the observer. When the
+    // pane becomes visible again, VirtualItem will re-run this setup and
+    // re-attach the observer.
+    const renderable = isRenderableRoot(targetRoot)
+    if (!renderable) {
+      setIsIntersecting(false)
+      return
+    }
+
+    // Compute an immediate best-effort visibility so switching tabs doesn't
+    // depend on the first IntersectionObserver callback.
+    try {
+      const rootRect =
+        targetRoot && !(targetRoot instanceof Document)
+          ? (targetRoot as Element).getBoundingClientRect()
+          : null
+      const bounds = rootRect ? { top: rootRect.top, bottom: rootRect.bottom } : getViewportRect()
+      setIsIntersecting(
+        shouldRenderByRects({ wrapperRect: wrapperRef.getBoundingClientRect(), rootRect: bounds, margin }),
+      )
+    } catch {
+      // Ignore measurement failures; IntersectionObserver will correct us.
+    }
+
     intersectionCleanup = subscribeToSharedObserver(wrapperRef, targetRoot, margin, (entry) => {
       const nextVisible = shouldRenderEntry(entry)
       queueVisibility(nextVisible)
@@ -340,4 +409,3 @@ export default function VirtualItem(props: VirtualItemProps) {
     </div>
   )
 }
-