diff --git a/.github/workflows/pr-build.yml b/.github/workflows/pr-build.yml
index a50c3f5e..aa599e28 100644
--- a/.github/workflows/pr-build.yml
+++ b/.github/workflows/pr-build.yml
@@ -104,6 +104,7 @@ jobs:
       - name: Test changed runnable UI behavior
         run: >-
           node --import tsx --test
+          packages/ui/src/components/session-list-visibility.test.ts
           packages/ui/src/lib/hooks/use-app-session-capture.test.ts
           packages/ui/src/lib/trailing-resync.test.ts
           packages/ui/src/stores/abort-created-workspace-cleanup.test.ts
diff --git a/packages/ui/src/components/instance/shell/SessionSidebar.tsx b/packages/ui/src/components/instance/shell/SessionSidebar.tsx
index 38628e80..fa80c1db 100644
--- a/packages/ui/src/components/instance/shell/SessionSidebar.tsx
+++ b/packages/ui/src/components/instance/shell/SessionSidebar.tsx
@@ -18,6 +18,7 @@ import AgentSelector from "../../agent-selector"
 import ModelSelector from "../../model-selector"
 import ThinkingSelector from "../../thinking-selector"
 import { getLogger } from "../../../lib/logger"
+import { shouldMountSessionList } from "../../session-list-visibility"
 
 const log = getLogger("session")
 
@@ -130,21 +131,23 @@ const SessionSidebar: Component<SessionSidebarProps> = (props) => (
       </div>
 
       <div class="session-sidebar flex flex-col flex-1 min-h-0">
-        <SessionList
-          instanceId={props.instanceId}
-          threads={props.threads()}
-          activeSessionId={props.activeSessionId()}
-          onSelect={props.onSelectSession}
-          onNew={() => {
-            const result = props.onNewSession()
-            if (result instanceof Promise) {
-              void result.catch((error) => log.error("Failed to create session:", error))
-            }
-          }}
-          enableFilterBar={props.showSearch()}
-          showHeader={false}
-          showFooter={false}
-        />
+        <Show when={shouldMountSessionList(props.drawerState())}>
+          <SessionList
+            instanceId={props.instanceId}
+            threads={props.threads()}
+            activeSessionId={props.activeSessionId()}
+            onSelect={props.onSelectSession}
+            onNew={() => {
+              const result = props.onNewSession()
+              if (result instanceof Promise) {
+                void result.catch((error) => log.error("Failed to create session:", error))
+              }
+            }}
+            enableFilterBar={props.showSearch()}
+            showHeader={false}
+            showFooter={false}
+          />
+        </Show>
 
         <div class="session-sidebar-separator" />
         <Show
diff --git a/packages/ui/src/components/session-list-visibility.test.ts b/packages/ui/src/components/session-list-visibility.test.ts
new file mode 100644
index 00000000..b863c651
--- /dev/null
+++ b/packages/ui/src/components/session-list-visibility.test.ts
@@ -0,0 +1,29 @@
+import assert from "node:assert/strict"
+import { describe, it } from "node:test"
+
+import { isSessionListViewportAttached, shouldMountSessionList, shouldRenderSessionRows } from "./session-list-visibility"
+
+describe("session list visibility", () => {
+  it("does not mount inside a closed floating drawer", () => {
+    assert.equal(shouldMountSessionList("floating-closed"), false)
+    assert.equal(shouldMountSessionList("floating-open"), true)
+    assert.equal(shouldMountSessionList("pinned"), true)
+  })
+
+  it("keeps the error state exclusive from session rows", () => {
+    assert.equal(shouldRenderSessionRows(true, true), false)
+    assert.equal(shouldRenderSessionRows(false, true), true)
+    assert.equal(shouldRenderSessionRows(false, false), false)
+  })
+
+  it("waits for the drawer viewport to enter a live window", () => {
+    const viewport = (isConnected: boolean, defaultView: unknown) => ({
+      isConnected,
+      ownerDocument: { defaultView },
+    }) as Pick<HTMLElement, "isConnected" | "ownerDocument">
+
+    assert.equal(isSessionListViewportAttached(viewport(true, null)), false)
+    assert.equal(isSessionListViewportAttached(viewport(false, {})), false)
+    assert.equal(isSessionListViewportAttached(viewport(true, {})), true)
+  })
+})
diff --git a/packages/ui/src/components/session-list-visibility.ts b/packages/ui/src/components/session-list-visibility.ts
new file mode 100644
index 00000000..a7826eaf
--- /dev/null
+++ b/packages/ui/src/components/session-list-visibility.ts
@@ -0,0 +1,15 @@
+import type { DrawerViewState } from "./instance/shell/types"
+
+export function shouldMountSessionList(drawerState: DrawerViewState): boolean {
+  return drawerState !== "floating-closed"
+}
+
+export function isSessionListViewportAttached(
+  viewport: Pick<HTMLElement, "isConnected" | "ownerDocument">,
+): boolean {
+  return viewport.isConnected && Boolean(viewport.ownerDocument.defaultView)
+}
+
+export function shouldRenderSessionRows(hasError: boolean, hasContent: boolean): boolean {
+  return !hasError && hasContent
+}
diff --git a/packages/ui/src/components/session-list.tsx b/packages/ui/src/components/session-list.tsx
index dde63d0a..db154943 100644
--- a/packages/ui/src/components/session-list.tsx
+++ b/packages/ui/src/components/session-list.tsx
@@ -37,6 +37,7 @@ import { collectSessionThreadIds, findSessionThread, flattenVisibleSessionThread
 import { getLogger } from "../lib/logger"
 import { copyToClipboard } from "../lib/clipboard"
 import { useConfig } from "../stores/preferences"
+import { isSessionListViewportAttached, shouldRenderSessionRows } from "./session-list-visibility"
 const log = getLogger("session")
 
 
@@ -71,8 +72,28 @@ const SessionList: Component<SessionListProps> = (props) => {
   const [reloadingSessionIds, setReloadingSessionIds] = createSignal<Set<string>>(new Set())
   const [now, setNow] = createSignal(Date.now())
   const [listEl, setListEl] = createSignal<HTMLDivElement>()
+  const [listViewportAttached, setListViewportAttached] = createSignal(false)
   const [virtualizerHandle, setVirtualizerHandle] = createSignal<VirtualizerHandle>()
   const [focusedSessionId, setFocusedSessionId] = createSignal<string>()
+  let attachmentFrame: number | undefined
+
+  const setListElement = (element: HTMLDivElement) => {
+    setListEl(element)
+    const detectAttachment = () => {
+      if (isSessionListViewportAttached(element)) {
+        attachmentFrame = undefined
+        setListViewportAttached(true)
+        return
+      }
+      setListViewportAttached(false)
+      if (typeof requestAnimationFrame !== "undefined") attachmentFrame = requestAnimationFrame(detectAttachment)
+    }
+    detectAttachment()
+  }
+
+  onCleanup(() => {
+    if (attachmentFrame !== undefined) cancelAnimationFrame(attachmentFrame)
+  })
 
   createEffect(() => {
     if (typeof window === "undefined") return
@@ -843,7 +864,7 @@ const SessionList: Component<SessionListProps> = (props) => {
 
        <div
          class="session-list flex-1 overflow-y-auto"
-         ref={setListEl}
+         ref={setListElement}
          onFocusIn={(event) => {
            const target = event.target
            if (!(target instanceof Element)) return
@@ -875,7 +896,10 @@ const SessionList: Component<SessionListProps> = (props) => {
             </div>
           </Show>
 
-          <Show when={visibleProjection().ids.length > 0 || hasMore() || isFetchingSessions()}>
+          <Show when={shouldRenderSessionRows(
+            Boolean(sessionListError()),
+            listViewportAttached() && (visibleProjection().ids.length > 0 || hasMore() || isFetchingSessions()),
+          )}>
            <div class="session-section">
              <Show when={visibleProjection().ids.length > 0}>
                <Virtualizer