fix(ui): 浮窗内 tab 的 visible 恒为 true，不再跟随面板开合

renderTab 的可见性原为 panelOpen && active——浮窗内容随侧边栏收起被判定为
不可见，遵守 visible 契约（不可见时暂停/空渲染）的消费插件 tab 会在浮窗里
白屏。改为 placement 三态（top/bottom/float），float 恒 true（兑现 AGENTS
§7.5 契约）；回归测试断言面板收起后浮窗内的注册插件 tab 仍收到
visible=true。

src/client/Sidebar.tsx

@@ -1191,7 +1191,13 @@ export function Sidebar(props: { ctx: Context; store: SidebarStore }) {
    * polling while the page is not actually visible). The pane id travels
    * with the tab so diff tabs can split below their source pane.
    */
-  const renderTab = (tab: SidebarTab, active: boolean, paneId: string, bottom = false) => (
+  // `placement` decides the visibility contract handed to the tab component:
+  // pane tabs are visible while their panel is open and they are active, but
+  // a free window is its own surface — its tab stays visible no matter what
+  // the panels do (the AGENTS §7.5 contract; plugin components honor
+  // `visible` to pause work, so tying floats to panelOpen would blank them
+  // the moment the sidebar collapses).
+  const renderTab = (tab: SidebarTab, active: boolean, paneId: string, placement: 'top' | 'bottom' | 'float' = 'top') => (
     <TabContent
       tab={tab}
       paneId={paneId}
@@ -1202,7 +1208,13 @@ export function Sidebar(props: { ctx: Context; store: SidebarStore }) {
       onReferenceFile={referenceInChat}
       ctx={ctx}
       store={store}
-      visible={bottom ? state.bottomOpen && active : state.panelOpen && active}
+      visible={
+        placement === 'float'
+          ? true
+          : placement === 'bottom'
+            ? state.bottomOpen && active
+            : state.panelOpen && active
+      }
       onSubagentJump={(childSessionId) => { subagentJumpRef.current = childSessionId }}
       onOpenDiff={(diffTab) => { store.reduce(s => openDiffTab(s, paneId, diffTab)) }}
       localeRevision={localeRevision}
@@ -1469,7 +1481,7 @@ export function Sidebar(props: { ctx: Context; store: SidebarStore }) {
             newTabOptions={buildNewTabOptions(state, ctx, { sessionId, cwd })}
             actions={actions}
             onNewTab={onNewTab}
-            renderTab={(tab, active, paneId) => renderTab(tab, active, paneId, true)}
+            renderTab={(tab, active, paneId) => renderTab(tab, active, paneId, 'bottom')}
             getTabIcon={tabIconOf}
             getTabBadge={tabBadgeOf}
           />
@@ -1488,7 +1500,7 @@ export function Sidebar(props: { ctx: Context; store: SidebarStore }) {
         <FreeWindow
           key={float.id}
           float={float}
-          renderTab={renderTab}
+          renderTab={(tab, active, paneId) => renderTab(tab, active, paneId, 'float')}
           getTabIcon={tabIconOf}
           onRaise={() => { store.reduce(s => raiseFloat(s, float.id)) }}
           onMove={(x, y) => { store.reduce(s => moveFloat(s, float.id, x, y)) }}

tests/free-window.spec.tsx

@@ -23,7 +23,7 @@ import { act } from 'react-dom/test-utils'
 ;(globalThis as Record<string, unknown>).IS_REACT_ACT_ENVIRONMENT = true
 
 import { Sidebar } from '../src/client/Sidebar.tsx'
-import { createSidebarStore, type SidebarStore, floatTab as floatTabReducer } from '../src/client/state.ts'
+import { createSidebarStore, togglePanel, type SidebarStore, floatTab as floatTabReducer } from '../src/client/state.ts'
 import { createBetterSidebarService, type BetterSidebarService } from '../src/client/service.ts'
 import { serializeDrag } from '../src/client/TabBar.tsx'
 
@@ -73,7 +73,15 @@ function mountSidebar(): Mounted {
   document.body.append(container)
   const store = createSidebarStore()
   const service = createBetterSidebarService(store)
-  service.registerTab({ id: 'notes', title: 'Notes', single: true, component: () => createElement('div', null, 'notes body') })
+  service.registerTab({
+    id: 'notes',
+    title: 'Notes',
+    single: true,
+    // Pins the visibility contract into the DOM: plugin components honor
+    // `visible` to pause work, so the float tests can assert what a
+    // registering plugin would actually receive.
+    component: ({ visible }) => createElement('div', { 'data-visible': String(visible) }, 'notes body'),
+  })
   store.setPrefs({ ...store.getPrefs(), openByDefault: true })
   store.setSession('s1')
   const localeSnapshot = { active: 'en' }
@@ -294,6 +302,26 @@ describe('free windows: the window', () => {
     }
   })
 
+  it('a floated plugin tab stays visible while the panel collapses (placement contract)', () => {
+    fakeConversationColumn()
+    const { container, store, service, unmount } = mountSidebar()
+    try {
+      act(() => { service.openTab({ type: 'notes', title: 'Notes' }) })
+      act(() => { store.reduce(s => floatTabReducer(s, 'notes', 512, 384)) })
+      const win = container.querySelector<HTMLElement>('[data-dsh-float-window]')!
+      expect(win.querySelector<HTMLElement>('[data-visible]')!.dataset.visible).toBe('true')
+      // Collapse the sidebar: the pane's seeded home tab goes invisible
+      // (panel closed), but the float is its own surface — a registering
+      // plugin's component must keep receiving visible=true (AGENTS §7.5).
+      act(() => { store.reduce(togglePanel) })
+      expect(win.querySelector<HTMLElement>('[data-visible]')!.dataset.visible).toBe('true')
+      // (No pane-side control: the pane's remaining tab is the builtin editor
+      // home tab, which does not surface `visible` into the DOM.)
+    } finally {
+      unmount()
+    }
+  })
+
   it('releasing the header over a pane docks the tab back (live pane highlight)', async () => {
     fakeConversationColumn()
     const { container, store, service, unmount } = mountSidebar()