diff --git a/apps/web/tests/plugin-manager.e2e.ts b/apps/web/tests/plugin-manager.e2e.ts
index 1f7ee7ee70..ff6871e4af 100644
--- a/apps/web/tests/plugin-manager.e2e.ts
+++ b/apps/web/tests/plugin-manager.e2e.ts
@@ -293,6 +293,38 @@ describe('web e2e: plugin manager', () => {
     }
   })
 
+  it('keeps the collapsed bar steady when collapsing lets a barely scrolling list fit', async () => {
+    await page.setViewportSize({ width: 1000, height: 480 })
+    const panel = await openPluginsPanel()
+    const body = panel.locator('[data-plugin-body]')
+    const compact = () => panel.locator('[data-plugin-bar]').evaluate(element => element.hasAttribute('data-compact'))
+    try {
+      await panel.locator('[data-plugin-package]').first().waitFor()
+      // Size the window so the resting bar leaves the list 12px of scroll range, less than collapsing frees.
+      const range = await body.evaluate(element => element.scrollHeight - element.clientHeight)
+      await page.setViewportSize({ width: 1000, height: 480 + range - 12 })
+      await expect.poll(() => body.evaluate(element => element.scrollHeight - element.clientHeight)).toBe(12)
+      await page.mouse.move(500, 400)
+      await page.mouse.wheel(0, 200)
+      await expect.poll(compact).toBe(true)
+      expect(await body.evaluate(element => element.scrollHeight - element.clientHeight)).toBeLessThanOrEqual(0)
+      // The clamp back to the top must not reopen the bar and loop the layout.
+      const flips = await panel.locator('[data-plugin-bar]').evaluate(element => new Promise<number>((resolve) => {
+        let count = 0
+        const observer = new MutationObserver(() => { count++ })
+        observer.observe(element, { attributes: true, attributeFilter: ['data-compact'] })
+        setTimeout(() => { observer.disconnect(); resolve(count) }, 500)
+      }))
+      expect(flips).toBe(0)
+      expect(await compact()).toBe(true)
+      await page.mouse.wheel(0, -200)
+      await expect.poll(compact).toBe(false)
+    } finally {
+      await body.evaluate((element) => { element.scrollTop = 0 })
+      await page.setViewportSize({ width: 1680, height: 1000 })
+    }
+  })
+
   it('keeps the plugin page title and actions pinned while the cards scroll', async () => {
     await page.setViewportSize({ width: 1000, height: 480 })
     const panel = await openPluginsPanel()
diff --git a/packages/client/ui-plugin-manager/src/client/PluginManagerPage.tsx b/packages/client/ui-plugin-manager/src/client/PluginManagerPage.tsx
index da8dde9af4..54d6508255 100644
--- a/packages/client/ui-plugin-manager/src/client/PluginManagerPage.tsx
+++ b/packages/client/ui-plugin-manager/src/client/PluginManagerPage.tsx
@@ -1728,7 +1728,18 @@ export function PluginManagerPage(props: PluginManagerPageProps): ReactNode {
       ) : null}
       {/* The page bar and the More list's head stay put; everything below scrolls. */}
       <div className={css.pageBody} data-plugin-body
-        onScroll={(event) => { setCompactBar(event.currentTarget.scrollTop > COMPACT_BAR_SCROLL_TOP) }}>
+        onScroll={(event) => {
+          const body = event.currentTarget
+          if (body.scrollTop > COMPACT_BAR_SCROLL_TOP) setCompactBar(true)
+          // Collapsing grows the body, so a list that only just scrolled can fit and clamp to 0 without
+          // the person scrolling up; the bar reopens at the top only while the list still scrolls.
+          else if (body.scrollTop === 0 && body.scrollHeight > body.clientHeight) setCompactBar(false)
+        }}
+        onWheel={(event) => {
+          // A list that fits only under the collapsed bar has no scroll range; scrolling up reopens the bar.
+          const body = event.currentTarget
+          if (event.deltaY < 0 && body.scrollHeight <= body.clientHeight) setCompactBar(false)
+        }}>
         {!onInstalled
           ? (
             <div role="tabpanel" id={`${tabsId}-${tab}-panel`} aria-labelledby={`${tabsId}-${tab}`} className={css.tabPanel}>
diff --git a/packages/client/ui-plugin-manager/tests/components.client.spec.tsx b/packages/client/ui-plugin-manager/tests/components.client.spec.tsx
index b28e6c4e44..5ee9c63ad7 100644
--- a/packages/client/ui-plugin-manager/tests/components.client.spec.tsx
+++ b/packages/client/ui-plugin-manager/tests/components.client.spec.tsx
@@ -292,12 +292,42 @@ describe('PluginManagerPage', () => {
     expect(bar?.hasAttribute('data-compact')).toBe(true)
     act(() => {
       if (body === null) throw new Error('the page body is missing')
+      Object.defineProperties(body, { scrollHeight: { configurable: true, value: 900 }, clientHeight: { configurable: true, value: 400 } })
       body.scrollTop = 0
       fireEvent.scroll(body)
     })
     expect(bar?.hasAttribute('data-compact')).toBe(false)
   })
 
+  it('keeps the collapsed bar when collapsing makes the list fit, and reopens it on an upward scroll', () => {
+    renderTab({ packages: [pkg()] })
+    const bar = document.querySelector('[data-plugin-bar]')
+    const body = document.querySelector<HTMLElement>('[data-plugin-body]')
+    if (body === null) throw new Error('the page body is missing')
+    const range = (scrollHeight: number): void => {
+      Object.defineProperties(body, {
+        scrollHeight: { configurable: true, value: scrollHeight },
+        clientHeight: { configurable: true, value: 400 },
+      })
+    }
+    range(440)
+    act(() => { body.scrollTop = 20; fireEvent.scroll(body) })
+    expect(bar?.hasAttribute('data-compact')).toBe(true)
+    // The taller body now holds the whole list, and the browser clamps the scroll position to 0.
+    range(400)
+    act(() => { body.scrollTop = 0; fireEvent.scroll(body) })
+    expect(bar?.hasAttribute('data-compact')).toBe(true)
+    act(() => { fireEvent.wheel(body, { deltaY: 40 }) })
+    expect(bar?.hasAttribute('data-compact')).toBe(true)
+    act(() => { fireEvent.wheel(body, { deltaY: -40 }) })
+    expect(bar?.hasAttribute('data-compact')).toBe(false)
+    // A wheel over a list that still scrolls leaves the bar to the scroll position.
+    range(900)
+    act(() => { body.scrollTop = 20; fireEvent.scroll(body) })
+    act(() => { fireEvent.wheel(body, { deltaY: -40 }) })
+    expect(bar?.hasAttribute('data-compact')).toBe(true)
+  })
+
   it('keeps the Installed tab panel through loading and the More list out of the tabs', () => {
     const worktree = '@deepseek-ai/dsh-experimental-tool-worktree'
     const { navigation, set } = renderTab({ status: 'loading', packages: [] }, {}, {