Vertical scrollbar never reappears after resizing window from non-scrollable back to scrollable (ScrollBarComponent::updateVisibility latch)

从 Microsoft.ReactNative/Fabric/Composition/ScrollViewComponentView.cpp 中的 ScrollBarComponent::updateVisibility 开始，然后跟踪 ContentSize、updateLayoutMetrics 和 updateShowsVerticalScrollIndicator。使用最小的 ScrollView 示例重现调整大小序列，并将 indicator prop 状态与计算出的可见性分离；完成标准是内容再次大于 viewport 时，垂直和水平滚动条都能重新出现。

Problem Description

Problem Description

A ScrollView's vertical scrollbar gets permanently latched hidden once the content has fit the viewport, and never comes back even when a later resize makes the content scrollable again.

Sequence:

1. Content is larger than the viewport → scrollbar shows (on hover). ✅
2. Enlarge the window until the content fits the viewport → scrollbar correctly disappears. ✅
3. Shrink the window again so the content is larger than the viewport → scrollbar does not come back. ❌ (mouse-wheel scrolling still works)

Once the bar has been hidden because the content fit the viewport, no later resize shows it again. The only way to recover is to change the showsVerticalScrollIndicator prop.

Root cause

Microsoft.ReactNative/Fabric/Composition/ScrollViewComponentView.cpp, ScrollBarComponent::updateVisibility (line numbers from the 0.82.5 shipped source):

void updateVisibility(bool visible) noexcept {
  if ((m_size.Width <= 0.0f && m_size.Height <= 0.0f) ||
      (m_contentSize.Width <= 0.0f && m_contentSize.Height <= 0.0f)) {
    m_rootVisual.IsVisible(false);
    return;
  }

  if (!visible) {                 // (A)
    m_visible = false;
    m_rootVisual.IsVisible(visible);
    return;                       // early-return: content/size are NOT re-evaluated
  }

  bool newVisibility = false;     // (B) recompute only when visible == true
  if (m_vertical) {
    newVisibility = (m_contentSize.Height > m_size.Height);
  } else {
    newVisibility = (m_contentSize.Width > m_size.Width);
  }

  m_visible = newVisibility;
  m_rootVisual.IsVisible(m_visible);
}

The size/content-driven callers pass the previously computed state back in as the visible argument:

// ContentSize(...)         -> updateVisibility(m_visible);
// updateLayoutMetrics(...) -> updateVisibility(m_visible);

m_visible defaults to true, and updateShowsVerticalScrollIndicator(value) calls updateVisibility(true|false) from the prop. So m_visible conflates two concepts: the prop showsVerticalScrollIndicator and the computed result (content > viewport).

When the window is enlarged so content fits, branch (B) sets m_visible = false. From then on, every layout/content update calls updateVisibility(m_visible == false), which hits branch (A) and early-returns without re-evaluating content > viewport — so shrinking the window back never re-enables the bar.

Proposed fix

Track the prop separately from the computed visibility and always recompute on size/content changes:

bool m_indicatorEnabled{true};   // set from showsVerticalScrollIndicator
bool m_visible{true};            // computed: (content > viewport) && enabled

void updateVisibility() noexcept {
  if ((m_size.Width <= 0.0f && m_size.Height <= 0.0f) ||
      (m_contentSize.Width <= 0.0f && m_contentSize.Height <= 0.0f)) {
    m_rootVisual.IsVisible(false);
    return;
  }
  const bool scrollable = m_vertical
      ? (m_contentSize.Height > m_size.Height)
      : (m_contentSize.Width > m_size.Width);
  m_visible = m_indicatorEnabled && scrollable;
  m_rootVisual.IsVisible(m_visible);
}

• updateShowsVerticalScrollIndicator(value) → m_indicatorEnabled = value; updateVisibility();
• ContentSize(...) / updateLayoutMetrics(...) → updateVisibility();

The same applies to the horizontal scrollbar.

Workaround (for app authors)

Toggle showsVerticalScrollIndicator for one frame on every viewport size change, which forces updateShowsVerticalScrollIndicator(true) → updateVisibility(true) to recompute:

const [showVScroll, setShowVScroll] = useState(true);
const onLayout = useCallback(() => {
  setShowVScroll(false);
  requestAnimationFrame(() => setShowVScroll(true));
}, []);
// <ScrollView showsVerticalScrollIndicator={showVScroll} onLayout={onLayout} />

Steps To Reproduce

1. Create/run a Composition (Fabric) RNW 0.82 app whose root renders a vertically scrollable ScrollView:

   import { ScrollView, Text } from 'react-native';
   
   export default function App() {
     return (
       <ScrollView showsVerticalScrollIndicator>
         {Array.from({ length: 60 }).map((_, i) => (
           <Text key={i} style={{ height: 40 }}>{`row ${i}`}</Text>
         ))}
       </ScrollView>
     );
   }
   

2. Launch in a small window → vertical scrollbar appears on hover.
3. Enlarge / maximize the window until all rows fit → scrollbar disappears (correct).
4. Shrink / restore the window so the rows no longer fit → scrollbar stays hidden, even though the view is scrollable (mouse wheel still scrolls).

Expected Results

After step 4, the vertical scrollbar should be available again (on hover) because the content is once more larger than the viewport.

CLI version

20.0.0

Environment

System:
  OS: Windows 11
SDKs:
  Windows SDK:
    AllowDevelopmentWithoutDevLicense: Enabled
    Versions:
      - 10.0.22621.0
      - 10.0.26100.0
IDEs:
  Visual Studio:
    - 18.7.11903.348 (Visual Studio Community 2026)
npmPackages:
  "@react-native-community/cli":
    installed: 20.0.0
    wanted: 20.0.0
  react:
    installed: 19.1.1
    wanted: 19.1.1
  react-native:
    installed: 0.82.1
    wanted: 0.82.1
  react-native-windows:
    installed: 0.82.5
    wanted: 0.82.5

Community Modules

From dependencies in package.json:

@expo/react-native-action-sheet: ^4.1.1
@fluentui/react-native: ^0.43.1
@react-native/new-app-screen: 0.82.1
@react-navigation/native: ^7.2.2
@react-navigation/stack: ^7.4.2
@reduxjs/toolkit: ^2.3.0
axios: ^1.9.0
i18next: ^24.2.3
react-i18next: ^15.4.1
react-native-popup-menu: ^0.17.0
react-native-safe-area-context: ^5.5.2
react-native-svg: ^15.12.1
react-redux: ^9.2.0
redux-thunk: ^3.1.0

Note: the bug reproduces with a bare ScrollView and does not depend on any of these modules.

Target React Native Architecture

New Architecture (WinAppSDK) Only

Target Platform Version

10.0.22621

Visual Studio Version

Visual Studio 2026

Build Configuration

Debug

Snack, code example, screenshot, or link to a repository

Minimal repro is the ScrollView snippet under Steps To Reproduce (no community modules required). Repro is a window-resize interaction (enlarge until content fits, then shrink), so it is not reproducible in a Snack — it requires a desktop RNW window.