Fix ScrollView infinite scrolling bug by correcting pageUp/pageDown dimension usage

[Copilot]

The ScrollView component was experiencing infinite scrolling behavior in React Native Windows 0.80 preview 6, as reported in the React Native Gallery app. The root cause was incorrect dimension usage in the pageUp() and pageDown() functions for horizontal scrolling.

Problem

When a ScrollView had the horizontal prop set to true, the pageUp() and pageDown() functions were incorrectly using frame.size.height instead of frame.size.width to calculate the scroll delta. This caused:

• Scrolling by the wrong amount (viewport height instead of viewport width)
• Erratic or infinite scrolling behavior
• Poor user experience in horizontal ScrollViews

The bug was introduced in 0.80 preview 6 when custom scrollbar functionality was added. The pageUp() and pageDown() functions were created with the dimensional bug and are triggered when users interact with the PageUp/PageDown regions of the custom scrollbars.

Solution

Fixed the dimension calculation in two functions:

// pageUp() - line 1059
- return scrollLeft(m_layoutMetrics.frame.size.height * m_layoutMetrics.pointScaleFactor, animate);
+ return scrollLeft(m_layoutMetrics.frame.size.width * m_layoutMetrics.pointScaleFactor, animate);

// pageDown() - line 1066  
- return scrollRight(m_layoutMetrics.frame.size.height * m_layoutMetrics.pointScaleFactor, animate);
+ return scrollRight(m_layoutMetrics.frame.size.width * m_layoutMetrics.pointScaleFactor, animate);

Now horizontal ScrollViews correctly use viewport width for page-based scrolling, while vertical ScrollViews continue to use viewport height as expected.

Impact

• ✅ Horizontal ScrollViews now scroll by the correct amount (viewport width)
• ✅ Vertical ScrollViews remain unchanged (continue using viewport height)
• ✅ Resolves infinite scrolling issue in React Native Gallery
• ✅ Fixes scrollbar PageUp/PageDown interactions for horizontal ScrollViews
• ✅ Minimal change with no side effects on other functionality

Fixes #15171.

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