fix: Virtualizer should layout items using a consistent size

[devongovett]

Fixes an issue caught in our S2 example apps in small viewports where the card view would appear to scroll faster than the rest of the page. This was due to the visibleRect changing size as the window scrolls when the CardView is larger than the window viewport. We need this to implement window scrolling, but the layouts (e.g. GridLayout) also use the visibleRect's size to determine how many columns of items to display etc. This resulted in the layout's contentSize being updated during scrolling, causing the scroll position to be clamped.

To fix this we need to introduce a separate virtualizer.size property to hold the size of the virtualizer regardless of the window scroll position. This way item layout is consistent. ScrollView now has an onSizeChange callback and this gets propagated down to the virtualizer state.

To make this not a breaking change which would require updates to v3, window scrolling support is now opt-in in the hooks, and only enabled by default in RAC. We fall back to the visibleRect size in this case.

[snowystinger]

Tested mobile and desktop, looks like the CardView is now working as expected. Will wait for verdaccio to be disabled again and then I'll approve

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the S2 docs
• View the React Aria docs

[rspbot]

## API Changes

@react-aria/virtualizer

/@react-aria/virtualizer:ScrollView

 ScrollView extends HTMLAttributes {
+  allowsWindowScrolling?: boolean
   children?: ReactNode
   contentSize: Size
   innerStyle?: CSSProperties
   onScroll?: (Event) => void
   onScrollEnd?: () => void
   onScrollStart?: () => void
+  onSizeChange?: (Size) => void
   onVisibleRectChange: (Rect) => void
   scrollDirection?: 'horizontal' | 'vertical' | 'both'
 }

---

@react-stately/virtualizer

/@react-stately/virtualizer:VirtualizerState

 VirtualizerState <T extends {}, V> {
   contentSize: Size
   endScrolling: () => void
   isScrolling: boolean
+  setSize: (Size) => void
   setVisibleRect: (Rect) => void
+  size: Size
   startScrolling: () => void
   virtualizer: Virtualizer<{}, V>
   visibleViews: Array<ReusableView<{}, V>>
 }

---