feat(react-headless-components-preview): add Popover & positioning

[mainframev]

Adds a headless Popover (Popover, PopoverTrigger, PopoverSurface) to the @fluentui/react-headless-components-preview package, positioned via the native CSS Anchor Positioning API instead of floating-ui

Feature with v9 @fluentui/react-popover

v9 feature	Status
open / defaultOpen / onOpenChange	✅
openOnHover + mouseLeaveDelay	✅
openOnContext (right-click, cursor-anchored)	✅
withArrow (+ consumer-owned arrow CSS via [data-placement])	✅
trapFocus + aria-modal / role="dialog"	✅ (via useFocusScope)
disableAutoFocus	✅
closeOnScroll, closeOnIframeFocus	✅
inline (skip top-layer)	✅
mountNode (portal target)	✅
Nested popovers (per-instance Escape / dismiss)	✅
positioning.position + positioning.align	✅
positioning.offset (number or { mainAxis, crossAxis })	✅
positioning.coverTarget	✅
positioning.fallbackPositions	✅
positioning.autoSize (true / 'width' / 'height')	✅
positioning.overflowBoundary	✅
positioning.matchTargetSize: 'width'	✅
positioning.strategy: 'absolute' | 'fixed'	✅
positioning.pinned	✅
positioning.target (custom anchor)	✅
positioning.positioningRef (imperative setTarget)	✅
flipBoundary	❓
arrowPadding	TBA
shiftToCoverTarget	❓
onPositioningEnd	➖ Native API has no "settle" event
disableUpdateOnResize	➖ Native anchor positioning + targeted usage is always-on
useTransform	➖ Native anchor positioning doesn't use transform, no conceptual equivalent

Related Issue(s)

• Partially implements RFC: Headless Components (@fluentui/react-headless-components) #35984

[github-actions]

Pull request demo site: URL

[github-actions]

📊 Bundle size report

Package & Exports	Baseline (minified/GZIP)	PR	Change
react-headless-components-preview react-headless-components-preview: entire library	69.676 kB 20.575 kB	87.717 kB 26.402 kB	18.041 kB 5.827 kB

_{🤖 This report was generated against 85ba749380bcc7df6e1257ee636fa1192c58744f}