[scroll area] Fix overflow and scrolling state

[atomiks]

This fixes a batch of Scroll Area state bugs where delayed content, threshold updates, and custom scrollbar interactions could leave overflow or scrolling attributes out of sync.

Changes

• Fixed overflow state when ScrollArea.Content mounts after the viewport's first measurement.
• Recomputed overflow edge attributes when overflowEdgeThreshold changes.
• Allowed wheel events on scrollbar tracks to chain at scroll edges.
• Marked custom scrollbar wheel and track-click interactions as scrolling.
• Cleaned up thumb drag state on pointercancel without releasing stale pointer capture.
• Documented Content data attributes.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@4936
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@4936
  

commit: 307bd9d

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+379B(+0.08%)	🔺+133B(+0.09%)

Details of bundle changes

Performance

Total duration: 1,014.38 ms -135.07 ms^(-11.8%) | Renders: 50 ⁽⁺⁰⁾ | Paint: 1,520.92 ms -220.87 ms^(-12.7%)

Test	Duration	Renders
Scroll Area mount (300 instances)	74.58 ms ▼-20.57 ms(-21.6%)	3 (+0)

11 tests within noise — details

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Check out the code infra dashboard for more information about this PR.

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