[popups] Add transitionKey prop to Viewport

[atomiks]

Viewport previously only animated content changes caused by switching detached triggers. This adds transitionKey for content that changes in place while a popup stays open.

Changes

• Added transitionKey to Popover.Viewport, Menu.Viewport, Tooltip.Viewport, and PreviewCard.Viewport.
• Reused the existing previous/current snapshot and popup resize transition when the key changes.
• Preserved component-specific focus behavior: Popover recovers lost focus to the first tabbable element, Menu resets and restores keyboard highlights across views, and Tooltip and Preview Card leave focus unchanged.
• Exposed Menu's forward/back direction through data-activation-direction and handled multi-level returns.
• Prevented cleanup from an interrupted transition from ending a newer transition.
• Added tests and documentation demos for each component.

Demos

Popover, Menu, Tooltip, Preview Card

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@5215
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@5215
  

commit: ffbf0b9

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+501B(+0.11%)	🔺+163B(+0.11%)

Details of bundle changes

Performance

Total duration: 1,111.14 ms -47.59 ms^(-4.1%) | Renders: 78 ⁽⁺⁰⁾ | Paint: 1,764.14 ms -69.23 ms^(-3.8%)

No significant changes — details

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[flaviendelangle]

I think this closes #1894

[atomiks]

@flaviendelangle not quite, this just supports it if you control it externally, but it likely still needs new component parts for proper DX. It's more like an actual workaround now though.

[flaviendelangle]

Could be worth answering on the issue once we merge this one to let people know their is a workaround.
But OK if we think we need some higher level API to help them do it 👍