[combobox][popover] Add visually hidden close button support with improved modal focus trapping

[atomiks]

• I have followed (at least) the PR section of the contributing guide.

Improves modal popup accessibility by ensuring there is a reachable escape hatch for the flows that trap assistive-technology focus.

Closes #2722
Closes #3693
Closes #4096

Changes in this PR:

• Popover now traps focus when modal is enabled and a Popover.Close is rendered inside Popover.Popup
• Combobox and Autocomplete now render an internal dismiss button after the popup whenever the popup is modal to assistive technology
• The internal combobox dismiss button is tracked as part of the popup subtree by FloatingFocusManager, even though it is rendered outside the popup element
• FloatingFocusManager no longer inserts the hidden fallback span after the reference element
• Relevant combobox and popover coverage was added for modal focus management and dismiss behavior

finalFocus remains the preferred way to restore focus when the trigger itself gets removed.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@4084
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@4084
  

commit: a8ab7ad

[mui-bot]

Bundle size report

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+1.02KB(+0.22%)	🔺+403B(+0.27%)

Details of bundle changes

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[atomiks]

Codex Review

Overview

This patch improves trapped popup accessibility in two places. Popovers only enable trapped modal behavior when a Popover.Close is rendered inside the popup, and comboboxes now render internal hidden dismiss controls before the input and after the popup so touch screen readers can leave the modal flow in either direction.

Findings (None)

No blocking issues found in this patch.

Confidence: 4/5

High confidence from a full branch pass and focused verification of the updated combobox, popover, and focus-manager paths with pnpm test:jsdom ComboboxRoot PopoverClose FloatingFocusManager --no-watch. The remaining risk is device-specific assistive technology behavior, since the key user-facing change here is how touch screen readers discover the hidden dismiss controls.

Notes

• Since the previous review, the combobox internal dismiss control was narrowed further: it is now an internal span with button semantics, and the generated API artifact for it was removed.
• The current branch is a single squashed commit on top of master, so this review covers the whole PR diff as it stands today.

[michaldudak]

I found an issue when testing it with VoiceOver on an iPad: after I go through the whole list of items and arrive at the hidden close button, VoiceOver focus moves to the input right after the close button is announced. This happens just once per open combobox (focusing the close button again works as expected).