Popover: PopoverContent throws "Primitive.div failed to slot onto its children" on mount (React 19)

Bug report

Current Behavior

PopoverContent throws immediately on mount:

Error: Primitive.div failed to slot onto its children. Expected a single React element child or `Slottable`.
    at Primitive.div.Slot
    at updateForwardRef
    at beginWork
    ...

This happens with a minimal, standard usage — a single valid-element child, no fragments, no arrays, nothing unusual:

import { Popover, PopoverContent, PopoverTrigger } from "radix-ui" // (also reproduces via the standalone @radix-ui/react-popover package)

function Demo() {
  return (
    <Popover.Root>
      <Popover.Trigger asChild>
        <button type="button">Open popover</button>
      </Popover.Trigger>
      <Popover.Portal>
        <Popover.Content>
          <div>Hello</div>
        </Popover.Content>
      </Popover.Portal>
    </Popover.Root>
  )
}

Clicking the trigger throws the error above the moment Popover.Content mounts. The trigger itself renders fine before that — the crash is specifically on Content mount, not on Trigger/Anchor composition.

Expected behavior

The popover opens and renders its content, same as Tooltip and Dialog do with an equivalent structure.

Reproduction / isolation notes

I spent a while isolating this before filing, since I initially assumed it was a mistake in my own code:

• Reproduces with a single child — not a multiple-children-into-Slot issue (ruled out the Slottable scenario from #3747, which is a different pattern).
• Reproduces with modal both true and false — not specific to the non-modal code path.
• Tooltip and Dialog both work fine in the exact same app/dependency tree with structurally similar composition (Trigger asChild + Portal + focus-managed Content).
• Pinning @radix-ui/react-slot to 1.2.5 or 1.2.3 (full clean reinstalls each time) does not fix it — so this isn't the Slot regression described in #3747 (which was fixed/worked around at the Slot level for a different pattern).
• Both radix-ui (1.6.7) and @radix-ui/react-popover (1.1.23) are already the latest stable releases at the time of filing — no newer version available to try (only 1.2.0-rc.* prereleases for @radix-ui/react-popover, and 1.7.0-rc.* for the radix-ui meta-package).

Root cause (best guess from reading source)

PopoverContentImpl composes:

FocusScope (asChild) > DismissableLayer (asChild) > PopperPrimitive.Content

Dialog composes FocusScope (asChild) > DismissableLayer (asChild) > Primitive.div (no Popper layer) — works fine.

Tooltip composes PopperPrimitive.Content directly, with no FocusScope/DismissableLayer wrapping (tooltips aren't focusable, so this makes sense) — works fine.

Popover is the one primitive combining both FocusScope/DismissableLayer and PopperPrimitive.Content underneath — and it's the one that's broken. That intersection is my best guess at the actual fault line, though I haven't dug further into exactly which prop/ref is losing its single-child shape across that specific chain.

Environment

• react: 19.2.8
• react-dom: 19.2.8
• radix-ui: 1.6.7 (unified package)
• @radix-ui/react-popover (resolved): 1.1.23
• @radix-ui/react-slot (resolved): 1.3.3 (also tried 1.2.5 and 1.2.3 via overrides — same failure both times)
• Bundler: Vite 8 (also reproduces under Storybook 10.5's Vite builder, separately from the app's own build)
• OS: Windows

Workaround in use

Not using Popover for now — using Dialog with modal={false} and manually-positioned Content (no Popper anchoring) instead, which sidesteps the broken combination entirely.