[Bug]: headless MenuSplitGroup renders without a contexts argument, so useIsInMenuSplitGroup can never return true #36651

Component

Menu

Package version

@fluentui/react-headless-components-preview 0.2.5

React version

18.3.1

Environment

node v22.12.0
@fluentui/react-headless-components-preview 0.2.5

Current Behavior

components/Menu/MenuSplitGroup/MenuSplitGroup.tsx calls

renderMenuSplitGroup(state);

with no contexts argument. useIsInMenuSplitGroup compares the value it receives against the
module-level default by identity. With nothing provided, the default is what every descendant
reads, the identity comparison is always false, and no descendant half can tell that it is inside
a split group.

The user-visible consequence is that the gutter suppression a split group is supposed to apply —
the icon and checkmark columns that the trigger half drops — never fires, because that suppression
is gated on exactly this hook.

Expected Behavior

useIsInMenuSplitGroup returns true for descendants of a MenuSplitGroup, matching the
behaviour of the Griffel-styled MenuSplitGroup the headless one mirrors.

Reproduction

Render a headless MenuSplitGroup containing a trigger half and call useIsInMenuSplitGroup from
any descendant: it returns false. Visually, in a styled layer over the headless package, the
trigger half keeps the icon and checkmark gutters that the Griffel-styled equivalent drops.

Steps to reproduce

1. Render a MenuSplitGroup with a menu item as its trigger half.
2. From a descendant, read useIsInMenuSplitGroup().
3. Observe false.

Discovery context

Found during pixel-level verification of a styling layer built over
@fluentui/react-headless-components-preview: the styled trigger half kept the icon and checkmark
gutters that the @fluentui/react-components twin drops, and tracing the gutter rule led back to a
hook that could never return true.

Proposed fix

Provide the contexts argument, with a useMemo keyed on state.setMultiline so the identity is
stable across renders (identity stability is the whole point — an inline object literal would defeat
the comparison in the other direction, re-firing on every render):

const contexts = React.useMemo(
  () => ({ menuSplitGroup: { setMultiline: state.setMultiline } }),
  [state.setMultiline],
);
return renderMenuSplitGroup(state, contexts);

setMultiline itself stays the documented no-op — this changes what descendants can observe, not
what the group does.

A working implementation is included in PR #[WINDMOD-PR-NUMBER] (commit 568297d44a); happy to
split it into a standalone PR against this issue.

Suggested severity

Medium - Has workaround (a styling layer can re-declare its own split-group context, at the cost of
duplicating a contract the package already owns).

Are you willing to submit a PR to fix?

yes