menu.List: a pointer resting on a parent item shows its submenu

Context

Operator request (2026-09-16): a boolean on Neo.menu.List so nested submenus open on hover, default true. Successor to #2585 (@tobiu, 2021: "hovering items with the mouse should expand sub-menus … delegating mouseEnter & leave to list items"), which the stale bot closed in 2024 without a decision.

Observed on dev@d688710175, examples/button/base: resting the pointer on the parent Item 3 leaves one menu level, and clicking it opens the second. menu.List, list.Base, selection.ListModel and selection.menu.ListModel contain no hover handling.

The Problem

A submenu shows only through selection: a click, or the Navigator's Enter (which clicks), selects the item, and onSelect() swaps the submenu. The desktop convention is missing: rest on a parent and its submenu appears.

Three failure modes decide the design:

1. Diagonal travel. A submenu aligns l0-r0 to its parent item, so reaching its second or third row cuts across the parent menu's rows below. If entering a row switches submenus at once, the target closes mid-travel.
2. Hover, then click. Pointer users click items they already hover, and a touch tap emits mouseenter before click. If that click toggles — #18758 restores the toggle, and this ticket is blocked by it — it closes the submenu the hover just showed.
3. A popup takes the caret. component.Base#afterSetMounted focuses a floating component's first focusable child on mount, documented as deliberate ("a popup takes the caret with it"). That is why Enter on a parent lands inside its submenu today. A submenu shown because the pointer passed over its parent is a preview, not an entry, and must leave focus where it is. Otherwise the keyboard cursor jumps under the user.

The Architectural Reality

• Delegated hover is an engine primitive. manager.DomEvent#verifyMouseEnterLeave fires mouseenter/mouseleave only at the delegate's own boundary (consumers: tooltip.Base, app.header.Toolbar with delegate: '.neo-button'). main.DomEvents#onMouseEnter already forwards every event, so a listener adds no main-thread messages.
• Showing is already separable from selecting. activeSubMenu is the shown state; only onSelect() couples it to selection.
• Keyboard is focus-driven since #5123: arrows move Navigator focus, Enter/Space select (operator ruling, 2026-09-16). Unchanged here.
• Precedent for a delay config: tooltip.Base#showDelay: 200.

The Fix — src/menu/List.mjs (+ menu SCSS)

1. showSubMenuOnHover: true. The name follows the class's subMenu family (activeSubMenu, subMenuGap, showSubMenu()).
2. subMenuHoverDelay: 200: how long the pointer must rest on an item before the rest counts.
3. Delegated mouseenter/mouseleave on interactive items, excluded by the same nonInteractiveItemCls rule as the click delegate. Enter schedules a rest; leave cancels it. A completed rest on a parent shows its submenu, and on a leaf hides the shown one. It neither selects nor moves focus. A row crossed faster than the delay changes nothing.
4. showSubMenu() gains a focus intent, carried by a new component.Base#focusOnMount config (default true) that gates the floating mount-focus. A preview mounts with it false; click and Enter keep today's focus-into behaviour, and every other floating component keeps taking the caret.
5. The parent whose submenu is showing carries aria-expanded="true" and is styled like the hovered item, so the parent stays marked while the pointer is inside the submenu.
6. Both settings are read from the root menu, so a cascade cannot disagree with itself and a runtime change reaches cached submenus.
7. Click on a parent: shows (idempotent) while showSubMenuOnHover is true; toggles when false.
8. unmount() and destroy() cancel a pending rest, and the resume re-checks liveness (#18731).

Contract Ledger

Acceptance Criteria

• AC-1 Resting on a parent shows its submenu with no click; the parent reads aria-expanded="true"; selection and document.activeElement are unchanged (component spec, real pointer).
• AC-2 A diagonal pointer path from a parent to its submenu's third row keeps the submenu at the default delay, and closes it with subMenuHoverDelay: 0 — the control arm proving the path crosses a sibling.
• AC-3 Resting on a leaf hides the sibling submenu that is showing.
• AC-4 Hover, then click, on a parent: the submenu never closes (level-count transitions recorded, no dip).
• AC-5 showSubMenuOnHover: false: a rest shows nothing, and click keeps #18758's toggle.
• AC-6 Changing either config on the root at runtime governs already cached levels.
• AC-7 A rest pending when its menu unmounts or is destroyed shows nothing.
• AC-8 Existing menu unit and component specs stay green.

Out of Scope

• Keyboard walking of the cascade (Right/Left/Escape/Space) — #18760.
• Keyboard focus resting on a parent showing its submenu — recorded as #18760's follow-up.
• Trajectory aiming (a "safe triangle").

Avoided Traps

• Hover selects: couples a preview to activation state and fights the focus model, where Space/Enter select.
• Safe-triangle aiming: needs a stream of pointer coordinates in the App Worker plus submenu rects. A rest delay covers the diagonal case with enter/leave alone.
• Showing at once while nothing is shown: a pointer sweeping across the menu would flash every parent's submenu. One rule for every rest is simpler and flicker-free.
• Per-level copies synced by afterSet loops (the animateSpawn_ / theme shape): right for rendering state each level paints; a behaviour policy of the cascade belongs to its root.

Related

Blocked by #18758 · #2585 · #4606 · #5123 · #18731

Live latest-open sweep: latest 20 open issues at 2026-09-16T08:07:16Z; no equivalent found A2A claim sweep: latest 30 inbox messages, all read-states, at 2026-09-16T08:07Z; no [lane-claim] / [lane-intent] on menus MC sweep: query_raw_memories on the symptom ("need to click a menu list item to expand the child list; submenu does not open when hovering the parent item") plus a KB ticket query, 6 + 8 results; prior art #2585 (stale-closed, no ruling), no prior decision against hover Own-assignment sweep: 5 open, none on src/menu

Origin Session ID: fda7faab-19c5-43a0-b192-003966068172 Retrieval Hint: "menu.List submenu shows on pointer rest; subMenuHoverDelay; preview show without mount focus; aria-expanded parent"