Grid multi-body across windows: detach locked SubGrids into popups, sync their scroll over a direct MessageChannel

[neo-opus-vega]

Author's Note: Moved from issues #9493 and #9494 by Vega (Claude Opus 5.5) during the 13.4 householding, 2026-11-08. The operator handed the deferred grid set to me ("please sync with Vega"). Both were deferred follow-ons of the Multi-Body epic #9486, which closed on 2026-11-08 with its single-window scope delivered. Their history stays on the issues.

Scope: medium-blast (grid, main-thread addons, multi-window)

The Concept

1. Detach the locked SubGrids (Grid Multi-Body: Enable Cross-Window SubGrid Detachment (Pop-out) #9493). The locked: 'start' and locked: 'end' SubGrids move into their own popup windows, as LivePreview does, and return when that window closes.
   • A SubGrid stays fully functional without being DOM-adjacent to the others.
   • Connect and disconnect logic, likely on the grid container, moves a SubGrid instance into a spawned window and recovers it on close.
   • The App Worker stays the single source of truth for the Store and the SelectionModel, wherever the VDOM is projected.
   • Keyboard navigation works across windows (carried from Grid Multi-Body: Adapt Keyboard Navigation for Split Bodies #9496). CellModel#onNavKeyColumn already steps through every region's logical dataFields. What remains is routing the detached window's key events into the same SelectionModel.
2. Direct cross-window scroll sync (Grid Multi-Body: Implement Direct Main-Thread Scroll Sync via MessageChannel #9494). Scroll routed through the App Worker (Main A → App Worker → Main B) adds latency. Instead, a main-thread addon in window A creates a MessageChannel and passes port2 through the App Worker to the addon in window B, and the two sync vertical scrollTop over it directly. A slight delay across monitors is acceptable.

The Rationale

Checked at dev bc8a8c1bcb:

• The single-window base is shipped. Neo.grid.View owns bodyStart, body and bodyEnd (src/grid/View.mjs:42–71), and horizontal sync runs in src/main/addon/GridHorizontalScrollSync.mjs.
• Nothing cross-window exists for grids yet. There is no detach path and no port delegation.
• The base a detach would stand on is incomplete. That base is the dock tear-out work (Multi-window docking choreography — real OS windows on the shared heap #15239). Its portability matrix still lacks a macOS verdict for rows 1, 2, 3 and 6. The operator's rule of 2026-11-08: "we don't have windows, and if a windows ticket goes stale, that is a close. electron windows => FM."
• No owner, no milestone, no near-term plan. Under the Every open engine issue gets a premise read and a verdict #15000 rules, a valid idea without a plan lives here, not in the issue backlog.

Open Questions

• OQ-1: Does detachment reuse the dock tear-out path (DragCoordinator, window claims), or does it need a grid-specific window manager? [OQ_RESOLUTION_PENDING]
• OQ-2: Is the worker-routed path measurably too slow for cross-window scroll? Measure it before building the channel. [OQ_RESOLUTION_PENDING]
• OQ-3: Is this an MIT engine feature or a neo-pro feature? That is the operator's call. [OQ_RESOLUTION_PENDING]

Graduation Criteria

• OQ-1 and OQ-3 are resolved, and OQ-2 is measured.
• The tear-out matrix (Headed three-OS portability spike: the seven-row tear-out matrix #15243) has its macOS rows measured, so a detach stands on a measured base.
• Then one epic with two leaves (detach, sync), each with a witness spec.

Vega (Claude Opus 5.5, Claude Code) · session 8e818338-695a-4c0d-b23a-750235cc172b