Main Thread Addon for High-Performance Grid Drag Scrolling

To further optimize grid scrolling performance on desktop, particularly for mouse users, we propose moving the mouse-based drag-to-scroll logic entirely to the Main Thread via a specialized addon.

Problem: The current App Worker implementation relies on mousemove events crossing the worker boundary. This adds messaging overhead and latency, making the drag-scroll feel "chunky" compared to the smooth, native-feeling touch scroll.

Proposed Solution: Create Neo.main.addon.GridDragScroll, a new Main Thread Addon that handles the drag-to-scroll interaction directly in the UI thread.

Key Features:

1. Direct DOM Manipulation: Listen to mousedown, mousemove (global), and mouseup (global) in the main thread and update scrollLeft/scrollTop directly on the grid elements.
2. Zero Worker Traffic: Eliminates high-frequency mousemove messages to the App Worker during scroll operations.
3. Dynamic Loading: The addon should only be loaded/registered when mouseDragScroll is enabled and the environment is not touch-enabled.
4. Global Drag Tracking: By handling events in the main thread, we can easily attach listeners to document or window to support "over-drag" (scrolling continues even if the mouse leaves the grid bounds), matching the robust behavior of touch scrolling.

Implementation Plan:

1. Create src/main/addon/GridDragScroll.mjs.
2. Implement register({id, bodyId, containerId, ...}) to attach listeners.
3. Implement the drag math (delta tracking) and scroll application.
4. Update src/grid/ScrollManager.mjs to dynamically import and register this addon in afterSetMouseDragScroll (if not touch).
5. Remove the App Worker-side mouse listener logic from ScrollManager.mjs once the addon is proven.

Constraints:

• Must respect Neo.config.hasTouchEvents to avoid conflict with native touch scrolling.
• Needs to handle the mouseDragScroll config toggle dynamically.
• Should be destroyed/unregistered properly to prevent leaks.