Add timeline virtualization for large clip collections

[chdenat]

Context

When a timeline contains several hundred or several thousand clips, rendering every element in the DOM significantly increases initial render time, memory usage, and the cost of updates during scrolling or editing.

Requested feature

Add horizontal and vertical clip virtualization so that only clips visible in the viewport, plus an optional configurable overscan area, remain mounted in the DOM.

The solution must preserve the timeline’s current behavior:

• clip positioning and duration display;
• selection, keyboard navigation, context menus, and editing;
• moving, resizing, snapping, and drag-and-drop;
• track and legend synchronization;
• readonly and playback modes;
• public events and controlled snapshots.

Acceptance criteria

• Small timelines keep their current behavior and rendering.
• For timelines with several hundred or several thousand clips, only the elements required for the current viewport are mounted in the DOM.
• Horizontal and vertical scrolling do not rebuild the entire timeline.
• Selecting a clip outside the viewport scrolls it into view before focusing it.
• Moving and resizing remain reliable at the boundaries of the virtualized area.
• Tests cover scrolling, selection, keyboard navigation, drag-and-drop, resizing, readonly mode, and playback.
• A benchmark documents the improvement on representative datasets.
• The behavior for automatic virtualization, explicit configuration, and disabling virtualization is documented.