Expand description
Cocoon → window.setTextEditorDecorations notification.
Channel-drain batching: ~5-200 calls per extension per second during
scroll; one renderer event per frame (16 ms window, drain stragglers).
Uses Arc<dyn RendererEmitter> captured once from
VineHost::RendererEmitter() so the drain task never holds a reference
to the full host across await points.