insertChunk(chunk, before) {
		const oldIdx = this.chunks.indexOf(chunk);
		const idx = this.chunks.indexOf(before);
		if (idx < 0) {
			throw new Error("before chunk not found");
		}
		if (oldIdx >= 0 && oldIdx > idx) {
			this.chunks.splice(oldIdx, 1);
			this.chunks.splice(idx, 0, chunk);
		} else if (oldIdx < 0) {
			this.chunks.splice(idx, 0, chunk);
			return true;
		}
		return false;
	}
inserts a chunk before another existing chunk in group
@param {Chunk} chunk Chunk being inserted
@param {Chunk} before Placeholder/target chunk marking new chunk insertion point
@returns {boolean} return true if insertion was successful