One theory about how to compute the edit context given a selection in XD

computeEditContextRoot.js

import { Artboard, Group } from 'scenegraph'

// Strictly speaking, promoteArtboards means follow the spec.
// For some usage, we don't want that behavior.
function computeEditContextRoot(selection, promoteArtboards = true) {
  // Number of nodes above the given node in the artwork tree.
  function computeDepthToRoot(node) {
    let depth = 0
    while (node.parent != null) {
      node = node.parent
      ++depth
    }
    return depth
  }

  // If no selection, no edit context.
  if (selection.items.length == 0) return null
  // contextNode is the eventual edit context root.
  let contextNode, contextDepth
  selection.items.forEach((node, idx) => {
    if (idx == 0) {
      const origNode = node
      // Prime the pump on the first iteration.
      contextNode = node.parent
      contextDepth = computeDepthToRoot(contextNode)
    } else {
      const origNode = node
      node = node.parent
      // Pull the deeper one up to the shallower's level,
      // and then search up the tree on both sides
      // for a common ancestor. We'll eventually hit
      // the root together, as a termination guarantee.
      let nodeDepth = computeDepthToRoot(node)
      while (nodeDepth > contextDepth) {
        node = node.parent
        --nodeDepth
      }
      while (nodeDepth < contextDepth) {
        contextNode = contextNode.parent
        --contextDepth
      }
      while (contextNode != node) {
        contextNode = contextNode.parent
        node = node.parent
        --contextDepth
        --nodeDepth
      }
    }
  })
  // Special case: if the edit context root would be an Artboard,
  // the result is promoted up to the document root.
  if (promoteArtboards && contextNode instanceof Artboard)
    contextNode = contextNode.parent
  return contextNode
}

export default computeEditContextRoot