fix getFragmentAtRange to use paths properly (#2012)

#### Is this adding or improving a _feature_ or fixing a _bug_?

Bug.

#### What's the new behavior?

Fixes `Node.getFragmentAtRange` to use the proper argument signature when calling `Node.splitNode`, and refactors it to use paths for all of its logic.

#### Have you checked that...?

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* [x] The new code matches the existing patterns and styles.
* [x] The tests pass with `yarn test`.
* [x] The linter passes with `yarn lint`. (Fix errors with `yarn prettier`.)
* [x] The relevant examples still work. (Run examples with `yarn watch`.)

#### Does this fix any issues or need any specific reviewers?

Fixes: #2009

packages/slate/src/models/node.js

@@ -673,64 +673,35 @@ class Node {
 
   getFragmentAtRange(range) {
     range = range.normalize(this)
-    if (range.isUnset) return Document.create()
 
+    if (range.isUnset) {
+      return Document.create()
+    }
+
+    const { startPath, startOffset, endPath, endOffset } = range
     let node = this
+    let targetPath = endPath
+    let targetPosition = endOffset
+    let mode = 'end'
 
-    // Make sure the children exist.
-    const { startKey, startOffset, endKey, endOffset } = range
-    const startText = node.assertDescendant(startKey)
-    const endText = node.assertDescendant(endKey)
+    while (targetPath.size) {
+      const index = targetPath.last()
+      node = node.splitNode(targetPath, targetPosition)
+      targetPosition = index + 1
+      targetPath = PathUtils.lift(targetPath)
 
-    // Split at the start and end.
-    let child = startText
-    let previous
-    let parent
-
-    while ((parent = node.getParent(child.key))) {
-      const index = parent.nodes.indexOf(child)
-      const position =
-        child.object == 'text' ? startOffset : child.nodes.indexOf(previous)
-
-      parent = parent.splitNode(index, position)
-      node = node.replaceNode(parent.key, parent)
-      previous = parent.nodes.get(index + 1)
-      child = parent
+      if (!targetPath.size && mode === 'end') {
+        targetPath = startPath
+        targetPosition = startOffset
+        mode = 'start'
+      }
     }
 
-    child = startKey == endKey ? node.getNextText(startKey) : endText
-
-    while ((parent = node.getParent(child.key))) {
-      const index = parent.nodes.indexOf(child)
-      const position =
-        child.object == 'text'
-          ? startKey == endKey ? endOffset - startOffset : endOffset
-          : child.nodes.indexOf(previous)
-
-      parent = parent.splitNode(index, position)
-      node = node.replaceNode(parent.key, parent)
-      previous = parent.nodes.get(index + 1)
-      child = parent
-    }
-
-    // Get the start and end nodes.
-    const startNode = node.getNextSibling(
-      node.getFurthestAncestor(startKey).key
-    )
-    const endNode =
-      startKey == endKey
-        ? node.getNextSibling(
-            node.getNextSibling(node.getFurthestAncestor(endKey).key).key
-          )
-        : node.getNextSibling(node.getFurthestAncestor(endKey).key)
-
-    // Get children range of nodes from start to end nodes
-    const startIndex = node.nodes.indexOf(startNode)
-    const endIndex = node.nodes.indexOf(endNode)
+    const startIndex = startPath.first() + 1
+    const endIndex = endPath.first() + 2
     const nodes = node.nodes.slice(startIndex, endIndex)
-
-    // Return a new document fragment.
-    return Document.create({ nodes })
+    const fragment = Document.create({ nodes })
+    return fragment
   }
 
   /**

packages/slate/test/models/node/get-fragment-at-range.js

@@ -0,0 +1,28 @@
+/** @jsx h */
+
+import h from '../../helpers/h'
+import assert from 'assert'
+
+export default function() {
+  const value = (
+    <value>
+      <document>
+        <paragraph>
+          one <anchor />two<focus /> three
+        </paragraph>
+      </document>
+    </value>
+  )
+
+  const { document, selection } = value
+  const actual = document.getFragmentAtRange(selection)
+  const expected = (
+    <document>
+      <paragraph>two</paragraph>
+    </document>
+  )
+
+  const a = actual.toJSON()
+  const e = expected.toJSON()
+  assert.deepEqual(a, e)
+}