Avoid mutating the selection in Transforms.setPoint (#3541)

Since Immer freezes the selection, we will get a TypeError if we try to mutate
the selection. Also, there's no need to mutate it directly in
Transforms.setPoint.

packages/slate/src/transforms/selection.ts

@@ -157,13 +157,10 @@ export const SelectionTransforms = {
 
     const { anchor, focus } = selection
     const point = edge === 'anchor' ? anchor : focus
-    const newPoint = Object.assign(point, props)
 
-    if (edge === 'anchor') {
-      Transforms.setSelection(editor, { anchor: newPoint })
-    } else {
-      Transforms.setSelection(editor, { focus: newPoint })
-    }
+    Transforms.setSelection(editor, {
+      [edge === 'anchor' ? 'anchor' : 'focus']: { ...point, ...props },
+    })
   },
 
   /**

packages/slate/test/transforms/setPoint/offset.js

@@ -0,0 +1,28 @@
+/** @jsx jsx */
+
+import { Editor, Transforms } from 'slate'
+import { jsx } from '../..'
+
+export const run = editor => {
+  Transforms.move(editor)
+  Transforms.setPoint(editor, { offset: 0 }, { edge: 'focus' })
+}
+
+export const input = (
+  <editor>
+    <block>
+      f<cursor />
+      oo
+    </block>
+  </editor>
+)
+
+export const output = (
+  <editor>
+    <block>
+      <focus />
+      fo
+      <anchor />o
+    </block>
+  </editor>
+)