fix cursor mark preservation logic

lib/models/transform.js

@@ -198,8 +198,8 @@ class Transform extends new Record(DEFAULT_PROPERTIES) {
     // Apply each of the steps in the transform, arriving at a new state.
     state = steps.reduce((memo, step) => this.applyStep(memo, step), state)
 
-    // If the selection has changed, clear any existing cursor marks.
-    if (state.selection != selection) {
+    // If there are cursor marks and they haven't changed, remove them.
+    if (state.cursorMarks && state.cursorMarks == cursorMarks) {
       state = state.merge({
         cursorMarks: null
       })