Allow editor-less documents to combine +-origin history events

Closes codemirror#5316

Author: marijnh

src/model/history.js

@@ -67,7 +67,7 @@ export function addChangeToHistory(doc, change, selAfter, opId) {
 
   if ((hist.lastOp == opId ||
        hist.lastOrigin == change.origin && change.origin &&
-       ((change.origin.charAt(0) == "+" && doc.cm && hist.lastModTime > time - doc.cm.options.historyEventDelay) ||
+       ((change.origin.charAt(0) == "+" && hist.lastModTime > time - (doc.cm ? doc.cm.options.historyEventDelay : 500)) ||
         change.origin.charAt(0) == "*")) &&
       (cur = lastChangeEvent(hist, hist.lastOp == opId))) {
     // Merge this change into the last event