Prevent Undo from clearing existing note

When loading the first note of a session in the right-hand pane or when
loading a note in the separate window, if you made a change and then
pressed Undo twice (or maybe only once in some situations), it could
undo to empty (though you could usually Redo to restore it).

chrome/content/zotero/bindings/styled-textbox.xml

@@ -605,6 +605,7 @@
 				<![CDATA[
 					if (this._editor) {
 						this._editor.undoManager.clear();
+						this._editor.undoManager.add();
 					}
 				]]>
 				</body>
@@ -690,6 +691,10 @@
 								}
 								if (self._value) {
 									self.value = self._value;
+									
+									// Prevent undoing to empty note after initialization
+									self._editor.undoManager.clear();
+									self._editor.undoManager.add();
 								}
 								if (self._focus) {
 									setTimeout(function () {

chrome/content/zotero/note.js

@@ -43,18 +43,7 @@ async function onLoad() {
 
 	if (itemID) {
 		var ref = await Zotero.Items.getAsync(itemID);
-
-		// If loading new or different note, disable undo while we repopulate the text field
-		// so Undo doesn't end up clearing the field. This also ensures that Undo doesn't
-		// undo content from another note into the current one.
-		let clearUndo = noteEditor.item ? noteEditor.item.id != itemID : false;
-
 		noteEditor.item = ref;
-
-		if (clearUndo) {
-			noteEditor.clearUndo();
-		}
-
 		document.title = ref.getNoteTitle();
 	}
 	else {
@@ -96,12 +85,9 @@ function onUnload() {
 
 var NotifyCallback = {
 	notify: function(action, type, ids){
-		if (noteEditor.item && ids.indexOf(noteEditor.item.id) != -1) {
-			// If the document title hasn't yet been set, reset undo so
-			// undoing to empty isn't possible
+		if (noteEditor.item && ids.includes(noteEditor.item.id)) {
 			var noteTitle = noteEditor.item.getNoteTitle();
 			if (!document.title && noteTitle != '') {
-				noteEditor.clearUndo();
 				document.title = noteTitle;
 			}