Fix: Undo/redo cursor position fixed (#1885)

lib/src/models/documents/history.dart

@@ -91,13 +91,12 @@ class History {
     }
     final delta = source.removeLast();
     // look for insert or delete
-    int? len = 0;
+    var len = 0;
     final ops = delta.toList();
     for (var i = 0; i < ops.length; i++) {
-      if (ops[i].key == Operation.insertKey) {
-        len = ops[i].length;
-      } else if (ops[i].key == Operation.deleteKey) {
-        len = ops[i].length! * -1;
+      if ((ops[i].key == Operation.insertKey) ||
+          (ops[i].key == Operation.retainKey)) {
+        len += ops[i].length ?? 0;
       }
     }
     final base = Delta.from(doc.toDelta());

lib/src/models/structs/history_changed.dart

@@ -8,5 +8,5 @@ class HistoryChanged {
   );
 
   final bool changed;
-  final int? len;
+  final int len;
 }

lib/src/widgets/quill/quill_controller.dart

@@ -215,26 +215,13 @@ class QuillController extends ChangeNotifier {
     }
   }
 
-  void _handleHistoryChange(int? len) {
-    // move cursor according to the length inserted or deleted from redo or undo
-    // operation. len is the length inserted or deleted.
-    if (len! != 0) {
-      // if (this.selection.extentOffset >= document.length) {
-      // // cursor exceeds the length of document, position it in the end
-      // updateSelection(
-      // TextSelection.collapsed(offset: document.length), ChangeSource.LOCAL);
-      updateSelection(
-        (selection.baseOffset + len) > 0
-            ? TextSelection.collapsed(
-                offset: selection.baseOffset + len,
-              )
-            : TextSelection.collapsed(offset: document.length),
-        ChangeSource.local,
-      );
-    } else {
-      // no need to move cursor
-      notifyListeners();
-    }
+  void _handleHistoryChange(int len) {
+    updateSelection(
+      TextSelection.collapsed(
+        offset: len,
+      ),
+      ChangeSource.local,
+    );
   }
 
   void redo() {