fix bug when replacing range starting at end of previous part

src/editor/offset.js

@@ -0,0 +1,26 @@
+/*
+Copyright 2019 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+export default class DocumentOffset {
+    constructor(offset, atEnd) {
+        this.offset = offset;
+        this.atEnd = atEnd;
+    }
+
+    asPosition(model) {
+        return model.positionForOffset(this.offset, this.atEnd);
+    }
+}