fix bug when replacing range starting at end of previous part

src/editor/model.js

@@ -408,16 +408,13 @@ export default class EditorModel {
 
     //mostly internal, called from Range.replace
     replaceRange(startPosition, endPosition, parts) {
+        // convert end position to offset, so it is independent of how the document is split into parts
+        // which we'll change when splitting up at the start position
+        const endOffset = endPosition.asOffset(this);
         const newStartPartIndex = this._splitAt(startPosition);
-        const idxDiff = newStartPartIndex - startPosition.index;
-        // if both position are in the same part, and we split it at start position,
-        // the offset of the end position needs to be decreased by the offset of the start position
-        const removedOffset = startPosition.index === endPosition.index ? startPosition.offset : 0;
-        const adjustedEndPosition = new DocumentPosition(
-            endPosition.index + idxDiff,
-            endPosition.offset - removedOffset,
-        );
-        const newEndPartIndex = this._splitAt(adjustedEndPosition);
+        // convert it back to position once split at start
+        endPosition = endOffset.asPosition(this);
+        const newEndPartIndex = this._splitAt(endPosition);
         for (let i = newEndPartIndex - 1; i >= newStartPartIndex; --i) {
             this._removePart(i);
         }

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);
+    }
+}

src/editor/position.js

@@ -14,6 +14,8 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+import DocumentOffset from "./offset";
+
 export default class DocumentPosition {
     constructor(index, offset) {
         this._index = index;
@@ -104,4 +106,18 @@ export default class DocumentPosition {
             }
         }
     }
+
+    asOffset(model) {
+        if (this.index === -1) {
+            return new DocumentOffset(0, true);
+        }
+        let offset = 0;
+        for (let i = 0; i < this.index; ++i) {
+            offset += model.parts[i].text.length;
+        }
+        offset += this.offset;
+        const lastPart = model.parts[this.index];
+        const atEnd = offset >= lastPart.text.length;
+        return new DocumentOffset(offset, atEnd);
+    }
 }