textLinesMutator: Fix insertions at the end of `lines`

The new insertions are directly pushed to curSplice now instead of
trying to incorporate an undefined line into the splice, which would
result in an error: TypeError: Cannot read property 'substring' of
undefined

src/static/js/Changeset.js

@@ -983,11 +983,17 @@ class TextLinesMutator {
         this._curSplice.push(...newLines);
         this._curLine += newLines.length;
       }
-    } else {
+    } else if (!this.hasMore()) {
       // There are no additional lines. Although the line is put into splice, curLine is not
-      // increased because there may be more chars in the line (newline is not reached).
+      // increased because there may be more chars in the line (newline is not reached). We are
+      // inserting at the end of lines. curCol is 0 as curLine is not in splice.
+      this._curSplice.push(text);
+      this._curCol += text.length;
+    } else {
+      // insert text after curCol
       const sline = this._putCurLineInSplice();
       if (!this._curSplice[sline]) {
+        // TODO should never happen now
         const err = new Error(
             'curSplice[sline] not populated, actual curSplice contents is ' +
             `${JSON.stringify(this._curSplice)}. Possibly related to ` +

src/tests/frontend/specs/easysync-mutations.js

@@ -160,6 +160,25 @@ describe('easysync-mutations', function () {
     ['insert', 'z'],
   ], ['fuz']);
 
+  // #2836, #5214, #3560 regressions
+  runMutationTest(11, ['\n'], [
+    ['remove', 1, 1, '\n'],
+    ['insert', 'c', 0],
+  ], ['c']);
+
+  runMutationTest(12, ['\n'], [
+    ['remove', 1, 1, '\n'],
+    ['insert', 'a\n', 1],
+    ['insert', 'c'],
+  ], ['a\n', 'c']);
+
+  runMutationTest(13, ['\n', 'fun\n', '\n'], [
+    ['remove', 1, 1, '\n'],
+    ['skip', 4, 1, false],
+    ['remove', 1, 1, '\n'],
+    ['insert', 'c'],
+  ], ['fun\n', 'c']);
+
   it('mutatorHasMore', async function () {
     const lines = ['1\n', '2\n', '3\n', '4\n'];
     let mu;