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

@@ -982,11 +982,17 @@ 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;