textLinesMutator: Fix removal at the end of the lines array

...in case we are in the middle of a line (curCol !== 0) and we remove
everything up to the end of the line.

Before this commit a string 'undefined' could make it into the splice.

Author: webzwo0i

src/static/js/Changeset.js

@@ -908,20 +908,24 @@ class TextLinesMutator {
     let removed = '';
     if (this._isCurLineInSplice()) {
       if (this._curCol === 0) {
+        // First line to be removed is in splice.
         removed = this._curSplice[this._curSplice.length - 1];
         this._curSplice.length--;
+        // Next lines to be removed are not in splice.
         removed += nextKLinesText(L - 1);
         this._curSplice[1] += L - 1;
       } else {
         removed = nextKLinesText(L - 1);
         this._curSplice[1] += L - 1;
         const sline = this._curSplice.length - 1;
         removed = this._curSplice[sline].substring(this._curCol) + removed;
-        this._curSplice[sline] = this._curSplice[sline].substring(0, this._curCol) +
-            this._linesGet(this._curSplice[0] + this._curSplice[1]);
-        this._curSplice[1] += 1;
+        // Is there a line left?
+        const remaining = this._linesGet(this._curSplice[0] + this._curSplice[1]) || '';
+        this._curSplice[sline] = this._curSplice[sline].substring(0, this._curCol) + remaining;
+        this._curSplice[1] += remaining ? 1 : 0;
       }
     } else {
+      // Nothing that is removed is in splice. Implies curCol === 0.
       removed = nextKLinesText(L);
       this._curSplice[1] += L;
     }

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

@@ -137,6 +137,29 @@ describe('easysync-mutations', function () {
     ['skip', 1, 1, true],
   ], ['banana\n', 'cabbage\n', 'duffle\n']);
 
+  runMutationTest(8, ['\n', 'fun\n', '\n'], [
+    ['remove', 1, 1, '\n'],
+    ['skip', 3, 0, false],
+    ['remove', 2, 2, '\n\n'],
+    ['insert', 'c'],
+  ], ['func']);
+
+  runMutationTest(9, ['\n', 'fun\n', '\n'], [
+    ['remove', 1, 1, '\n'],
+    ['skip', 3, 0, false],
+    ['remove', 2, 2, '\n\n'],
+    ['insert', 'c'],
+    ['insert', 'a\n', 1],
+    ['insert', 'c'],
+  ], ['funca\n', 'c']);
+
+  runMutationTest(10, ['\n', 'fun\n', '\n'], [
+    ['remove', 1, 1, '\n'],
+    ['skip', 2, 0, false],
+    ['remove', 3, 2, 'n\n\n'],
+    ['insert', 'z'],
+  ], ['fuz']);
+
   it('mutatorHasMore', async function () {
     const lines = ['1\n', '2\n', '3\n', '4\n'];
     let mu;