tests: Add regression tests for #2836

They test if it is possible to delete lines from the array that is
mutated by a changeset, and after no line is left insert some
characters (with or without new lines).

Author: webzwo0i

src/tests/frontend/specs/easysync.js

@@ -178,6 +178,17 @@ describe('easysync', function () {
     ['skip', 1, 1, true],
   ], ['banana\n', 'cabbage\n', 'duffle\n']);
 
+  // #2836 regressions
+  runMutationTest(8, ['\n'], [
+    ['remove', 1, 1, '\n'],
+    ['insert', 'c', 0],
+  ], ['c']);
+  runMutationTest(9, ['\n'], [
+    ['remove', 1, 1, '\n'],
+    ['insert', 'a'],
+    ['insert', 'c\n', 1],
+  ], ['ac\n']);
+
   const poolOrArray = (attribs) => {
     if (attribs.getAttrib) {
       return attribs; // it's already an attrib pool