fixup! textLinesMutator: Fix insertions with newlines at the end of a line

In such cases the remaining part of the old line is directly pushed to
the splice but we need to ensure it is not an empty string.
Before this commit an empty string was pushed to the splice.

src/static/js/Changeset.js

@@ -908,8 +908,10 @@ 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 {
@@ -917,11 +919,13 @@ class TextLinesMutator {
         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;
     }
@@ -973,17 +977,24 @@ class TextLinesMutator {
         this._curLine += newLines.length;
         // insert the remaining chars from the "old" line (e.g. the line we were in
         // when we started to insert new lines)
-        this._curSplice.push(theLine.substring(lineCol));
-        this._curCol = 0; // TODO(doc) why is this not set to the length of last line?
+        const remaining = theLine.substring(lineCol);
+        if (remaining !== '') this._curSplice.push(remaining);
+        this._curCol = 0;
       } else {
         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-assembler.js

@@ -92,6 +92,50 @@ describe('easysync-assembler', function () {
     expect(assem.toString()).to.equal('-k');
   });
 
+  it('smartOpAssembler append - op with text', async function () {
+    const assem = Changeset.smartOpAssembler();
+    const pool = poolOrArray([
+      'attr1,1',
+      'attr2,2',
+      'attr3,3',
+      'attr4,4',
+      'attr5,5',
+    ]);
+
+    padutils.warnDeprecated.disabledForTestingOnly = true;
+    try {
+      assem.appendOpWithText('-', 'test', '*3*4*5', pool);
+      assem.appendOpWithText('-', 'test', '*3*4*5', pool);
+      assem.appendOpWithText('-', 'test', '*1*4*5', pool);
+    } finally {
+      delete padutils.warnDeprecated.disabledForTestingOnly;
+    }
+    assem.endDocument();
+    expect(assem.toString()).to.equal('*3*4*5-8*1*4*5-4');
+  });
+
+  it('smartOpAssembler append - op with multiline text', async function () {
+    const assem = Changeset.smartOpAssembler();
+    const pool = poolOrArray([
+      'attr1,1',
+      'attr2,2',
+      'attr3,3',
+      'attr4,4',
+      'attr5,5',
+    ]);
+
+    padutils.warnDeprecated.disabledForTestingOnly = true;
+    try {
+      assem.appendOpWithText('-', 'test\ntest', '*3*4*5', pool);
+      assem.appendOpWithText('-', '\ntest\n', '*3*4*5', pool);
+      assem.appendOpWithText('-', '\ntest', '*1*4*5', pool);
+    } finally {
+      delete padutils.warnDeprecated.disabledForTestingOnly;
+    }
+    assem.endDocument();
+    expect(assem.toString()).to.equal('*3*4*5|3-f*1*4*5|1-1*1*4*5-4');
+  });
+
   it('smartOpAssembler append + op with text', async function () {
     const assem = Changeset.smartOpAssembler();
     const pool = poolOrArray([

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

@@ -137,6 +137,54 @@ 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']);
+
+  // #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']);
+
+  runMutationTest(14, ['\n'], [
+    ['remove', 1, 1, '\n'],
+    ['insert', 'a'],
+    ['insert', 'c\n', 1],
+  ], ['ac\n']);
+
   it('mutatorHasMore', async function () {
     const lines = ['1\n', '2\n', '3\n', '4\n'];
     let mu;