diff --git a/pkgs/sass_language_services/lib/src/lsp/text_document.dart b/pkgs/sass_language_services/lib/src/lsp/text_document.dart
index f2c6cde..ff3bdb6 100644
--- a/pkgs/sass_language_services/lib/src/lsp/text_document.dart
+++ b/pkgs/sass_language_services/lib/src/lsp/text_document.dart
@@ -115,8 +115,10 @@ class TextDocument {
     }
 
     var line = low - 1;
-    offset =
-        _ensureBeforeEndOfLine(offset: offset, lineOffset: lineOffsets[line]);
+    offset = _ensureBeforeEndOfLine(
+      offset: offset,
+      lineOffset: lineOffsets[line],
+    );
 
     return Position(character: offset - lineOffsets[line], line: line);
   }
@@ -151,8 +153,12 @@ class TextDocument {
             lineOffsets[i + startLine + 1] = addedLineOffsets[i];
           }
         } else {
+          // Avoid going outside the range on weird range inputs.
           lineOffsets.replaceRange(
-              startLine + 1, endLine - startLine, addedLineOffsets);
+            min(startLine + 1, lineOffsets.length),
+            min(endLine + 1, lineOffsets.length),
+            addedLineOffsets,
+          );
         }
 
         var diff = text.length - (endOffset - startOffset);
diff --git a/pkgs/sass_language_services/test/lsp/text_document_test.dart b/pkgs/sass_language_services/test/lsp/text_document_test.dart
index 7849035..da27077 100644
--- a/pkgs/sass_language_services/test/lsp/text_document_test.dart
+++ b/pkgs/sass_language_services/test/lsp/text_document_test.dart
@@ -16,20 +16,25 @@ lsp.Either2<lsp.TextDocumentContentChangeEvent1,
 lsp.Either2<lsp.TextDocumentContentChangeEvent1,
         lsp.TextDocumentContentChangeEvent2>
     updateIncremental(String text, lsp.Range range) {
-  return lsp.Either2.t2(lsp.TextDocumentContentChangeEvent2(text: text));
+  return lsp.Either2.t1(
+    lsp.TextDocumentContentChangeEvent1(
+      text: text,
+      range: range,
+    ),
+  );
 }
 
 lsp.Range forSubstring(TextDocument document, String substring) {
   var i = document.getText().indexOf(substring);
-  return lsp.Range(
-    start: document.positionAt(i),
-    end: document.positionAt(i + substring.length),
-  );
+  var start = document.positionAt(i);
+  var end = document.positionAt(i + substring.length);
+  var range = lsp.Range(start: start, end: end);
+  return range;
 }
 
 lsp.Range afterSubstring(TextDocument document, String substring) {
   var i = document.getText().indexOf(substring);
-  var pos = document.positionAt(i);
+  var pos = document.positionAt(i + substring.length);
   return lsp.Range(start: pos, end: pos);
 }
 
@@ -72,10 +77,7 @@ void main() {
     });
 
     test('multiple lines', () {
-      var content = '''abcde
-fghij
-klmno
-''';
+      var content = 'abcde\nfghij\nklmno\n';
       var document = createDocument(content);
       expect(document.lineCount, equals(4));
 
@@ -201,7 +203,7 @@ klmno
   });
 
   group('incremental updates', () {
-    void checkLineNumbers(TextDocument document) {
+    void expectLineAtOffsets(TextDocument document) {
       // Assuming \n.
       var text = document.getText();
       var characters = text.split('');
@@ -212,10 +214,325 @@ klmno
           expected += 1;
         }
       }
-      expect(document.positionAt(text.length), equals(expected));
+      expect(document.positionAt(text.length).line, equals(expected));
     }
 
-    test('incrementally removing content', () {});
+    test('removing content', () {
+      var document = createDocument('abcde\nfghij\nklmno');
+      expect(document.lineCount, equals(3));
+      expect(document.version, equals(0));
+      expectLineAtOffsets(document);
+      document.update(
+        [updateIncremental('', forSubstring(document, 'abcde'))],
+        1,
+      );
+      expect(document.version, equals(1));
+      expect(document.getText(), equals('\nfghij\nklmno'));
+      expect(document.lineCount, equals(3));
+      expectLineAtOffsets(document);
+    });
+
+    test('removing content over multiple lines', () {
+      var document = createDocument('abcde\nfghij\nklmno\npqrst');
+      expect(document.version, equals(0));
+      expect(document.lineCount, equals(4));
+      expectLineAtOffsets(document);
+      document.update(
+        [updateIncremental('', forSubstring(document, 'fghij\nklmno'))],
+        1,
+      );
+      expect(document.version, equals(1));
+      expect(document.getText(), 'abcde\n\npqrst');
+      expect(document.lineCount, equals(3));
+      expectLineAtOffsets(document);
+    });
+
+    test('adding content', () {
+      var document = createDocument('abcde\nfghij\nklmno');
+      expect(document.lineCount, equals(3));
+      expect(document.version, equals(0));
+      expectLineAtOffsets(document);
+      document.update(
+        [updateIncremental('12345', afterSubstring(document, 'abcde\n'))],
+        1,
+      );
+      expect(document.version, equals(1));
+      expect(document.getText(), equals('abcde\n12345fghij\nklmno'));
+      expect(document.lineCount, equals(3));
+      expectLineAtOffsets(document);
+    });
+
+    test('adding content over multiple lines', () {
+      var document = createDocument('abcde\nfghij\nklmno');
+      expect(document.lineCount, equals(3));
+      expect(document.version, equals(0));
+      expectLineAtOffsets(document);
+      document.update(
+        [
+          updateIncremental(
+            '12345\n67890\n',
+            afterSubstring(document, 'abcde\n'),
+          )
+        ],
+        1,
+      );
+      expect(document.version, equals(1));
+      expect(document.getText(), equals('abcde\n12345\n67890\nfghij\nklmno'));
+      expect(document.lineCount, equals(5));
+      expectLineAtOffsets(document);
+    });
+
+    test('replacing single-line content with more content', () {
+      var document = createDocument('abcde\nfghij\nklmno');
+      expect(document.lineCount, equals(3));
+      expect(document.version, equals(0));
+      expectLineAtOffsets(document);
+      document.update(
+        [
+          updateIncremental(
+            '1234567890',
+            forSubstring(document, 'abcde'),
+          )
+        ],
+        1,
+      );
+      expect(document.version, equals(1));
+      expect(document.getText(), equals('1234567890\nfghij\nklmno'));
+      expect(document.lineCount, equals(3));
+      expectLineAtOffsets(document);
+    });
+
+    test('replacing single-line content with less content', () {
+      var document = createDocument('abcde\nfghij\nklmno');
+      expect(document.lineCount, equals(3));
+      expect(document.version, equals(0));
+      expectLineAtOffsets(document);
+      document.update(
+        [
+          updateIncremental(
+            '1',
+            forSubstring(document, 'abcde'),
+          )
+        ],
+        1,
+      );
+      expect(document.version, equals(1));
+      expect(document.getText(), equals('1\nfghij\nklmno'));
+      expect(document.lineCount, equals(3));
+      expectLineAtOffsets(document);
+    });
+
+    test('replacing single-line content with same amount of characters', () {
+      var document = createDocument('abcde\nfghij\nklmno');
+      expect(document.lineCount, equals(3));
+      expect(document.version, equals(0));
+      expectLineAtOffsets(document);
+      document.update(
+        [
+          updateIncremental(
+            '12345',
+            forSubstring(document, 'abcde'),
+          )
+        ],
+        1,
+      );
+      expect(document.version, equals(1));
+      expect(document.getText(), equals('12345\nfghij\nklmno'));
+      expect(document.lineCount, equals(3));
+      expectLineAtOffsets(document);
+    });
+
+    test('replacing multi-line content with more lines', () {
+      var document = createDocument('abcde\nfghij\nklmno');
+      expect(document.lineCount, equals(3));
+      expect(document.version, equals(0));
+      expectLineAtOffsets(document);
+      document.update(
+        [
+          updateIncremental(
+            '12345\n67890\nABCDE\n',
+            forSubstring(document, 'abcde\n'),
+          )
+        ],
+        1,
+      );
+      expect(document.version, equals(1));
+      expect(document.getText(), equals('12345\n67890\nABCDE\nfghij\nklmno'));
+      expect(document.lineCount, equals(5));
+      expectLineAtOffsets(document);
+    });
+
+    test('replacing multi-line content with fewer lines', () {
+      var document = createDocument('12345\n67890\nABCDE\nfghij\nklmno');
+      expect(document.lineCount, equals(5));
+      expect(document.version, equals(0));
+      expectLineAtOffsets(document);
+      document.update(
+        [
+          updateIncremental(
+            'abcde\n',
+            forSubstring(document, '12345\n67890\nABCDE\n'),
+          )
+        ],
+        1,
+      );
+      expect(document.version, equals(1));
+      expect(document.getText(), equals('abcde\nfghij\nklmno'));
+      expect(document.lineCount, equals(3));
+      expectLineAtOffsets(document);
+    });
+
+    test('replacing multi-line content with same amounts', () {
+      var document = createDocument('12345\n67890\nABCDE\nfghij\nklmno');
+      expect(document.lineCount, equals(5));
+      expect(document.version, equals(0));
+      expectLineAtOffsets(document);
+      document.update(
+        [
+          updateIncremental(
+            'abcde\nFGHJI',
+            forSubstring(document, 'ABCDE\nfghij'),
+          )
+        ],
+        1,
+      );
+      expect(document.version, equals(1));
+      expect(document.getText(), equals('12345\n67890\nabcde\nFGHJI\nklmno'));
+      expect(document.lineCount, equals(5));
+      expectLineAtOffsets(document);
+    });
+
+    test('replace large number of lines', () {
+      var document = createDocument('12345\n67890\nasdf');
+      expect(document.lineCount, equals(3));
+      expect(document.version, equals(0));
+      expectLineAtOffsets(document);
+      var text = '';
+      for (var i = 0; i < 20000; i++) {
+        text += 'asdf\n';
+      }
+      document.update(
+        [updateIncremental(text, forSubstring(document, '67890\n'))],
+        1,
+      );
+      expect(document.version, equals(1));
+      expect(document.getText(), '12345\n${text}asdf');
+      expect(document.lineCount, 20002);
+      expectLineAtOffsets(document);
+    });
+
+    test('several incremental changes', () {
+      var document = createDocument('abcde\nfghij\nklmno');
+      expect(document.lineCount, equals(3));
+      expect(document.version, equals(0));
+      expectLineAtOffsets(document);
+      document.update(
+        [
+          updateIncremental('BCD', forSubstring(document, 'bcd')),
+          updateIncremental('LMN', forSubstring(document, 'lmn')),
+          updateIncremental('GHI', forSubstring(document, 'ghi')),
+        ],
+        1,
+      );
+      expect(document.version, equals(1));
+      expect(document.getText(), equals('aBCDe\nfGHIj\nkLMNo'));
+      expect(document.lineCount, equals(3));
+      expectLineAtOffsets(document);
+    });
+
+    test('append', () {
+      var document = createDocument('abcde');
+      expect(document.lineCount, equals(1));
+      document.update(
+        [
+          updateIncremental(
+            '\nfghij\nklmno',
+            afterSubstring(document, 'abcde'),
+          ),
+        ],
+        1,
+      );
+      expect(document.getText(), equals('abcde\nfghij\nklmno'));
+      expect(document.lineCount, equals(3));
+    });
+
+    test('delete', () {
+      var document = createDocument('abcde\nfghij\nklmno');
+      expect(document.lineCount, equals(3));
+      document.update(
+        [
+          updateIncremental(
+            '',
+            forSubstring(document, 'o'),
+          ),
+        ],
+        1,
+      );
+      expect(document.getText(), equals('abcde\nfghij\nklmn'));
+      expect(document.version, equals(1));
+      expect(document.lineCount, equals(3));
+      expectLineAtOffsets(document);
+      document.update(
+        [
+          updateIncremental(
+            '',
+            forSubstring(document, 'fghij\nklmn'),
+          ),
+        ],
+        2,
+      );
+      expect(document.getText(), equals('abcde\n'));
+      expect(document.version, equals(2));
+      expect(document.lineCount, equals(2));
+      expectLineAtOffsets(document);
+    });
+
+    test('handles weird update ranges', () {
+      var document = createDocument('abcde\nfghij');
+      document.update(
+        [
+          updateIncremental('1234', range(-4, 0, -2, 3)),
+        ],
+        1,
+      );
+      expect(document.getText(), equals('1234abcde\nfghij'));
+
+      document = createDocument('abcde\nfghij');
+      document.update(
+        [
+          updateIncremental('1234', range(-1, 0, 0, 5)),
+        ],
+        1,
+      );
+      expect(document.getText(), equals('1234\nfghij'));
+
+      document = createDocument('abcde\nfghij');
+      document.update(
+        [
+          updateIncremental('1234', range(1, 0, 13, 14)),
+        ],
+        1,
+      );
+      expect(document.getText(), equals('abcde\n1234'));
+
+      document = createDocument('abcde\nfghij');
+      document.update(
+        [
+          updateIncremental('1234', range(13, 0, 35, 14)),
+        ],
+        1,
+      );
+      expect(document.getText(), equals('abcde\nfghij1234'));
+
+      document = createDocument('abcde\nfghij');
+      document.update(
+        [
+          updateIncremental('1234', range(-13, 0, 35, 14)),
+        ],
+        1,
+      );
+      expect(document.getText(), equals('1234'));
+    });
   });
 
   group('applyEdits', () {