diff --git a/pkgs/sass_language_services/test/lsp/text_document_test.dart b/pkgs/sass_language_services/test/lsp/text_document_test.dart
new file mode 100644
index 0000000..a9efad2
--- /dev/null
+++ b/pkgs/sass_language_services/test/lsp/text_document_test.dart
@@ -0,0 +1,277 @@
+import 'package:lsp_server/lsp_server.dart' as lsp;
+import 'package:sass_language_services/sass_language_services.dart';
+import 'package:test/test.dart';
+
+import '../position_utils.dart';
+
+TextDocument createDocument(String content) {
+  return TextDocument(Uri.parse('test://hello/world'), 'text', 0, content);
+}
+
+lsp.Either2<lsp.TextDocumentContentChangeEvent1,
+    lsp.TextDocumentContentChangeEvent2> updateFull(String text) {
+  return lsp.Either2.t2(lsp.TextDocumentContentChangeEvent2(text: text));
+}
+
+lsp.Either2<lsp.TextDocumentContentChangeEvent1,
+        lsp.TextDocumentContentChangeEvent2>
+    updateIncremental(String text, lsp.Range range) {
+  return lsp.Either2.t2(lsp.TextDocumentContentChangeEvent2(text: text));
+}
+
+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),
+  );
+}
+
+lsp.Range afterSubstring(TextDocument document, String substring) {
+  var i = document.getText().indexOf(substring);
+  var pos = document.positionAt(i);
+  return lsp.Range(start: pos, end: pos);
+}
+
+lsp.TextEdit insert(String text, lsp.Position at) {
+  return lsp.TextEdit(newText: text, range: lsp.Range(start: at, end: at));
+}
+
+lsp.TextEdit replace(String text, lsp.Range range) {
+  return lsp.TextEdit(newText: text, range: range);
+}
+
+lsp.TextEdit delete(lsp.Range range) {
+  return lsp.TextEdit(newText: '', range: range);
+}
+
+void main() {
+  group('lines, offsets and positions', () {
+    test('empty content', () {
+      var document = createDocument('');
+
+      expect(document.lineCount, equals(1));
+      expect(document.offsetAt(position(0, 0)), equals(0));
+
+      var pos = document.positionAt(0);
+      expect(pos.line, equals(0));
+      expect(pos.character, equals(0));
+    });
+
+    test('single line', () {
+      var content = 'Hello World';
+      var document = createDocument(content);
+      expect(document.lineCount, equals(1));
+
+      for (var i = 0; i < content.length; i++) {
+        expect(document.offsetAt(position(0, i)), equals(i));
+        var pos = document.positionAt(i);
+        expect(pos.line, equals(0));
+        expect(pos.character, equals(i));
+      }
+    });
+
+    test('multiple lines', () {
+      var content = '''abcde
+fghij
+klmno
+''';
+      var document = createDocument(content);
+      expect(document.lineCount, equals(4));
+
+      for (var i = 0; i < content.length; i++) {
+        var line = (i / 6).floor();
+        var char = i % 6;
+        expect(document.offsetAt(position(line, char)), equals(i));
+
+        var pos = document.positionAt(i);
+        expect(pos.line, equals(line));
+        expect(pos.character, equals(char));
+      }
+
+      // Out of bounds.
+      expect(document.offsetAt(position(3, 0)), content.length);
+      expect(document.offsetAt(position(3, 1)), content.length);
+
+      var pos = document.positionAt(18);
+      expect(pos.line, equals(3));
+      expect(pos.character, equals(0));
+
+      pos = document.positionAt(19);
+      expect(pos.line, equals(3));
+      expect(pos.character, equals(0));
+    });
+
+    test('starts with newline', () {
+      var content = '\nABCDE';
+      var document = createDocument(content);
+      expect(document.lineCount, equals(2));
+    });
+
+    test('newline characters', () {
+      var document = createDocument('\rABCDE');
+      expect(document.lineCount, equals(2));
+      document = createDocument('\nABCDE');
+      expect(document.lineCount, equals(2));
+
+      document = createDocument('\r\nABCDE');
+      expect(document.lineCount, equals(2));
+
+      document = createDocument('\n\nABCDE');
+      expect(document.lineCount, equals(3));
+
+      document = createDocument('\r\rABCDE');
+      expect(document.lineCount, equals(3));
+
+      document = createDocument('\n\rABCDE');
+      expect(document.lineCount, equals(3));
+    });
+
+    test('getText', () {
+      var content = 'abcde\nfghij\nklmno';
+      var document = createDocument(content);
+      expect(document.getText(), equals(content));
+
+      expect(
+        document.getText(range: range(0, 0, 0, 5)),
+        equals('asdf'),
+      );
+      expect(
+        document.getText(range: range(0, 4, 1, 1)),
+        equals('e\nf'),
+      );
+    });
+
+    test('invalid input at beginning of file', () {
+      var document = createDocument('asdf');
+      expect(document.offsetAt(position(-1, 0)), 0);
+      expect(document.offsetAt(position(0, -1)), 0);
+
+      var pos = document.positionAt(-1);
+      expect(pos.line, equals(0));
+      expect(pos.character, equals(0));
+    });
+
+    test('invalid input at end of file', () {
+      var document = createDocument('asdf');
+      expect(document.offsetAt(position(1, 1)), 4);
+
+      var pos = document.positionAt(8);
+      expect(pos.line, equals(0));
+      expect(pos.character, equals(4));
+    });
+
+    test('invalid input at beginning of line', () {
+      var document = createDocument('a\ns\nd\r\nf');
+      expect(document.offsetAt(position(0, -1)), 0);
+      expect(document.offsetAt(position(1, -1)), 2);
+      expect(document.offsetAt(position(2, -1)), 4);
+      expect(document.offsetAt(position(3, -1)), 7);
+    });
+
+    test('invalid input at end of line', () {
+      var document = createDocument('a\ns\nd\r\nf');
+      expect(document.offsetAt(position(0, 10)), 1);
+      expect(document.offsetAt(position(1, 10)), 3);
+      expect(document.offsetAt(position(2, 2)), 5);
+      expect(document.offsetAt(position(2, 3)), 5);
+      expect(document.offsetAt(position(2, 10)), 5);
+      expect(document.offsetAt(position(3, 10)), 8);
+
+      var pos = document.positionAt(6);
+      expect(pos.line, equals(2));
+      expect(pos.character, equals(1));
+    });
+  });
+
+  group('full updates', () {
+    test('one full update', () {
+      var document = createDocument('asdfqwer');
+      document.update([updateFull('hjklyuio')], 1);
+      expect(document.version, equals(1));
+      expect(document.getText(), equals('hjklyuio'));
+    });
+
+    test('several full updates', () {
+      var document = createDocument('asdfqwer');
+      document.update([updateFull('hjklyuio'), updateFull('12345')], 2);
+      expect(document.version, equals(2));
+      expect(document.getText(), equals('12345'));
+    });
+  });
+
+  group('incremental updates', () {
+    void checkLineNumbers(TextDocument document) {
+      // Assuming \n.
+      var text = document.getText();
+      var characters = text.split('');
+      var expected = 0;
+      for (var i = 0; i < text.length; i++) {
+        expect(document.positionAt(i).line, expected);
+        if (characters[i] == '\n') {
+          expected += 1;
+        }
+      }
+      expect(document.positionAt(text.length), equals(expected));
+    }
+
+    test('incrementally removing content', () {});
+  });
+
+  group('applyEdits', () {
+    test('inserts', () {
+      var input = createDocument('asdfasdfasdf');
+      expect(
+        input.applyEdits([insert('Hello', position(0, 0))]),
+        equals('Helloasdfasdfasdf'),
+      );
+      expect(
+        input.applyEdits([insert('Hello', position(0, 1))]),
+        equals('aHellosdfasdfasdf'),
+      );
+      expect(
+        input.applyEdits([
+          insert('Hello', position(0, 1)),
+          insert('World', position(0, 1)),
+        ]),
+        equals('aHelloWorldsdfasdfasdf'),
+      );
+      expect(
+        input.applyEdits([
+          insert('Mint', position(0, 2)),
+          insert('Hello', position(0, 1)),
+          insert('World', position(0, 1)),
+          insert('Jams', position(0, 2)),
+          insert('Casiopea', position(0, 2)),
+        ]),
+        equals('aHelloWorldsMintJamsCasiopeadfasdfasdf'),
+      );
+    });
+
+    test('replace', () {
+      var input = createDocument('0123456789');
+      expect(
+        input.applyEdits([replace('Hello', range(0, 3, 0, 5))]),
+        equals('012Hello56789'),
+      );
+      expect(
+        input.applyEdits([
+          replace('Hello', range(0, 3, 0, 5)),
+          replace('World', range(0, 5, 0, 7)),
+        ]),
+        equals('012HelloWorld789'),
+      );
+    });
+
+    test('mix', () {
+      var input = createDocument('0123456789');
+      expect(
+        input.applyEdits([
+          insert('Jams', position(0, 6)),
+          replace('Mint', range(0, 3, 0, 6)),
+        ]),
+        equals('012MintJams6789'),
+      );
+    });
+  });
+}
diff --git a/pkgs/sass_language_services/test/position_utils.dart b/pkgs/sass_language_services/test/position_utils.dart
index 6cdf787..7e293ea 100644
--- a/pkgs/sass_language_services/test/position_utils.dart
+++ b/pkgs/sass_language_services/test/position_utils.dart
@@ -3,3 +3,14 @@ import 'package:lsp_server/lsp_server.dart' as lsp;
 lsp.Position at({required int line, required int char}) {
   return lsp.Position(character: char, line: line);
 }
+
+lsp.Position position(int line, int char) {
+  return at(line: line, char: char);
+}
+
+lsp.Range range(int startLine, int startChar, int endLine, int endChar) {
+  return lsp.Range(
+    start: at(line: startLine, char: startChar),
+    end: at(line: startLine, char: startChar),
+  );
+}