diff --git a/Package.swift b/Package.swift
index 452e602..e49b1ef 100644
--- a/Package.swift
+++ b/Package.swift
@@ -62,6 +62,10 @@ let package = Package(
         .testTarget(
             name: "SaplingCoreTests",
             dependencies: ["SaplingCore"]
+        ),
+        .testTarget(
+            name: "SaplingEditorTests",
+            dependencies: ["SaplingCore", "SaplingEditor"]
         )
     ]
 )
diff --git a/Tests/SaplingEditorTests/EditorStateTests.swift b/Tests/SaplingEditorTests/EditorStateTests.swift
new file mode 100644
index 0000000..1fb9807
--- /dev/null
+++ b/Tests/SaplingEditorTests/EditorStateTests.swift
@@ -0,0 +1,59 @@
+import XCTest
+import SaplingCore
+@testable import SaplingEditor
+
+final class EditorStateTests: XCTestCase {
+    func testActiveLineFollowsSelection() {
+        let source = "# Heading\nRendered **line**\nRaw source line"
+        let document = EditorDocument(
+            url: URL(fileURLWithPath: "/tmp/EditorStateTests.md"),
+            title: "EditorStateTests",
+            source: source
+        )
+        var state = EditorState(document: document)
+
+        let secondLineLocation = (source as NSString).range(of: "Rendered").location
+        state.updateSelection(EditorSelection(location: secondLineLocation, length: 0))
+
+        XCTAssertEqual(state.activeLineIndex, 1)
+        XCTAssertEqual(state.lines[0].mode, .rendered)
+        XCTAssertEqual(state.lines[1].mode, .source)
+        XCTAssertEqual(state.lines[2].mode, .rendered)
+    }
+
+    func testUpdatingSourceTracksUnsavedChanges() {
+        let document = EditorDocument(
+            url: URL(fileURLWithPath: "/tmp/EditorStateTests.md"),
+            title: "EditorStateTests",
+            source: "Initial"
+        )
+        var state = EditorState(document: document)
+
+        state.updateSource("Initial\nChanged")
+
+        XCTAssertTrue(state.hasUnsavedChanges)
+        XCTAssertEqual(state.lines.count, 2)
+
+        state.markSaved()
+
+        XCTAssertFalse(state.hasUnsavedChanges)
+    }
+
+    @MainActor
+    func testViewModelSavesDocumentToDisk() throws {
+        let directory = FileManager.default.temporaryDirectory
+            .appendingPathComponent(UUID().uuidString, isDirectory: true)
+        try FileManager.default.createDirectory(at: directory, withIntermediateDirectories: true)
+        let url = directory.appendingPathComponent("Note.md")
+        try "# Note\n".write(to: url, atomically: true, encoding: .utf8)
+
+        let document = try HybridMarkdownEditorViewModel.loadDocument(at: url)
+        let viewModel = HybridMarkdownEditorViewModel(document: document)
+        viewModel.updateSource("# Note\n\nUpdated")
+        try viewModel.save()
+
+        let saved = try String(contentsOf: url, encoding: .utf8)
+        XCTAssertEqual(saved, "# Note\n\nUpdated")
+        XCTAssertFalse(viewModel.hasUnsavedChanges)
+    }
+}