From 5a2a63e2bb80d4adf374752a1bb31a9570caff19 Mon Sep 17 00:00:00 2001
From: Frederic Cabassut <cabassut.frederic@gmail.com>
Date: Sun, 22 Mar 2026 18:51:14 +0100
Subject: [PATCH] fix: prevent 3-way merge from reverting sequential
 single-user edits

The server's 3-way content merge was incorrectly triggering on sequential
edits by the same user. When a user checked then unchecked a checklist
item, the merge saw local === base (both unchecked) and kept the server's
checked state, silently discarding the uncheck.

Now the merge only runs when there's an actual concurrent edit
(baseVersion < existing.version), meaning another user saved between the
client's load and this save. For single-user sequential edits, the
client's content is used directly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
---
 src/lib/server/notes-service.ts | 29 ++++++++++++++---------------
 tests/e2e/checklist.spec.ts     | 25 +++++++++++++++++++++++++
 2 files changed, 39 insertions(+), 15 deletions(-)

diff --git a/src/lib/server/notes-service.ts b/src/lib/server/notes-service.ts
index b610c57..3fc8633 100644
--- a/src/lib/server/notes-service.ts
+++ b/src/lib/server/notes-service.ts
@@ -238,21 +238,20 @@ export function updateNote(db: Db, userId: number, id: string, input: UpdateNote
 
 	if (input.title !== undefined) { sharedUpdates.title = input.title; hasSharedUpdates = true; }
 	if (input.content !== undefined) {
-		// 3-way merge to preserve concurrent edits from other users
-		if (input.content !== existing.content) {
-			const baseSnapshot = input.baseVersion !== undefined
-				? db.select({ content: noteVersions.content })
-					.from(noteVersions)
-					.where(and(eq(noteVersions.noteId, id), lte(noteVersions.version, input.baseVersion)))
-					.orderBy(desc(noteVersions.version))
-					.limit(1)
-					.get()
-				: db.select({ content: noteVersions.content })
-					.from(noteVersions)
-					.where(eq(noteVersions.noteId, id))
-					.orderBy(desc(noteVersions.version))
-					.limit(1)
-					.get();
+		// 3-way merge to preserve concurrent edits from other users.
+		// Only attempt merge when there's a concurrent edit: the client's
+		// baseVersion is behind the current version, meaning someone else
+		// saved between the client's load and this save.
+		const hasConcurrentEdit = input.baseVersion !== undefined
+			&& input.baseVersion < existing.version;
+
+		if (hasConcurrentEdit && input.content !== existing.content) {
+			const baseSnapshot = db.select({ content: noteVersions.content })
+				.from(noteVersions)
+				.where(and(eq(noteVersions.noteId, id), lte(noteVersions.version, input.baseVersion!)))
+				.orderBy(desc(noteVersions.version))
+				.limit(1)
+				.get();
 
 			if (baseSnapshot && baseSnapshot.content !== existing.content) {
 				sharedUpdates.content = mergeContent(baseSnapshot.content, input.content, existing.content);
diff --git a/tests/e2e/checklist.spec.ts b/tests/e2e/checklist.spec.ts
index 47ead8e..0f4c9a5 100644
--- a/tests/e2e/checklist.spec.ts
+++ b/tests/e2e/checklist.spec.ts
@@ -60,6 +60,31 @@ test.describe('Checklist', () => {
 		await expect(page.getByTestId('checklist-done-checkbox').first()).toBeChecked();
 	});
 
+	test('Scenario: Sequential check and uncheck both persist correctly', async ({ authenticatedPage: page }) => {
+		// Given a checklist note with an item "Buy milk" exists
+		await createChecklistNote(page, 'Tasks2', ['Buy milk']);
+
+		// When the user checks the item and closes
+		await noteCard(page, 'Tasks2').click();
+		await page.getByTestId('checklist-checkbox').first().click();
+		await expect(page.getByTestId('checklist-toggle-done')).toContainText('1 done');
+		await page.getByTestId('close-editor-btn').click();
+
+		// Then the checked state persists
+		await noteCard(page, 'Tasks2').click();
+		await expect(page.getByTestId('checklist-done-checkbox').first()).toBeChecked();
+
+		// When the user unchecks the item from the done section and closes
+		await page.getByTestId('checklist-done-checkbox').first().click();
+		await expect(page.getByTestId('checklist-checkbox')).toHaveCount(1);
+		await page.getByTestId('close-editor-btn').click();
+
+		// Then the unchecked state also persists (not reverted by 3-way merge)
+		await noteCard(page, 'Tasks2').click();
+		await expect(page.getByTestId('checklist-checkbox')).toHaveCount(1);
+		await expect(page.getByTestId('checklist-input').first()).toHaveValue('Buy milk');
+	});
+
 	test('Scenario: Enter key adds a new checklist item', async ({ authenticatedPage: page }) => {
 		// Given a checklist with one item
 		await page.getByTestId('new-note-btn').click();