Migrate collaborative editing from simple diffing to CRDT

[tnkuehne]

Replace the simple character-by-character diffing approach with Yjs CRDT
for more robust collaborative editing:

• Add Yjs, y-protocols, and y-codemirror.next dependencies
• Rewrite CollaborativeDocument Durable Object to use Yjs sync protocol
• Update CodeMirror component to use yCollab extension for collaboration
• Create YjsWebSocketProvider for client-side Yjs sync
• Remove manual cursor/selection tracking (now handled by Yjs awareness)
• Update page component to use Yjs document and awareness

Benefits:

• Automatic conflict resolution instead of "last write wins"
• Built-in offline support capability
• Better scalability with many concurrent editors
• Industry-proven approach (used by Notion, Linear, etc.)

---

Summary by cubic

Migrate collaborative editing to Yjs CRDT for conflict-free syncing, real-time awareness, and better scalability. Replaces custom diffs and manual cursor tracking with an industry-standard approach.

• New Features

  • Server: Durable Object now speaks Yjs sync/awareness over WebSocket, persists a Yjs state blob, and checkpoints every 30s; fixes include protocol-driven awareness cleanup, race-free init with a documentInitialized flag, and randomUUID connection IDs.
  • Client: New YjsWebSocketProvider and y-codemirror.next yCollab integration; cursors and selections are handled by Yjs awareness; collaborative mode only enables when both yText and awareness are provided.
  • UI: Blob page moves from bespoke WS/diffing to a shared Y.Doc; user colors via awareness; content updates flow through Y.Text.
  • Added dependencies: yjs, y-protocols, y-codemirror.next, lib0.

• Migration

  • CodeMirror.svelte API change: remove remoteCursors/remoteSelections props and pass yText and awareness instead.

^{Written for commit 58d90b0. Summary will update on new commits.}

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	gether	58d90b0	Commit Preview URL Branch Preview URL	Jan 10 2026, 11:23 PM

[cubic-dev-ai]

5 issues found across 8 files

Prompt for AI agents (all issues)

Check if these issues are valid — if so, understand the root cause of each and fix them.


<file name="packages/ui/src/lib/components/editor/CodeMirror.svelte">

<violation number="1" location="packages/ui/src/lib/components/editor/CodeMirror.svelte:53">
P1: The `isCollaborative` derived state only checks for `yText`, but the yCollab extension requires both `yText` AND `awareness`. If `yText` is provided without `awareness`, the editor will be non-functional: collaborative sync won't work (yCollab not added), and manual value sync is also disabled.</violation>
</file>

<file name="packages/collab/src/index.ts">

<violation number="1" location="packages/collab/src/index.ts:28">
P3: The `clientIdToWebSocket` map is populated but never read. This appears to be dead code. Either remove it or document its intended use.</violation>

<violation number="2" location="packages/collab/src/index.ts:114">
P2: The client ID generation formula `this.doc.clientID + this.ctx.getWebSockets().length` is prone to collisions. Two simultaneous connections could get the same ID, and IDs can be reused after disconnections. Consider using `crypto.randomUUID()` or an incrementing counter stored in the DO state for unique IDs.</violation>

<violation number="3" location="packages/collab/src/index.ts:129">
P2: Race condition: The initial content check `text.length === 0 && initialContent` is not protected by `blockConcurrencyWhile`. If multiple clients connect simultaneously with initial content, they could all see the document as empty and insert duplicate content.</violation>

<violation number="4" location="packages/collab/src/index.ts:247">
P1: The `removeAwarenessStates` call uses server-assigned `clientId` which doesn't match the actual awareness client IDs. Awareness states are keyed by each client's own Y.Doc clientID (set on the client side), not the server-assigned ID. This will fail to clean up stale awareness states, causing ghost cursors/presence for disconnected users. You need to track the client's actual awareness clientID (received in awareness messages) or query `awareness.getStates()` to find which clientId to remove.</violation>
</file>

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