fix(sync): no conflicts with one's own text, one copy per conflict

- Leaving the tab sent pending text with keepalive but kept it queued;
  typing on folded it with newer keystrokes under the old base version,
  which then conflicted with the user's own text. The flushed batch now
  becomes the batch in flight.
- After a conflict, the focused field still showed the losing text and
  every further batch made another copy. Typing on goes into the one
  copy until the cursor leaves; on leaving, the field shows the block's
  current text, and entering a block bases edits on what it shows.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/lib/docSync/__tests__/engine.test.ts

@@ -358,6 +358,58 @@ describe('DocSyncEngine', () => {
     expect(contents(engine)).toEqual(['Schreibe']);
   });
 
+  it('typing on after leaving the tab causes no conflict with oneself', async () => {
+    const server = new FakeServer();
+    const engine = await startEngine(new FakeNet(server));
+    engine.focusBlock('b1');
+    edit(engine, (b) => [{ ...b[0], content: 'Start und' }]);
+    // Tab hidden (window switch, app switch on the phone): pending text goes out with keepalive
+    (engine as any).flushOnExit();
+    await new Promise((r) => setTimeout(r, 0));
+    expect(serverContents(server)).toEqual(['Start und']);
+
+    // Back in the tab, typing on in the same block
+    edit(engine, (b) => [{ ...b[0], content: 'Start und weiter' }]);
+    await engine.syncNow();
+    edit(engine, (b) => [{ ...b[0], content: 'Start und weiter geht es' }]);
+    await engine.syncNow();
+
+    expect(engine.getState().conflicts).toEqual([]);
+    expect(serverContents(server)).toEqual(['Start und weiter geht es']);
+  });
+
+  it('typing on in a block after a conflict updates the one conflict copy', async () => {
+    const server = new FakeServer();
+    const a = await startEngine(new FakeNet(server));
+    const b = await startEngine(new FakeNet(server));
+
+    a.focusBlock('b1');
+    edit(b, (blocks) => [{ ...blocks[0], content: 'B hat geändert' }]);
+    await b.syncNow();
+    await a.syncNow(); // the focused field keeps showing A's own text
+
+    a.applyFlat([{ id: 'b1', type: 'bullet', content: 'Start + A', level: 0 }]);
+    await a.syncNow(); // conflict: A's text becomes a copy
+    // A keeps typing in the same field, which still shows A's text
+    a.applyFlat([{ ...flatOf(a)[0], content: 'Start + A tippt' }, flatOf(a)[1]]);
+    await a.syncNow();
+    a.applyFlat([{ ...flatOf(a)[0], content: 'Start + A tippt weiter' }, flatOf(a)[1]]);
+    await a.syncNow();
+    await b.syncNow();
+
+    expect(a.getState().conflicts).toHaveLength(1);
+    expect(serverContents(server)).toEqual(['B hat geändert', 'Start + A tippt weiter']);
+    expect(contents(b)).toEqual(['B hat geändert', 'Start + A tippt weiter']);
+
+    // Once the cursor left, the field shows B's version again: editing it changes the block itself
+    a.focusBlock(null);
+    a.focusBlock('b1');
+    edit(a, (blocks) => [{ ...blocks[0], content: 'B hat geändert, A ergänzt' }, blocks[1]]);
+    await a.syncNow();
+    expect(a.getState().conflicts).toHaveLength(1);
+    expect(serverContents(server)).toEqual(['B hat geändert, A ergänzt', 'Start + A tippt weiter']);
+  });
+
   it('viewers cannot change anything', async () => {
     const server = new FakeServer();
     const viewer = await startEngine(new FakeNet(server, 'viewer'));

frontend/src/lib/docSync/engine.ts

@@ -140,6 +140,10 @@ export class DocSyncEngine {
   // Version of each block the editor last showed in agreement with the engine. The focused text field does
   // not take over remote changes while typing, so its edits are based on this version, not the newest one.
   private uiSeen = new Map<string, number>();
+  // Focused block -> its conflict copy: the field still shows the user's losing text, so typing on there
+  // goes into that copy instead of making a new copy per keystroke batch. Ends when the field shows the
+  // engine's text again or the cursor leaves the block.
+  private conflictRedirects = new Map<string, string>();
   private undoStack: UndoStep[] = [];
   private redoStack: UndoStep[] = [];
 
@@ -267,15 +271,19 @@ export class DocSyncEngine {
     }
     for (const b of blocks) {
       const cur = this.local.blocks.get(b.id);
-      if (cur && !cur.deleted && cur.content === (b.content || '')) this.uiSeen.set(b.id, cur.version);
+      if (cur && !cur.deleted && cur.content === (b.content || '')) {
+        this.uiSeen.set(b.id, cur.version);
+        this.conflictRedirects.delete(b.id);
+      }
     }
     const ops = diffFlat(this.local, blocks, () => this.newOpId());
     if (ops.length === 0) return;
     const before = this.local;
     const t = this.now();
     for (const op of ops) {
-      if ((op.kind === 'insert' || op.kind === 'set_content') && op.blockId) this.activity.set(op.blockId, t);
       if (op.kind === 'set_content' && this.uiSeen.has(op.blockId!)) op.baseVersion = this.uiSeen.get(op.blockId!);
+      if (op.kind === 'set_content') this.redirectToConflictCopy(op, this.local);
+      if ((op.kind === 'insert' || op.kind === 'set_content') && op.blockId) this.activity.set(op.blockId, t);
     }
     this.enqueue(ops);
     this.record(before, ops);
@@ -350,7 +358,12 @@ export class DocSyncEngine {
     const t = this.now();
     const changed = blockId !== this.focused;
     if (this.focused) this.activity.set(this.focused, t);
+    if (changed && this.focused) this.conflictRedirects.delete(this.focused);
     this.focused = blockId;
+    // Entering a block, its field shows the engine's text (unfocused fields take it over), so edits from
+    // here on build on that version, not on one an earlier visit of the field saw
+    const entered = changed && blockId ? this.local.blocks.get(blockId) : undefined;
+    if (entered && !entered.deleted) this.uiSeen.set(entered.id, entered.version);
     if (blockId) this.activity.set(blockId, t);
     // Announce the lease right away: with push the next regular sync may be up to 30 s away
     if (changed && blockId && this.canEdit && this.collaborative()) {
@@ -387,7 +400,7 @@ export class DocSyncEngine {
         ? this.status === 'error'
           ? this.pollDelay()
           : this.nextBackoff()
-        : this.syncAgain || this.pending.length > 0
+        : this.syncAgain || this.pending.length > 0 || this.inflight
         ? 0
         : this.pollDelay();
       this.syncAgain = false;
@@ -462,6 +475,19 @@ export class DocSyncEngine {
     return { ok: true };
   }
 
+  /** Sends a text change into the conflict copy of its block (see conflictRedirects). */
+  private redirectToConflictCopy(op: Op, doc: Doc, copyId = this.conflictRedirects.get(op.blockId!)): void {
+    if (!copyId) return;
+    const copy = doc.blocks.get(copyId);
+    if (!copy || copy.deleted || copy.props.conflictOf !== op.blockId) {
+      // Resolved or discarded meanwhile: a new change is a new conflict
+      this.conflictRedirects.delete(op.blockId!);
+      return;
+    }
+    op.blockId = copyId;
+    op.baseVersion = copy.version;
+  }
+
   private recompute(): void {
     const v = cloneDoc(this.confirmed);
     // Pending ops count the way they will be sent: formBatch folds all text changes of a block into one,
@@ -537,20 +563,27 @@ export class DocSyncEngine {
       this.pending = [];
       await this.persist();
     }
-    const ops = this.canEdit ? this.inflight || [] : [];
-    if (ops.length > 0) this.status = 'saving';
+    // flushOnExit may start a new batch while this request is on the wire; only this one is answered here
+    let batch = this.canEdit ? this.inflight : null;
+    if (batch && batch.length > 0) this.status = 'saving';
 
     for (;;) {
       const res = await this.transport.sync(this.docId, {
         clientId: this.clientId,
         lastSeq: this.lastSeq,
-        ops: this.inflight ? ops : [],
+        ops: batch || [],
         leases: this.canEdit && this.collaborative() ? { hold: this.holds(), release: [] } : undefined,
       });
-      if (this.inflight) {
-        const sent = new Map(this.inflight.map((op) => [op.opId, op]));
+      const lostTo = new Map<string, string>(); // block -> conflict copy of its losing text
+      if (batch) {
+        const sent = new Map(batch.map((op) => [op.opId, op]));
         for (const r of res.results) {
           if (r.status === 'conflict') this.conflicts = [...this.conflicts, r];
+          const conflicted = sent.get(r.opId);
+          if (r.status === 'conflict' && r.conflictBlockId && conflicted?.kind === 'set_content') {
+            lostTo.set(conflicted.blockId!, r.conflictBlockId);
+            if (this.focused === conflicted.blockId) this.conflictRedirects.set(conflicted.blockId!, r.conflictBlockId);
+          }
           // Someone else created a block with the same ID first (e.g. the first block of an empty note):
           // keep the typed text by sending it as a regular text change of that block
           const op = sent.get(r.opId);
@@ -559,7 +592,8 @@ export class DocSyncEngine {
             this.pending.unshift({ opId: this.newOpId(), kind: 'set_content', blockId: op.blockId, content: op.content, baseVersion: 0 });
           }
         }
-        this.inflight = null;
+        if (this.inflight === batch) this.inflight = null;
+        batch = null;
       }
       this.role = res.role;
       this.shared = res.shared;
@@ -581,6 +615,11 @@ export class DocSyncEngine {
         this.lastSeq = entry.seq;
       }
       if (gap) await this.loadSnapshot();
+      // Text typed into the same field while the losing batch was on the wire builds on the losing text
+      // and belongs into the copy too
+      for (const op of this.pending) {
+        if (op.kind === 'set_content' && lostTo.has(op.blockId!)) this.redirectToConflictCopy(op, this.confirmed, lostTo.get(op.blockId!));
+      }
       await this.persist();
       if (!res.hasMore || gap) break;
     }
@@ -690,7 +729,16 @@ export class DocSyncEngine {
   /** Sends what is pending with keepalive (survives page unload) and releases leases. */
   private flushOnExit(): void {
     if (this.canEdit && (this.inflight || this.pending.length > 0)) {
-      const ops = this.inflight || formBatch(this.pending);
+      // The batch becomes the one in flight: the next sync resends exactly these ops (same IDs, applied
+      // once), and text typed afterwards starts a new batch on top instead of being folded into this one
+      // with its older base version, which would conflict with the user's own text
+      if (!this.inflight) {
+        this.inflight = formBatch(this.pending);
+        this.pending = [];
+        this.recompute();
+        void this.persist();
+      }
+      const ops = this.inflight;
       this.transport
         .sync(this.docId, { clientId: this.clientId, lastSeq: this.lastSeq, ops }, { keepalive: true })
         .catch(() => {