fix: prevent deleted toolbar bookmarks from being reverted by auto-sync

[Preshy]

Summary

Fixes a race condition where deleting a bookmark from the toolbar gets reverted when auto-sync runs.

Root Cause

A tombstone overwrite race condition in performSync():

1. performSync reads localTombstones = [] at sync start
2. User deletes a bookmark → onRemoved fires → addTombstone() writes new tombstone to storage
3. performSync computes mergedTombstones from the stale tombstones read in step 1
4. performSync calls storeTombstones(mergedTombstones) → overwrites the new tombstone
5. Follow-up sync has no tombstone → re-adds bookmark from cloud → deletion reverted

Fix

Before writing merged tombstones, re-read current tombstones from storage to preserve any tombstones written concurrently by onRemoved:

const mergedTombstones = mergeTombstonesLocal(localTombstones, cloudTombstones);
const currentTombstones = await getTombstones();
const safeMergedTombstones = mergeTombstonesLocal(currentTombstones, mergedTombstones);
await storeTombstones(safeMergedTombstones);

Tests

Unit tests (19 tests) — toolbar-delete-revert.test.js

• Tombstone filtering, full sync flow, race condition, server-side push, cross-browser paths, edge cases
• Uses copied functions for isolated unit testing

Integration tests (10 tests) — toolbar-delete-race-integration.test.js

• Calls the REAL performSync code (not copied/reimplemented functions) with fully mocked browser.* and fetch APIs
• Simulates the race condition: delays cloud GET, fires the real onRemoved handler mid-sync, verifies the tombstone is preserved and the bookmark isn't re-added
• Tests: basic sync flow, tombstone preservation during race, tombstone merge correctness, categorization filtering, concurrent sync guard, state management

Test-only exports (__test__ in background/index.js)

• Guarded behind import.meta.env?.VITEST — tree-shaken in production builds
• Exports performSync, addTombstone, categorizeCloudBookmarks, resetState, etc.
• No impact on production bundle size (verified: Chrome build 54.64 kB unchanged)

Results

• 527 tests pass (517 pre-existing + 10 new integration tests)
• 61 pre-existing failures in store.test.js (unrelated storage.setItem issue)
• 0 regressions

[Copilot]

Pull request overview

This PR fixes a race condition where deleting a bookmark from the toolbar gets reverted by auto-sync. The root cause was that performSync() read tombstones at the start of the sync, but if a user deleted a bookmark mid-sync, the onRemoved handler wrote a new tombstone to storage that would be overwritten when performSync later called storeTombstones() with its stale merge result.

Changes:

• Re-reads current tombstones from storage before writing merged tombstones in performSync(), preserving any concurrent tombstones written by onRemoved during the sync.
• Adds comprehensive unit tests (toolbar-delete-revert.test.js) covering tombstone filtering, race condition scenarios, and cross-browser edge cases using copied functions.
• Adds integration tests (toolbar-delete-race-integration.test.js) that call the real performSync code with mocked browser APIs and simulates the exact race condition.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
apps/extension/src/background/index.js	Re-reads tombstones before writing to avoid overwriting concurrent tombstones; updates all downstream references to use safeMergedTombstones; adds __test__ exports guarded by VITEST for integration testing
apps/extension/__tests__/toolbar-delete-revert.test.js	Unit tests for tombstone filtering, full sync flow simulation, race condition demonstration, server-side push, and edge cases using copied helper functions
apps/extension/__tests__/toolbar-delete-race-integration.test.js	Integration tests calling real performSync with mocked browser/fetch APIs, simulating the race condition by blocking the cloud GET and firing onRemoved mid-sync

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