fix electric collection progressive mode

.changeset/fix-progressive-mode-fetchsnapshot.md

@@ -0,0 +1,32 @@
+---
+"@tanstack/electric-db-collection": patch
+"@tanstack/db-collection-e2e": patch
+"@tanstack/db": patch
+---
+
+Fix progressive mode to use fetchSnapshot and atomic swap
+
+Progressive mode was broken because `requestSnapshot()` injected snapshots into the stream in causally correct position, which didn't work properly with the `full` mode stream. This release fixes progressive mode by:
+
+**Core Changes:**
+
+- Use `fetchSnapshot()` during initial sync to fetch and apply snapshots immediately in sync transactions
+- Buffer all stream messages during initial sync (renamed flag to `isBufferingInitialSync`)
+- Perform atomic swap on first `up-to-date`: truncate snapshot data → apply buffered messages → mark ready
+- Track txids/snapshots only after atomic swap (enables correct optimistic transaction confirmation)
+
+**Test Infrastructure:**
+
+- Added `ELECTRIC_TEST_HOOKS` symbol for test control (hidden from public API)
+- Added `progressiveTestControl.releaseInitialSync()` to E2E test config for explicit transition control
+- Created comprehensive progressive mode E2E test suite (8 tests):
+  - Explicit snapshot phase and atomic swap validation
+  - Txid tracking behavior (Electric-only)
+  - Multiple concurrent snapshots with deduplication
+  - Incremental updates after swap
+  - Predicate handling and resilience tests
+
+**Bug Fixes:**
+
+- Fixed type errors in test files
+- All 166 unit tests + 95 E2E tests passing

packages/db-collection-e2e/src/index.ts

@@ -25,3 +25,4 @@ export { createDeduplicationTestSuite } from "./suites/deduplication.suite"
 export { createCollationTestSuite } from "./suites/collation.suite"
 export { createMutationsTestSuite } from "./suites/mutations.suite"
 export { createLiveUpdatesTestSuite } from "./suites/live-updates.suite"
+export { createProgressiveTestSuite } from "./suites/progressive.suite"