diff --git a/src/core/services/immersion-tracker/lexical-rollup-worker-runtime.test.ts b/src/core/services/immersion-tracker/lexical-rollup-worker-runtime.test.ts
index 802df029..dbfa8ad1 100644
--- a/src/core/services/immersion-tracker/lexical-rollup-worker-runtime.test.ts
+++ b/src/core/services/immersion-tracker/lexical-rollup-worker-runtime.test.ts
@@ -54,3 +54,16 @@ test('lexical rollup worker module resolves in the current layout', () => {
   assert.ok(workerPath, 'expected the lexical rollup worker module to resolve');
   assert.ok(workerPath.endsWith(__filename.endsWith('.ts') ? '.ts' : '.js'));
 });
+
+test('lexical rollup worker leaves a backfill pending when no worker can start', async () => {
+  const runtime = new LexicalRollupWorkerRuntime({
+    resolveWorkerPath: () => null,
+    warn: () => {},
+  } as never);
+
+  try {
+    await assert.doesNotReject(runtime.run('/tmp/not-used.sqlite'));
+  } finally {
+    runtime.destroy();
+  }
+});
diff --git a/src/core/services/immersion-tracker/lexical-rollup-worker-runtime.ts b/src/core/services/immersion-tracker/lexical-rollup-worker-runtime.ts
index 5c320873..e83c0146 100644
--- a/src/core/services/immersion-tracker/lexical-rollup-worker-runtime.ts
+++ b/src/core/services/immersion-tracker/lexical-rollup-worker-runtime.ts
@@ -1,7 +1,6 @@
 import fs from 'node:fs';
 import path from 'node:path';
 import { createLogger } from '../../../logger';
-import { executeLexicalRollupBackfillTask } from './lexical-rollup-worker';
 
 interface WorkerResponse {
   ok?: boolean;
@@ -15,6 +14,12 @@ interface WorkerHandle {
   terminate(): Promise<number>;
 }
 
+interface LexicalRollupWorkerRuntimeOptions {
+  resolveWorkerPath?: () => string | null;
+  createWorker?: (workerPath: string, workerData: { dbPath: string }) => Promise<WorkerHandle>;
+  warn?: (message: string, ...meta: unknown[]) => void;
+}
+
 const logger = createLogger('main:immersion-tracker:lexical-rollup-worker');
 
 export function resolveLexicalRollupWorkerPath(): string | null {
@@ -29,21 +34,27 @@ export class LexicalRollupWorkerRuntime {
   private readonly activeWorkers = new Set<WorkerHandle>();
   private destroyed = false;
 
+  constructor(private readonly options: LexicalRollupWorkerRuntimeOptions = {}) {}
+
   async run(dbPath: string): Promise<void> {
     if (this.destroyed) throw new Error('Lexical rollup worker is shut down');
     let worker: WorkerHandle;
     try {
-      const workerPath = resolveLexicalRollupWorkerPath();
+      const workerPath = (this.options.resolveWorkerPath ?? resolveLexicalRollupWorkerPath)();
       if (!workerPath) throw new Error('Emitted lexical rollup worker module was not found');
-      const { Worker } = await import('node:worker_threads');
-      worker = new Worker(workerPath, { workerData: { dbPath } });
+      const createWorker =
+        this.options.createWorker ??
+        (async (resolvedPath, workerData) => {
+          const { Worker } = await import('node:worker_threads');
+          return new Worker(resolvedPath, { workerData });
+        });
+      worker = await createWorker(workerPath, { dbPath });
     } catch (error) {
       if (this.destroyed) throw new Error('Lexical rollup worker is shut down');
-      logger.warn(
-        'Lexical rollup worker unavailable; running backfill on the current thread',
+      (this.options.warn ?? logger.warn)(
+        'Lexical rollup worker unavailable; leaving backfill pending for a later startup',
         error,
       );
-      executeLexicalRollupBackfillTask(dbPath);
       return;
     }
 
diff --git a/src/core/services/immersion-tracker/lexical-rollups.ts b/src/core/services/immersion-tracker/lexical-rollups.ts
index 7052df36..1c02987e 100644
--- a/src/core/services/immersion-tracker/lexical-rollups.ts
+++ b/src/core/services/immersion-tracker/lexical-rollups.ts
@@ -11,7 +11,7 @@ const LOCAL_EPOCH_DAY_SQL = `
   CAST(julianday(CAST(%VALUE% AS REAL), 'unixepoch', 'localtime') - 2440587.5 AS INTEGER)
 `;
 
-function localEpochDaySql(value: string): string {
+export function localEpochDaySql(value: string): string {
   return LOCAL_EPOCH_DAY_SQL.replace('%VALUE%', value);
 }
 
@@ -131,8 +131,10 @@ export function markLexicalDailyRollupsReady(db: DatabaseSync): void {
 
 /** Rebuild from the first-seen source of truth; run off the UI/main DB thread. */
 export function rebuildLexicalDailyRollups(db: DatabaseSync): void {
-  db.exec('BEGIN IMMEDIATE');
+  let transactionStarted = false;
   try {
+    db.exec('BEGIN IMMEDIATE');
+    transactionStarted = true;
     db.exec('DELETE FROM imm_lexical_daily_rollups');
     db.exec(`
       INSERT INTO imm_lexical_daily_rollups(epoch_day, word_count, word_count_without_names, kanji_count)
@@ -151,7 +153,7 @@ export function rebuildLexicalDailyRollups(db: DatabaseSync): void {
     markLexicalDailyRollupsReady(db);
     db.exec('COMMIT');
   } catch (error) {
-    db.exec('ROLLBACK');
+    if (transactionStarted) db.exec('ROLLBACK');
     throw error;
   }
 }
diff --git a/src/core/services/immersion-tracker/query-lexical.ts b/src/core/services/immersion-tracker/query-lexical.ts
index f31e96fd..de1dfce9 100644
--- a/src/core/services/immersion-tracker/query-lexical.ts
+++ b/src/core/services/immersion-tracker/query-lexical.ts
@@ -20,7 +20,11 @@ import type {
 } from './types';
 import { fromDbTimestamp, toDbTimestamp } from './query-shared';
 import { nowMs } from './time';
-import { areLexicalDailyRollupsReady, getLexicalDailyRollups } from './lexical-rollups';
+import {
+  areLexicalDailyRollupsReady,
+  getLexicalDailyRollups,
+  localEpochDaySql,
+} from './lexical-rollups';
 
 const VOCABULARY_STATS_FILTER_OVERSAMPLE_FACTOR = 4;
 const VOCABULARY_STATS_FILTER_OVERSAMPLE_MIN = 100;
@@ -185,7 +189,7 @@ export function getVocabularyChartData(db: DatabaseSync): VocabularyChartData {
       .prepare(
         `
           SELECT headword, word, reading, pos2,
-            CAST(julianday(CAST(first_seen AS REAL), 'unixepoch', 'localtime') - 2440587.5 AS INTEGER) AS epochDay
+            ${localEpochDaySql('first_seen')} AS epochDay
           FROM imm_words
           WHERE headword IN (${placeholders}) OR word IN (${placeholders}) OR reading IN (${placeholders})
         `,
diff --git a/src/core/services/immersion-tracker/query-trends.ts b/src/core/services/immersion-tracker/query-trends.ts
index 0ff7fbdc..dfa3c3e2 100644
--- a/src/core/services/immersion-tracker/query-trends.ts
+++ b/src/core/services/immersion-tracker/query-trends.ts
@@ -662,6 +662,8 @@ function buildNewWordsPerDay(
   axis: number[] | null,
 ): TrendChartPoint[] {
   if (areLexicalDailyRollupsReady(db)) {
+    // A trend range is defined in calendar buckets, so the rollup includes the
+    // complete local cutoff day rather than applying a time-of-day boundary.
     const cutoffDay = cutoffMs === null ? null : getLocalEpochDay(db, cutoffMs);
     const rows = getLexicalDailyRollups(db).filter(
       (row) => cutoffDay === null || row.epochDay >= cutoffDay,
diff --git a/stats/src/hooks/useVocabulary.ts b/stats/src/hooks/useVocabulary.ts
index 471b5e89..e5afebdf 100644
--- a/stats/src/hooks/useVocabulary.ts
+++ b/stats/src/hooks/useVocabulary.ts
@@ -76,6 +76,7 @@ export function useVocabulary() {
         })
         .catch((chartError: unknown) => {
           console.error('Failed to load vocabulary charts', chartError);
+          if (!cancelled) chartRetryTimer = setTimeout(loadCharts, 1_000);
         });
     };
     loadCharts();