fix(renderer): stabilize preserveLineBreaks whitespace and newline rendering

docs/subagents/INDEX.md

@@ -25,3 +25,4 @@ Read first. Keep concise.
 | `codex-preserve-linebreak-display-20260220T110436Z-r8f1` | `codex-preserve-linebreak-display` | `Fix visible overlay display artifact when subtitleStyle.preserveLineBreaks is disabled` | `completed` | `docs/subagents/agents/codex-preserve-linebreak-display-20260220T110436Z-r8f1.md` | `2026-02-20T11:10:51Z` |
 | `codex-review-refactor-cleanup-20260220T113818Z-i2ov` | `codex-review-refactor-cleanup` | `Review recent TASK-85 refactor effort and identify remaining cleanup work` | `handoff` | `docs/subagents/agents/codex-review-refactor-cleanup-20260220T113818Z-i2ov.md` | `2026-02-20T11:48:28Z` |
 | `codex-commit-unstaged-20260220T115057Z-k7q2` | `codex-commit-unstaged` | `Commit all current unstaged repository changes with content-derived conventional message` | `in_progress` | `docs/subagents/agents/codex-commit-unstaged-20260220T115057Z-k7q2.md` | `2026-02-20T11:51:18Z` |
+| `codex-overlay-whitespace-newline-20260221T040705Z-aw2j` | `codex-overlay-whitespace-newline` | `Fix visible overlay whitespace/newline token rendering bug with TDD regression coverage` | `completed` | `docs/subagents/agents/codex-overlay-whitespace-newline-20260221T040705Z-aw2j.md` | `2026-02-21T04:18:16Z` |

docs/subagents/agents/codex-overlay-whitespace-newline-20260221T040705Z-aw2j.md

@@ -0,0 +1,38 @@
+# Agent Log: codex-overlay-whitespace-newline-20260221T040705Z-aw2j
+
+- alias: codex-overlay-whitespace-newline
+- mission: Fix visible overlay whitespace/newline token rendering bug (blank hoverable tokens) with regression tests.
+- status: completed
+- started_utc: 2026-02-21T04:07:05Z
+- last_update_utc: 2026-02-21T04:14:30Z
+
+## Intent
+- Reproduce bug around spaces/newlines + `preserveLineBreaks` in visible overlay.
+- Add failing renderer/tokenization regression tests first.
+- Patch render/token mapping to avoid hoverable blank tokens and preserve-mode duplication.
+
+## Touched Files
+- src/renderer/subtitle-render.ts
+- src/renderer/subtitle-render.test.ts
+- docs/subagents/INDEX.md
+- docs/subagents/collaboration.md
+- docs/subagents/agents/codex-overlay-whitespace-newline-20260221T040705Z-aw2j.md
+
+## Decisions
+- Ignore whitespace-only token surfaces in source alignment.
+- Non-preserve mode: flatten token newlines to spaces, whitespace-only surfaces as text nodes.
+- Preserve mode alignment: skip unmatched token surfaces (do not append fallback token) to avoid duplicate tail text.
+
+## Verification
+- RED: new whitespace-separator test failed before fix.
+- GREEN: renderer suite pass after first fix.
+- RED: new preserve-mode duplicate-tail mismatch test failed before fix.
+- GREEN: `bun run build && node dist/renderer/subtitle-render.test.js` pass (11/11).
+- GREEN: `node --test dist/renderer/subtitle-render.test.js` pass.
+
+## Blockers
+- none
+
+## Next Step
+- User visual verify with `preserveLineBreaks=true` + known mixed-width numeral lines.
+- [2026-02-21T04:18:16Z] follow-up: fixed non-token first-paint behavior to collapse line breaks when `preserveLineBreaks=false` (`normalizeSubtitle(..., collapseLineBreaks=true)` in fallback path); added regression test for normalize collapse.

docs/subagents/collaboration.md

@@ -16,3 +16,7 @@ Shared notes. Append-only.
 - [2026-02-20T11:04:36Z] [codex-preserve-linebreak-display-20260220T110436Z-r8f1|codex-preserve-linebreak-display] overlap note: touching `src/renderer/subtitle-render.ts` + renderer tests to fix preserve-linebreaks disabled display artifact while preserving TASK-91 behavior.
 - [2026-02-20T11:07:29Z] [codex-preserve-linebreak-display-20260220T110436Z-r8f1|codex-preserve-linebreak-display] completed follow-up for TASK-91: non-preserve mode now flattens token CR/LF to spaces instead of emitting `<br>` from token surfaces; regression test added.
 - [2026-02-20T11:10:51Z] [codex-preserve-linebreak-display-20260220T110436Z-r8f1|codex-preserve-linebreak-display] second follow-up: handle overlap token streams by aligning non-preserve rendering to normalized source text and skipping unmatched tail tokens (prevents duplicated second-line phrase).
+- [2026-02-21T04:07:05Z] [codex-overlay-whitespace-newline-20260221T040705Z-aw2j|codex-overlay-whitespace-newline] overlap note: touching `src/renderer/subtitle-render.ts` + renderer tests for whitespace/newline display/token hover regression around `preserveLineBreaks`.
+- [2026-02-21T04:09:02Z] [codex-overlay-whitespace-newline-20260221T040705Z-aw2j|codex-overlay-whitespace-newline] completed: whitespace-only token surfaces no longer become token segments; non-preserve mode now flattens token newlines to spaces and renders whitespace as text nodes; added regression test in renderer suite.
+- [2026-02-21T04:14:30Z] [codex-overlay-whitespace-newline-20260221T040705Z-aw2j|codex-overlay-whitespace-newline] preserve-line-breaks follow-up: when token surface mismatches source (e.g., `1` vs `１`), alignment now skips unmatched token instead of appending both source tail + token; fixes duplicated no-break line artifact.
+- [2026-02-21T04:18:16Z] [codex-overlay-whitespace-newline-20260221T040705Z-aw2j|codex-overlay-whitespace-newline] follow-up fix: non-token fallback now honors preserveLineBreaks flag by collapsing line breaks when disabled; prevents visible multi-line -> single-line transition while tokenized payload arrives.

src/renderer/subtitle-render.test.ts

@@ -5,7 +5,7 @@ import path from 'node:path';
 
 import type { MergedToken } from '../types';
 import { PartOfSpeech } from '../types.js';
-import { alignTokensToSourceText, computeWordClass } from './subtitle-render.js';
+import { alignTokensToSourceText, computeWordClass, normalizeSubtitle } from './subtitle-render.js';
 
 function createToken(overrides: Partial<MergedToken>): MergedToken {
   return {
@@ -216,6 +216,45 @@ test('alignTokensToSourceText preserves newline separators between adjacent toke
   );
 });
 
+test('alignTokensToSourceText treats whitespace-only token surfaces as plain text separators', () => {
+  const tokens = [
+    createToken({ surface: '常人が使えば' }),
+    createToken({ surface: ' ' }),
+    createToken({ surface: 'その圧倒的な力に' }),
+    createToken({ surface: '\n' }),
+    createToken({ surface: '体が耐えきれず死に至るが…' }),
+  ];
+
+  const segments = alignTokensToSourceText(tokens, '常人が使えば その圧倒的な力に\n体が耐えきれず死に至るが…');
+  assert.deepEqual(
+    segments.map((segment) => (segment.kind === 'text' ? `text:${segment.text}` : 'token')),
+    ['token', 'text: ', 'token', 'text:\n', 'token'],
+  );
+});
+
+test('alignTokensToSourceText avoids duplicate tail when later token surface does not match source', () => {
+  const tokens = [
+    createToken({ surface: '君たちが潰した拠点に' }),
+    createToken({ surface: '教団の主力は1人もいない' }),
+  ];
+
+  const segments = alignTokensToSourceText(
+    tokens,
+    '君たちが潰した拠点に\n教団の主力は１人もいない',
+  );
+  assert.deepEqual(
+    segments.map((segment) => (segment.kind === 'text' ? `text:${segment.text}` : 'token')),
+    ['token', 'text:\n教団の主力は１人もいない'],
+  );
+});
+
+test('normalizeSubtitle collapses explicit line breaks when collapseLineBreaks is enabled', () => {
+  assert.equal(
+    normalizeSubtitle('常人が使えば\\Nその圧倒的な力に\\n体が耐えきれず死に至るが…', true, true),
+    '常人が使えば その圧倒的な力に 体が耐えきれず死に至るが…',
+  );
+});
+
 test('JLPT CSS rules use underline-only styling in renderer stylesheet', () => {
   const distCssPath = path.join(process.cwd(), 'dist', 'renderer', 'style.css');
   const srcCssPath = path.join(process.cwd(), 'src', 'renderer', 'style.css');

src/renderer/subtitle-render.ts

@@ -9,7 +9,11 @@ type FrequencyRenderSettings = {
   bandedColors: [string, string, string, string, string];
 };
 
-function normalizeSubtitle(text: string, trim = true, collapseLineBreaks = false): string {
+function isWhitespaceOnly(value: string): boolean {
+  return value.trim().length === 0;
+}
+
+export function normalizeSubtitle(text: string, trim = true, collapseLineBreaks = false): string {
   if (!text) return '';
 
   let normalized = text.replace(/\\N/g, '\n').replace(/\\n/g, '\n');
@@ -140,24 +144,13 @@ function renderWithTokens(
   }
 
   for (const token of tokens) {
-    const surface = token.surface;
+    const surface = token.surface.replace(/\n/g, ' ');
+    if (!surface) {
+      continue;
+    }
 
-    if (surface.includes('\n')) {
-      const parts = surface.split('\n');
-      for (let i = 0; i < parts.length; i += 1) {
-        const part = parts[i];
-        if (part) {
-          const span = document.createElement('span');
-          span.className = computeWordClass(token, resolvedFrequencyRenderSettings);
-          span.textContent = part;
-          if (token.reading) span.dataset.reading = token.reading;
-          if (token.headword) span.dataset.headword = token.headword;
-          fragment.appendChild(span);
-        }
-        if (i < parts.length - 1) {
-          fragment.appendChild(document.createElement('br'));
-        }
-      }
+    if (isWhitespaceOnly(surface)) {
+      fragment.appendChild(document.createTextNode(surface));
       continue;
     }
 
@@ -187,17 +180,14 @@ export function alignTokensToSourceText(
 
   for (const token of tokens) {
     const surface = token.surface;
-    if (!surface) {
+    if (!surface || isWhitespaceOnly(surface)) {
       continue;
     }
 
     const foundIndex = sourceText.indexOf(surface, cursor);
     if (foundIndex < 0) {
-      if (cursor < sourceText.length) {
-        segments.push({ kind: 'text', text: sourceText.slice(cursor) });
-      }
-      segments.push({ kind: 'token', token });
-      cursor = sourceText.length;
+      // Token text can diverge from source normalization (e.g., half/full-width forms).
+      // Skip unmatched token to avoid duplicating visible tail text in preserve-line-break mode.
       continue;
     }
 
@@ -318,7 +308,7 @@ export function createSubtitleRenderer(ctx: RendererContext) {
       return;
     }
 
-    const normalized = normalizeSubtitle(text);
+    const normalized = normalizeSubtitle(text, true, !ctx.state.preserveSubtitleLineBreaks);
     if (tokens && tokens.length > 0) {
       renderWithTokens(
         ctx.dom.subtitleRoot,