feat(diagnostics): persist full stderr detail behind a "Show details" toggle

describeDownloadError condenses a postprocessing failure to one line for
the queue/notification, discarding the rest of yt-dlp's stderr. The
persisted error log now also keeps the fuller non-noise transcript
(stderrDetail) as an optional `detail` field, and the Diagnostics card
gets a collapsible "Show details" expander per row so a failure like
"Postprocessing: Conversion failed!" can be inspected -- and copied
into a bug report -- without re-running the download from a terminal.

src/main/core/validation.ts

@@ -74,7 +74,8 @@ export function isValidErrorLogEntry(o: unknown): o is ErrorLogEntry {
     (e.kind === 'video' || e.kind === 'audio') &&
     typeof e.error === 'string' &&
     typeof e.occurredAt === 'number' &&
-    isOptionalString(e.title)
+    isOptionalString(e.title) &&
+    isOptionalString(e.detail)
   )
 }
 

src/main/download.ts

@@ -33,7 +33,7 @@ import {
   FILEPATH_MARKER,
   DEST_MARKER
 } from './core/buildArgs'
-import { describeDownloadError } from './log'
+import { describeDownloadError, stderrDetail } from './log'
 import { addErrorLog } from './errorlog'
 import {
   STALL_TIMEOUT_MS,
@@ -117,7 +117,12 @@ function notify(wc: WebContents, title: string, body: string, incognito = false)
   if (flashWin && !flashWin.isDestroyed() && !flashWin.isFocused()) flashWin.flashFrame(true)
 }
 
-function logFailure(opts: StartDownloadOptions, title: string | undefined, error: string): void {
+function logFailure(
+  opts: StartDownloadOptions,
+  title: string | undefined,
+  error: string,
+  detail?: string
+): void {
   // L136: incognito downloads are never recorded — not even a failure entry, which
   // would persist the title + URL in the user-facing diagnostics log.
   if (opts.incognito) return
@@ -127,6 +132,7 @@ function logFailure(opts: StartDownloadOptions, title: string | undefined, error
     url: opts.url,
     kind: opts.kind,
     error,
+    detail,
     occurredAt: Date.now()
   })
 }
@@ -532,7 +538,7 @@ function wireChildProcess(params: {
     } else {
       const msg = describeDownloadError(stderrTail) || `yt-dlp exited with code ${code}`
       send(wc, { type: 'error', id: opts.id, error: msg })
-      logFailure(opts, getTitle(), msg)
+      logFailure(opts, getTitle(), msg, stderrDetail(stderrTail))
       notify(
         wc,
         opts.incognito ? 'Download failed' : (getTitle() ?? 'Download failed'),

src/main/log.ts

@@ -53,3 +53,20 @@ export function describeDownloadError(stderr: string): string {
   }
   return context.length > 0 ? `${headline} — ${context.join(' · ')}` : headline
 }
+
+/**
+ * Full non-noise stderr, for the persisted error log's "Show details" expander
+ * (Diagnostics card) — everything describeDownloadError condenses to one line,
+ * kept in full (progress ticks and delete-bookkeeping still stripped) so a bug
+ * report carries the real yt-dlp/ffmpeg transcript instead of a paraphrase.
+ * Returns undefined when there's nothing beyond a single line — the headline
+ * the caller already shows — so a plain, self-explanatory error doesn't grow a
+ * details toggle with nothing new in it.
+ */
+export function stderrDetail(stderr: string): string | undefined {
+  const lines = stderr
+    .split('\n')
+    .map((l) => l.trim())
+    .filter((l) => l && !CONTEXT_NOISE.test(l))
+  return lines.length > 1 ? lines.join('\n') : undefined
+}

src/renderer/src/store/errorlog.ts

@@ -13,6 +13,23 @@ const seed: ErrorLogEntry[] = PREVIEW
         kind: 'video',
         error: '[youtube] err0r: Private video. Sign in if you have access.',
         occurredAt: Date.now() - 1000 * 60 * 12
+      },
+      {
+        id: 'e2',
+        title: 'you’re about to get hacked!! (7 reasons why)',
+        url: 'https://youtube.com/watch?v=q2err0r',
+        kind: 'video',
+        error:
+          'Postprocessing: Conversion failed! — [ModifyChapters] Re-encoding "video.mp4" with appropriate keyframes',
+        detail: [
+          '[info] q2err0r: Downloading 1 format(s): 137+251',
+          '[SponsorBlock] Found 1 segments in the SponsorBlock database',
+          '[Merger] Merging formats into "video.mp4"',
+          '[ModifyChapters] Re-encoding "video.mp4" with appropriate keyframes',
+          '[ModifyChapters] Removing chapters from video.mp4',
+          'ERROR: Postprocessing: Conversion failed!'
+        ].join('\n'),
+        occurredAt: Date.now() - 1000 * 60 * 5
       }
     ]
   : []

src/renderer/src/views/settings/DiagnosticsCard.tsx

@@ -1,6 +1,12 @@
 import { useState } from 'react'
 import { Button, Card, Subtitle2, Caption1, Text } from '@fluentui/react-components'
-import { BugRegular, CopyRegular, DeleteRegular } from '@fluentui/react-icons'
+import {
+  BugRegular,
+  ChevronDownRegular,
+  ChevronRightRegular,
+  CopyRegular,
+  DeleteRegular
+} from '@fluentui/react-icons'
 import { useErrorLog } from '../../store/errorlog'
 import { useErrorTextStyles } from '../../components/ui/errorText'
 import { EmptyState } from '../../components/ui/EmptyState'
@@ -13,13 +19,25 @@ export function DiagnosticsCard(): React.JSX.Element {
   const clearErrorLog = useErrorLog((s) => s.clear)
 
   const [confirmClearLog, setConfirmClearLog] = useState(false)
+  // Which rows have their "Show details" transcript expanded; keyed by entry id.
+  const [expanded, setExpanded] = useState<Set<string>>(new Set())
+  function toggleExpanded(id: string): void {
+    setExpanded((s) => {
+      const next = new Set(s)
+      if (next.has(id)) next.delete(id)
+      else next.add(id)
+      return next
+    })
+  }
 
   function copyErrorReport(): void {
     // The button is disabled when there are no entries, so `report` is always
     // non-empty here -- no need for an unreachable empty-report fallback (L159).
     const report = errorEntries
       .map((e) =>
-        [new Date(e.occurredAt).toLocaleString(), e.title ?? e.url, e.url, e.error].join('\n')
+        [new Date(e.occurredAt).toLocaleString(), e.title ?? e.url, e.url, e.error, e.detail]
+          .filter(Boolean)
+          .join('\n')
       )
       .join('\n\n---\n\n')
     navigator.clipboard.writeText(report).catch(() => {})
@@ -78,17 +96,33 @@ export function DiagnosticsCard(): React.JSX.Element {
         <EmptyState compact message="No errors yet." />
       ) : (
         <div className={styles.errorList}>
-          {errorEntries.slice(0, 20).map((e) => (
-            <div key={e.id} className={styles.errorRow}>
-              <div className={styles.errorRowHeader}>
-                <Text className={styles.errorRowTitle}>{e.title ?? e.url}</Text>
-                <Caption1 className={styles.hint}>
-                  {new Date(e.occurredAt).toLocaleString()}
-                </Caption1>
+          {errorEntries.slice(0, 20).map((e) => {
+            const isExpanded = expanded.has(e.id)
+            return (
+              <div key={e.id} className={styles.errorRow}>
+                <div className={styles.errorRowHeader}>
+                  <Text className={styles.errorRowTitle}>{e.title ?? e.url}</Text>
+                  <Caption1 className={styles.hint}>
+                    {new Date(e.occurredAt).toLocaleString()}
+                  </Caption1>
+                </div>
+                <Caption1 className={errText.errorPre}>{e.error}</Caption1>
+                {e.detail && (
+                  <>
+                    <Button
+                      appearance="transparent"
+                      size="small"
+                      icon={isExpanded ? <ChevronDownRegular /> : <ChevronRightRegular />}
+                      onClick={() => toggleExpanded(e.id)}
+                    >
+                      {isExpanded ? 'Hide details' : 'Show details'}
+                    </Button>
+                    {isExpanded && <pre className={styles.errorDetail}>{e.detail}</pre>}
+                  </>
+                )}
               </div>
-              <Caption1 className={errText.errorPre}>{e.error}</Caption1>
-            </div>
-          ))}
+            )
+          })}
           {errorEntries.length > 20 && (
             <Caption1 className={styles.hint}>Showing 20 of {errorEntries.length} errors.</Caption1>
           )}

src/renderer/src/views/settings/settingsStyles.ts

@@ -113,5 +113,21 @@ export const useSettingsStyles = makeStyles({
     overflow: 'hidden',
     textOverflow: 'ellipsis',
     whiteSpace: 'nowrap'
+  },
+  // The Diagnostics card's "Show details" expander -- the fuller non-noise
+  // stderr transcript behind a failure's one-line headline. Scrollable and
+  // capped so a long SponsorBlock/ffmpeg transcript can't blow out the card.
+  errorDetail: {
+    fontFamily: tokens.fontFamilyMonospace,
+    fontSize: tokens.fontSizeBase200,
+    whiteSpace: 'pre-wrap',
+    wordBreak: 'break-word',
+    color: tokens.colorNeutralForeground3,
+    backgroundColor: tokens.colorNeutralBackground1,
+    marginTop: '6px',
+    padding: '8px',
+    maxHeight: '200px',
+    overflowY: 'auto',
+    ...shorthands.borderRadius(tokens.borderRadiusMedium)
   }
 })

src/shared/ipc.ts

@@ -845,6 +845,13 @@ export interface ErrorLogEntry {
   url: string
   kind: MediaKind
   error: string
+  /**
+   * Fuller non-noise stderr context behind the Diagnostics card's "Show
+   * details" toggle — undefined when `error` already says everything (most
+   * failures), present for the generic "Postprocessing: Conversion failed!"
+   * wrapper where the useful detail is the tagged step lines that preceded it.
+   */
+  detail?: string
   /** epoch milliseconds */
   occurredAt: number
 }

test/log.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect } from 'vitest'
-import { cleanError, describeDownloadError } from '../src/main/log'
+import { cleanError, describeDownloadError, stderrDetail } from '../src/main/log'
 
 describe('cleanError', () => {
   it('returns the last ERROR line, stripped of its prefix', () => {
@@ -76,3 +76,38 @@ describe('describeDownloadError', () => {
     expect(describeDownloadError('')).toBe('')
   })
 })
+
+describe('stderrDetail', () => {
+  it('keeps the full non-noise transcript for the Diagnostics "Show details" expander', () => {
+    const stderr = [
+      '[info] qkUTB65OfAc: Downloading 1 format(s): 137+251',
+      '[SponsorBlock] Found 1 segments in the SponsorBlock database',
+      '[ModifyChapters] Re-encoding "video.mp4" with appropriate keyframes',
+      'ERROR: Postprocessing: Conversion failed!'
+    ].join('\n')
+    const detail = stderrDetail(stderr)
+    expect(detail).toContain('[SponsorBlock] Found 1 segments')
+    expect(detail).toContain('[ModifyChapters] Re-encoding')
+    expect(detail).toContain('ERROR: Postprocessing: Conversion failed!')
+  })
+
+  it('strips [download] progress ticks and "Deleting original file" bookkeeping', () => {
+    const stderr = [
+      '[download]  42.0% of   23.21MiB at   15.11MiB/s ETA 00:00',
+      'Deleting original file video.f251.webm (pass -k to keep)',
+      '[ModifyChapters] Re-encoding "video.mp4" with appropriate keyframes',
+      'ERROR: Postprocessing: Conversion failed!'
+    ].join('\n')
+    const detail = stderrDetail(stderr)
+    expect(detail).not.toMatch(/\[download\]/)
+    expect(detail).not.toContain('Deleting original file')
+  })
+
+  it('returns undefined when there is nothing beyond a single headline', () => {
+    expect(stderrDetail('ERROR: Postprocessing: Conversion failed!')).toBeUndefined()
+  })
+
+  it('returns undefined for blank stderr', () => {
+    expect(stderrDetail('')).toBeUndefined()
+  })
+})

test/validation.test.ts

@@ -175,6 +175,11 @@ describe('isValidErrorLogEntry', () => {
     expect(isValidErrorLogEntry({ ...validError, title: 'optional' })).toBe(true)
   })
 
+  it('accepts an optional detail string and rejects a non-string one', () => {
+    expect(isValidErrorLogEntry({ ...validError, detail: 'the full transcript' })).toBe(true)
+    expect(isValidErrorLogEntry({ ...validError, detail: 42 })).toBe(false)
+  })
+
   it('rejects a missing error message', () => {
     const { error: _error, ...noError } = validError
     expect(isValidErrorLogEntry(noError)).toBe(false)