web/desktop: grid + subtitle fixes, comment toggle, faster startup

Several related UI and startup fixes from one session:

- Web grid: channel and music views rendered in a single column because
  their content was wrapped in one nested div that became a single cell of
  the outer CSS grid. Span the wrappers (and .empty) across all columns
  with grid-column:1/-1.

- Subtitles: route every track through /api/sub-vtt and strip per-cue
  position/alignment settings (align:start position:0% etc.) from both SRT
  and VTT, so auto-generated captions render centered instead of left-
  aligned. New strip_cue_settings/normalize_vtt + sub_vtt_url helpers.

- Comment downloads: add a global backup.fetch_comments toggle (the
  per-channel option already existed; it's now a tri-state override that
  defers to the global). Full five-touchpoint wiring across config,
  download_options, the downloader's apply_comments resolver, both UIs'
  global + per-channel controls, and downloader seeding.

- Desktop startup: run the initial library scan + search-index sync on a
  background thread (mirrors the web server's deferred bind) so the window
  appears immediately instead of blocking on a cold-cache scan; update()
  swaps the library in when it lands.

- Perceptual dedup: cap worker threads via fingerprint::default_workers()
  (~half the cores, always leaving at least one free) instead of using
  every core, so the hashing pass doesn't starve the UI.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app.rs

@@ -116,6 +116,11 @@ pub struct App {
     /// non-YouTube content is included.
     library_root: PathBuf,
     library: Vec<library::Channel>,
+    /// Receiver for the initial library scan, which runs on a background
+    /// thread so the window appears immediately instead of blocking on a
+    /// cold-cache scan of a large library. `None` once the scan has landed
+    /// (or was never deferred). Polled in `update()`.
+    library_load_rx: Option<std::sync::mpsc::Receiver<Vec<library::Channel>>>,
     sidebar_view: SidebarView,
     selected_video: Option<String>,
     /// Edit buffer for the selected video's note, plus the id it belongs
@@ -256,7 +261,9 @@ pub struct App {
 struct ChannelOptionsForm {
     quality_idx: usize,           // 0=default, 1=Best, 2=1080p, 3=720p, 4=480p, 5=360p
     audio_only: bool,
-    fetch_comments: bool,
+    // Per-channel comment-fetch override: 0=Default(global), 1=On, 2=Off.
+    // Maps to Option<bool> on save (same tri-state as the subtitle indices).
+    comments_idx: usize,
     skip_auth_check: bool,
     limit_rate_kb: String,
     min_filesize_mb: String,
@@ -369,14 +376,25 @@ impl App {
         let db_path = channels_root.join("yt-offline.db");
         let db = Database::open(&db_path)
             .unwrap_or_else(|_| Database::open_in_memory().expect("in-memory db failed"));
+        // Defer the (potentially multi-minute, disk-bound) library scan +
+        // search-index sync to a background thread so the window appears
+        // immediately instead of blocking on a cold-cache scan of a large
+        // library. The UI renders an empty library until the scan lands and
+        // update() swaps it in. The folder/watched/flag reads below are cheap
+        // DB lookups that don't depend on the scan, so they stay synchronous.
+        let (library_load_tx, library_load_rx) =
+            std::sync::mpsc::channel::<Vec<library::Channel>>();
+        let status = "Scanning library…".to_string();
+        {
+            let db = db.clone();
+            let channels_root = channels_root.clone();
+            let ctx = cc.egui_ctx.clone();
+            std::thread::Builder::new()
+                .name("yt-offline-libscan".into())
+                .spawn(move || {
                     let mut library = library::scan_channels_with_cache(&channels_root, Some(&db));
-        let status = format!(
+                    // Hydrate per-channel download options + folder assignments
-            "{} channels, {} videos",
+                    // from SQLite onto the scanned library before publishing it.
-            library.len(),
-            library.iter().map(|c| c.total_videos()).sum::<usize>()
-        );
-        // Hydrate per-channel download options + folder assignments from
-        // SQLite onto the scanned library before publishing it to the UI.
                     if let Ok(map) = db.get_all_channel_options() {
                         library::apply_channel_options(&mut library, &map);
                     }
@@ -386,6 +404,12 @@ impl App {
                     if let Err(e) = db.sync_search_index(&library::build_search_entries(&library)) {
                         eprintln!("search index sync failed: {e}");
                     }
+                    let _ = library_load_tx.send(library);
+                    ctx.request_repaint(); // wake update() to swap the result in
+                })
+                .ok();
+        }
+        let library: Vec<library::Channel> = Vec::new();
         let folders = db.list_folders().unwrap_or_default();
         let watched = db.get_watched().unwrap_or_default();
         let flags = db.get_video_flags().unwrap_or_default();
@@ -408,6 +432,7 @@ impl App {
         downloader.subtitle_defaults = config.subtitles.clone();
         downloader.youtube_player_clients = config.backup.youtube_player_clients.clone();
         downloader.sponsorblock_mode = config.backup.sponsorblock_mode.clone();
+        downloader.fetch_comments = config.backup.fetch_comments;
         downloader.convert_defaults = config.convert.clone();
         let config_bind = config.web.bind.clone();
         let password_set = db.get_setting("password_hash").ok().flatten().is_some();
@@ -473,6 +498,7 @@ impl App {
             channels_root: channels_root.clone(),
             library_root,
             library,
+            library_load_rx: Some(library_load_rx),
             sidebar_view: SidebarView::All,
             selected_video: None,
             note_buffer: String::new(),
@@ -575,7 +601,7 @@ impl App {
         ChannelOptionsForm {
             quality_idx,
             audio_only: opts.audio_only,
-            fetch_comments: opts.fetch_comments,
+            comments_idx: tri_to_idx(opts.fetch_comments),
             skip_auth_check: opts.skip_auth_check,
             limit_rate_kb: num(opts.limit_rate_kb),
             min_filesize_mb: num(opts.min_filesize_mb),
@@ -613,7 +639,7 @@ impl App {
         crate::download_options::DownloadOptions {
             quality,
             audio_only: f.audio_only,
-            fetch_comments: f.fetch_comments,
+            fetch_comments: idx_to_tri(f.comments_idx),
             skip_auth_check: f.skip_auth_check,
             limit_rate_kb: parse_num(&f.limit_rate_kb),
             min_filesize_mb: parse_num(&f.min_filesize_mb),
@@ -946,7 +972,7 @@ impl App {
         let progress = self.dedup_progress.clone();
         let (tx, rx) = std::sync::mpsc::channel();
         self.dedup_rx = Some(rx);
-        let workers = std::thread::available_parallelism().map(|n| n.get()).unwrap_or(4);
+        let workers = crate::fingerprint::default_workers();
         std::thread::spawn(move || {
             let res = crate::fingerprint::rebuild_and_group(
                 &db, inputs, &valid_paths, workers, &progress.0, &progress.1,
@@ -2038,6 +2064,19 @@ impl App {
     fn maintenance_screen(&mut self, ctx: &egui::Context) {
         let report = self.health_report.clone().unwrap_or_default();
 
+        // Swap in the library once the deferred startup scan finishes.
+        let loaded_library = self.library_load_rx.as_ref().and_then(|rx| rx.try_recv().ok());
+        if let Some(lib) = loaded_library {
+            self.library = lib;
+            self.status = format!(
+                "{} channels, {} videos",
+                self.library.len(),
+                self.library.iter().map(|c| c.total_videos()).sum::<usize>()
+            );
+            self.library_load_rx = None;
+            ctx.request_repaint();
+        }
+
         // Drain the background dedup result if it's ready.
         let mut dedup_done = None;
         if let Some(rx) = &self.dedup_rx {
@@ -2387,8 +2426,14 @@ impl App {
                     ui.end_row();
 
                     ui.label("Fetch comments");
-                    ui.checkbox(&mut form.fetch_comments, "Embed --write-comments into info.json")
+                    egui::ComboBox::from_id_salt("ch_comments")
-                        .on_hover_text("Slow on popular videos. Once captured, comments are browsable from the player modal in the web UI.");
+                        .selected_text(match form.comments_idx { 1 => "On", 2 => "Off", _ => "Default (global)" })
+                        .show_ui(ui, |ui| {
+                            ui.selectable_value(&mut form.comments_idx, 0, "Default (global)");
+                            ui.selectable_value(&mut form.comments_idx, 1, "On");
+                            ui.selectable_value(&mut form.comments_idx, 2, "Off");
+                        })
+                        .response.on_hover_text("Default = use the global Fetch comments setting. Adds --write-comments — slow on popular videos. Once captured, comments are browsable from the player modal in the web UI.");
                     ui.end_row();
 
                     ui.label("Skip auth check");
@@ -3020,6 +3065,15 @@ impl App {
                                  overrides live in each channel's options.");
                         ui.end_row();
 
+                        ui.label("Fetch comments:");
+                        ui.checkbox(&mut self.config.backup.fetch_comments, "Download comments (--write-comments)")
+                            .on_hover_text(
+                                "Fetch each video's comment tree into its info.json so the \
+                                 web player's Comments tab is populated. Slow on popular \
+                                 videos (yt-dlp paginates through thousands of replies). \
+                                 Per-channel overrides live in each channel's options.");
+                        ui.end_row();
+
                         ui.label("Web UI port:");
                         ui.add(
                             egui::DragValue::new(&mut self.config.web.port)
@@ -3429,6 +3483,7 @@ impl App {
                         self.downloader.subtitle_defaults = self.config.subtitles.clone();
                         self.downloader.youtube_player_clients = self.config.backup.youtube_player_clients.clone();
                         self.downloader.sponsorblock_mode = self.config.backup.sponsorblock_mode.clone();
+                        self.downloader.fetch_comments = self.config.backup.fetch_comments;
                         self.downloader.convert_defaults = self.config.convert.clone();
                         if dir_changed {
                             self.channels_root = new_dir.clone();

src/config.rs

@@ -134,6 +134,13 @@ pub struct BackupSection {
     /// overrides live in [`crate::download_options::DownloadOptions`].
     #[serde(default = "default_sponsorblock_mode")]
     pub sponsorblock_mode: String,
+    /// Download the comment tree into each video's info.json sidecar via
+    /// yt-dlp `--write-comments`. Off by default — comment download is slow
+    /// (yt-dlp paginates through thousands of replies on popular videos).
+    /// When on, the player's Comments tab is populated. Per-channel overrides
+    /// live in [`crate::download_options::DownloadOptions`].
+    #[serde(default)]
+    pub fetch_comments: bool,
 }
 
 fn default_max_concurrent() -> usize { 3 }
@@ -273,6 +280,7 @@ impl Config {
                 use_pot_provider: false,
                 youtube_player_clients: String::new(),
                 sponsorblock_mode: default_sponsorblock_mode(),
+                fetch_comments: false,
             },
             player: PlayerSection::default(),
             ui: UiSection::default(),

src/download_options.rs

@@ -112,12 +112,14 @@ pub struct DownloadOptions {
     #[serde(default)]
     pub extra_args: Vec<String>,
 
-    /// Embed video comments into the info.json sidecar (yt-dlp
+    /// Per-channel override for fetching video comments into the info.json
-    /// `--write-comments`). Off by default because comment download is slow
+    /// sidecar (yt-dlp `--write-comments`). `None` defers to the global
-    /// — yt-dlp paginates through thousands of replies for popular videos.
+    /// `backup.fetch_comments`; `Some(true)`/`Some(false)` forces it on/off
-    /// When on, the web UI's player modal exposes a Comments tab.
+    /// for this channel. Resolved in the downloader's comment resolver, which
+    /// merges this with the global default. When on, the player's Comments
+    /// tab is populated (comment download is slow on popular videos).
     #[serde(default)]
-    pub fetch_comments: bool,
+    pub fetch_comments: Option<bool>,
 
     /// Skip yt-dlp's channel-tab authentication sanity check by passing
     /// `--extractor-args youtubetab:skip=authcheck`.
@@ -179,9 +181,9 @@ impl DownloadOptions {
                 cmd.arg(arg);
             }
         }
-        if self.fetch_comments {
+        // Comment fetching (--write-comments) is emitted by the downloader's
-            cmd.arg("--write-comments");
+        // comment resolver, which merges this per-channel override with the
-        }
+        // global backup.fetch_comments default. apply() handles the rest.
         if self.skip_auth_check {
             // Its own --extractor-args flag; the youtubetab: namespace is
             // distinct from the POT provider's youtubepot-bgutilhttp: one,

src/downloader.rs

@@ -18,6 +18,7 @@
 //! | `--embed-metadata --embed-info-json --embed-chapters` | Embed rich metadata into the MKV |
 //! | `--xattrs` | Store metadata in filesystem extended attributes |
 //! | `--sponsorblock-mark/-remove all` | SponsorBlock handling, per `sponsorblock_mode` (off/mark/remove; see [`Self::apply_sponsorblock`]) |
+//! | `--write-comments` | Fetch the comment tree, per `fetch_comments` global + per-channel override (see [`Self::apply_comments`]) |
 //! | `--impersonate <target>` | Browser TLS fingerprint per source platform (see [`crate::platform::Platform::impersonate_target`]) |
 //! | `--break-on-existing` | Stop when the archive file records the video as already downloaded |
 //! | `--download-archive archive.txt` | Record downloaded IDs to avoid re-downloading |
@@ -317,6 +318,10 @@ pub struct Downloader {
     /// Global `backup.sponsorblock_mode` ("off" / "mark" / "remove").
     /// Per-channel options can override. Set at construction + on save.
     pub sponsorblock_mode: String,
+    /// Global `backup.fetch_comments`. When true, downloads pass
+    /// `--write-comments`. Per-channel options can override. Set at
+    /// construction + on settings save.
+    pub fetch_comments: bool,
     /// Global `[convert]` config. Drives the post-download ffmpeg pass.
     /// Per-channel options override the mode. Set at construction + save.
     pub convert_defaults: crate::config::ConvertSection,
@@ -363,6 +368,7 @@ impl Downloader {
             subtitle_defaults: crate::config::SubtitlesSection::default(),
             youtube_player_clients: String::new(),
             sponsorblock_mode: "mark".to_string(),
+            fetch_comments: false,
             convert_defaults: crate::config::ConvertSection::default(),
             retry_queue: Vec::new(),
             rate_limited_backoff: false,
@@ -411,6 +417,18 @@ impl Downloader {
         }
     }
 
+    /// Add `--write-comments` when comment fetching is enabled, merging the
+    /// per-channel override (`Some(true)`/`Some(false)`) with the global
+    /// `fetch_comments` default (used when the override is `None`).
+    fn apply_comments(&self, cmd: &mut Command, opts: Option<&DownloadOptions>) {
+        let enabled = opts
+            .and_then(|o| o.fetch_comments)
+            .unwrap_or(self.fetch_comments);
+        if enabled {
+            cmd.arg("--write-comments");
+        }
+    }
+
     fn apply_subtitle_flags(&self, cmd: &mut Command, opts: Option<&DownloadOptions>) {
         let g = &self.subtitle_defaults;
         // Per-channel override-or-global for each knob.
@@ -763,6 +781,8 @@ impl Downloader {
         self.apply_player_client(&mut cmd, channel_options);
         // SponsorBlock: global default + per-channel override (off/mark/remove).
         self.apply_sponsorblock(&mut cmd, channel_options);
+        // Comment fetching: global backup.fetch_comments + per-channel override.
+        self.apply_comments(&mut cmd, channel_options);
         // Post-download conversion: resolve global [convert] + per-channel
         // override. When active, ask yt-dlp to print each finished file's
         // final path (after_move) so we can enqueue an ffmpeg pass on it.

src/fingerprint.rs

@@ -114,6 +114,18 @@ pub struct FpComputed {
     pub hashes: Vec<u64>,
 }
 
+/// How many worker threads the dedup/fingerprint pass should use by default.
+/// Deliberately leaves CPU headroom — each worker also spawns an ffmpeg
+/// process for frame extraction, and the pass is a background maintenance job
+/// that must not starve the UI, downloads, or scans. Uses roughly half the
+/// cores, clamped to `[1, cores-1]` so at least one core stays free.
+pub fn default_workers() -> usize {
+    let cores = std::thread::available_parallelism()
+        .map(|n| n.get())
+        .unwrap_or(4);
+    (cores / 2).clamp(1, cores.saturating_sub(1).max(1))
+}
+
 /// Fingerprint many videos in parallel, bumping `progress` after each one so a
 /// UI can show "N / total". Mirrors the library scanner's hand-rolled worker
 /// pool (no rayon dependency).
@@ -312,6 +324,17 @@ pub fn rebuild_and_group(
 mod tests {
     use super::*;
 
+    #[test]
+    fn default_workers_leaves_headroom() {
+        let w = default_workers();
+        let cores = std::thread::available_parallelism().map(|n| n.get()).unwrap_or(4);
+        assert!(w >= 1, "must use at least one worker");
+        // On any multi-core machine at least one core stays free.
+        if cores > 1 {
+            assert!(w < cores, "should leave a core free (w={w}, cores={cores})");
+        }
+    }
+
     #[test]
     fn dhash_gradients() {
         // Strictly increasing rows → every pixel < its right neighbour → no

src/web.rs

@@ -416,6 +416,10 @@ struct SettingsPayload {
     /// SponsorBlock mode: "off" / "mark" / "remove".
     #[serde(default)]
     sponsorblock_mode: String,
+    /// Global comment fetching (`--write-comments`). Per-channel overrides
+    /// live in each channel's DownloadOptions, not here.
+    #[serde(default)]
+    fetch_comments: bool,
     /// Global [convert] settings, round-tripped on GET + POST.
     #[serde(default)]
     convert_mode: String,
@@ -453,6 +457,22 @@ fn file_url(library_root: &StdPath, full: &StdPath) -> Option<String> {
     Some(format!("/files/{}", parts.join("/")))
 }
 
+/// Like `file_url` but targets the `/api/sub-vtt/…` endpoint. All subtitle
+/// tracks are routed through that handler (not the raw `/files/` mount) so
+/// their cue position/alignment settings get stripped before the player
+/// renders them — otherwise auto-generated captions render left-aligned.
+fn sub_vtt_url(library_root: &StdPath, full: &StdPath) -> Option<String> {
+    let rel = full.strip_prefix(library_root).ok()?;
+    let mut parts: Vec<String> = Vec::new();
+    for c in rel.components() {
+        if let std::path::Component::Normal(s) = c {
+            parts.push(percent_encode_segment(s.to_str()?));
+        }
+    }
+    if parts.is_empty() { return None; }
+    Some(format!("/api/sub-vtt/{}", parts.join("/")))
+}
+
 fn percent_encode_segment(s: &str) -> String {
     use std::fmt::Write;
     let mut out = String::with_capacity(s.len());
@@ -603,7 +623,42 @@ fn srt_to_vtt(srt: &str) -> String {
     let mut out = String::from("WEBVTT\n\n");
     for line in srt.lines() {
         if line.contains("-->") {
-            out.push_str(&line.replace(',', "."));
+            out.push_str(&strip_cue_settings(&line.replace(',', ".")));
+        } else {
+            out.push_str(line);
+        }
+        out.push('\n');
+    }
+    out
+}
+
+/// Drop the per-cue position/alignment settings ("align:start position:0%",
+/// "line:…", "size:…") that yt-dlp's auto-generated captions append to every
+/// cue timing line. Browsers honor them and left-align/offset the text;
+/// removing them restores the default centered, bottom rendering. Only the
+/// settings *after* the "start --> end" timestamps are removed — the two
+/// timestamps are preserved verbatim.
+fn strip_cue_settings(timing_line: &str) -> String {
+    match timing_line.find("-->") {
+        Some(arrow) => {
+            let start = timing_line[..arrow].trim_end();
+            let after = timing_line[arrow + 3..].trim_start();
+            // The end timestamp is the first whitespace-delimited token; any
+            // cue settings follow it and are dropped.
+            let end = after.split_whitespace().next().unwrap_or("");
+            format!("{start} --> {end}")
+        }
+        None => timing_line.to_string(),
+    }
+}
+
+/// Strip cue settings from an already-WebVTT document (leaves the header,
+/// cue identifiers, and text untouched — only timing lines are normalized).
+fn normalize_vtt(vtt: &str) -> String {
+    let mut out = String::with_capacity(vtt.len());
+    for line in vtt.lines() {
+        if line.contains("-->") {
+            out.push_str(&strip_cue_settings(line));
         } else {
             out.push_str(line);
         }
@@ -817,6 +872,27 @@ mod tests {
         assert!(vtt.contains("Hello, world"));
     }
 
+    #[test]
+    fn strip_cue_settings_drops_alignment_keeps_timestamps() {
+        let line = "00:00:01.000 --> 00:00:03.000 align:start position:0%";
+        assert_eq!(strip_cue_settings(line), "00:00:01.000 --> 00:00:03.000");
+        // A plain timing line is unchanged.
+        let plain = "00:00:01.000 --> 00:00:03.000";
+        assert_eq!(strip_cue_settings(plain), plain);
+        // Non-timing lines pass through untouched.
+        assert_eq!(strip_cue_settings("Hello align:start"), "Hello align:start");
+    }
+
+    #[test]
+    fn normalize_vtt_strips_settings_only_on_timing_lines() {
+        let vtt = "WEBVTT\n\n00:00:01.000 --> 00:00:03.000 align:start position:10%\nHello\n";
+        let out = normalize_vtt(vtt);
+        assert!(out.contains("00:00:01.000 --> 00:00:03.000\n"));
+        assert!(!out.contains("align:start"));
+        assert!(out.contains("Hello"));
+        assert!(out.starts_with("WEBVTT"));
+    }
+
     #[test]
     fn count_cookies_only_counts_seven_field_lines() {
         let body = "# Netscape HTTP Cookie File\n\
@@ -1214,14 +1290,9 @@ async fn build_library_payload(state: &Arc<WebState>) -> LibraryResponse {
         let thumb_url = v.thumb_path.as_deref().and_then(|p| file_url(root, p));
         let subtitles: Vec<SubtitleInfo> = v.subtitles.iter()
             .filter_map(|s| {
-                let is_srt = s.path.extension().and_then(|e| e.to_str()) == Some("srt");
+                // Route every track (SRT and VTT alike) through /api/sub-vtt so
-                let url = if is_srt {
+                // cue settings are stripped and SRT is converted on the fly.
-                    // Route SRT through the on-the-fly conversion endpoint.
+                let url = sub_vtt_url(root, &s.path)?;
-                    let rel = s.path.strip_prefix(root).ok()?;
-                    Some(format!("/api/sub-vtt/{}", rel.display()))
-                } else {
-                    file_url(root, &s.path)
-                }?;
                 Some(SubtitleInfo {
                     lang: s.lang.clone(),
                     label: lang_label(&s.lang),
@@ -1444,6 +1515,7 @@ async fn get_settings(State(state): State<Arc<WebState>>) -> impl IntoResponse {
     let subs = cfg.subtitles.clone();
     let player_clients_out = cfg.backup.youtube_player_clients.clone();
     let sponsorblock_out = cfg.backup.sponsorblock_mode.clone();
+    let fetch_comments_out = cfg.backup.fetch_comments;
     let convert = cfg.convert.clone();
     drop(cfg);
 
@@ -1483,6 +1555,7 @@ async fn get_settings(State(state): State<Arc<WebState>>) -> impl IntoResponse {
         subtitle_langs: subs.langs,
         youtube_player_clients: player_clients_out.clone(),
         sponsorblock_mode: sponsorblock_out.clone(),
+        fetch_comments: fetch_comments_out,
         convert_mode: convert.mode.clone(),
         convert_crf: convert.crf,
         convert_preset: convert.preset.clone(),
@@ -1529,6 +1602,7 @@ async fn post_settings(
     cfg.subtitles.langs = body.subtitle_langs.trim().to_string();
     cfg.backup.youtube_player_clients = body.youtube_player_clients.trim().to_string();
     cfg.backup.sponsorblock_mode = body.sponsorblock_mode.trim().to_string();
+    cfg.backup.fetch_comments = body.fetch_comments;
     // Global format-conversion defaults.
     cfg.convert.mode = body.convert_mode.trim().to_string();
     cfg.convert.crf = body.convert_crf;
@@ -1551,6 +1625,7 @@ async fn post_settings(
     let subs = cfg.subtitles.clone();
     let player_clients_out = cfg.backup.youtube_player_clients.clone();
     let sponsorblock_out = cfg.backup.sponsorblock_mode.clone();
+    let fetch_comments_out = cfg.backup.fetch_comments;
     let convert = cfg.convert.clone();
     drop(cfg);
 
@@ -1565,6 +1640,7 @@ async fn post_settings(
         dl.subtitle_defaults = subs.clone();
         dl.youtube_player_clients = player_clients_out.clone();
         dl.sponsorblock_mode = sponsorblock_out.clone();
+        dl.fetch_comments = fetch_comments_out;
         dl.convert_defaults = convert.clone();
     }
 
@@ -1622,6 +1698,7 @@ async fn post_settings(
         subtitle_langs: subs.langs,
         youtube_player_clients: player_clients_out.clone(),
         sponsorblock_mode: sponsorblock_out.clone(),
+        fetch_comments: fetch_comments_out,
         convert_mode: convert.mode.clone(),
         convert_crf: convert.crf,
         convert_preset: convert.preset.clone(),
@@ -1630,7 +1707,9 @@ async fn post_settings(
     }).into_response()
 }
 
-/// `GET /api/sub-vtt/*path` — serve an SRT subtitle file as WebVTT.
+/// `GET /api/sub-vtt/*path` — serve a subtitle file as WebVTT: SRT is
+/// converted on the fly, VTT is normalized; both have per-cue position/
+/// alignment settings stripped so captions render centered, not left-aligned.
 ///
 /// The path is relative to the channels root.  The file must be within the
 /// channels root (path traversal is rejected with 403).
@@ -1651,7 +1730,8 @@ async fn get_sub_vtt(
         Ok(s) => s,
         Err(_) => return StatusCode::NOT_FOUND.into_response(),
     };
-    let vtt = srt_to_vtt(&content);
+    let is_srt = path.extension().and_then(|e| e.to_str()) == Some("srt");
+    let vtt = if is_srt { srt_to_vtt(&content) } else { normalize_vtt(&content) };
     ([(header::CONTENT_TYPE, "text/vtt; charset=utf-8")], vtt).into_response()
 }
 
@@ -2226,7 +2306,7 @@ fn run_dedup(
     by_path: HashMap<String, SimilarVideo>,
     valid_paths: HashSet<String>,
 ) {
-    let workers = std::thread::available_parallelism().map(|n| n.get()).unwrap_or(4);
+    let workers = crate::fingerprint::default_workers();
     let outcome: Result<Vec<SimilarGroup>, String> =
         crate::fingerprint::rebuild_and_group(&db, inputs, &valid_paths, workers, &dedup.done, &dedup.total)
             .map(|path_groups| {
@@ -2853,6 +2933,7 @@ async fn serve(config: Config, shutdown_rx: std::sync::mpsc::Receiver<()>) {
     downloader.subtitle_defaults = config.subtitles.clone();
     downloader.youtube_player_clients = config.backup.youtube_player_clients.clone();
     downloader.sponsorblock_mode = config.backup.sponsorblock_mode.clone();
+    downloader.fetch_comments = config.backup.fetch_comments;
     downloader.convert_defaults = config.convert.clone();
     let music_root = downloader.music_root();
     let _ = std::fs::create_dir_all(&music_root);

src/web_ui/index.html

@@ -142,7 +142,7 @@
   .dl-new-flags{display:flex;gap:12px;flex-wrap:wrap;font-size:12px;color:var(--muted)}
   .dl-new-flags label{display:flex;align-items:center;gap:4px;cursor:pointer;white-space:nowrap}
   .preview-thumb{width:100%;max-width:280px;aspect-ratio:16/9;object-fit:cover;border-radius:4px;background:#000;flex-shrink:0}
-  .empty{text-align:center;color:var(--muted);padding:40px;font-size:13px}
+  .empty{grid-column:1/-1;text-align:center;color:var(--muted);padding:40px;font-size:13px}
   @media(max-width:640px){
     #menu-btn{display:block}
     aside{position:fixed;top:0;left:0;height:100%;z-index:50;transform:translateX(-100%);width:240px}
@@ -776,7 +776,7 @@ function renderChannelGrid(){
   const chs=q?library.filter(ch=>ch.name.toLowerCase().includes(q)||ch.uploader?.toLowerCase().includes(q)):library;
   setStatus(chs.length+' channel'+(chs.length!==1?'s':''));
   if(!chs.length){grid.innerHTML='<div class="empty">Nothing here.</div>';return}
-  grid.innerHTML='<div style="display:grid;grid-template-columns:repeat(auto-fill,minmax(160px,1fr));gap:12px;padding:4px">'+chs.map((ch,i)=>{
+  grid.innerHTML='<div style="grid-column:1/-1;display:grid;grid-template-columns:repeat(auto-fill,minmax(160px,1fr));gap:12px;padding:4px">'+chs.map((ch,i)=>{
     const idx=library.indexOf(ch);
     const thumb=ch.thumb_url?`<img src="${ch.thumb_url}" loading="lazy" alt="">`:'<div class="nothumb">📺</div>';
     const size=ch.size_bytes>0?` · ${fmtSize(ch.size_bytes)}`:'';
@@ -799,7 +799,7 @@ function renderMusicGrid(){
   const tracks=q?musicTracks.filter(t=>t.title.toLowerCase().includes(q)||t.artist.toLowerCase().includes(q)):musicTracks;
   setStatus(tracks.length+' track'+(tracks.length!==1?'s':''));
   if(!tracks.length){grid.innerHTML='<div class="empty">No music yet. Use 🎵 Music mode in the download bar.</div>';return}
-  let currentArtist='',h='<div style="width:100%;padding:4px">';
+  let currentArtist='',h='<div style="grid-column:1/-1;width:100%;padding:4px">';
   for(const t of tracks){
     if(t.artist!==currentArtist){
       currentArtist=t.artist;
@@ -994,9 +994,9 @@ async function openChannelOptions(idx){
     <div class="settings-row"><label>Audio-only</label>
       <input type="checkbox" id="opt-audio" ${opts.audio_only?'checked':''}></div>
     <div class="settings-hint" style="margin-bottom:4px">Useful for music channels — saves disk + skips video re-encode.</div>
-    <div class="settings-row"><label>Fetch comments</label>
+    <div class="settings-row"><label>Fetch comments (blank = global)</label>
-      <input type="checkbox" id="opt-comments" ${opts.fetch_comments?'checked':''}></div>
+      ${triSelect('opt-comments',opts.fetch_comments)}</div>
-    <div class="settings-hint" style="margin-bottom:4px">Adds <code>--write-comments</code>. Embeds the comment tree into info.json — slow for popular videos but enables the Comments tab in the player.</div>
+    <div class="settings-hint" style="margin-bottom:4px">Adds <code>--write-comments</code>. Embeds the comment tree into info.json — slow for popular videos but enables the Comments tab in the player. "Default" uses the global setting.</div>
     <div class="settings-row"><label>Skip auth check</label>
       <input type="checkbox" id="opt-skipauth" ${opts.skip_auth_check?'checked':''}></div>
     <div class="settings-hint" style="margin-bottom:4px">Adds <code>--extractor-args youtubetab:skip=authcheck</code>. Safe for <strong>public</strong> channels — silences the "playlists that require authentication may not extract correctly" warning without changing which videos are found. Leave off for members-only/private channels, where that warning means your cookies may not be working.</div>
@@ -1058,7 +1058,7 @@ function readChannelOptionsForm(){
   return {
     quality:q?q:null,
     audio_only:document.getElementById('opt-audio')?.checked||false,
-    fetch_comments:document.getElementById('opt-comments')?.checked||false,
+    fetch_comments:triValue('opt-comments'),
     skip_auth_check:document.getElementById('opt-skipauth')?.checked||false,
     limit_rate_kb:intOrNull('opt-rate'),
     min_filesize_mb:intOrNull('opt-minsz'),
@@ -1785,6 +1785,11 @@ async function openSettings(){
       </select>
     </div>
     <div class="settings-hint" style="margin-bottom:6px">Uses the community SponsorBlock database for sponsor / intro / self-promo segments. <b>Mark</b> adds skippable chapter markers; <b>Remove</b> cuts the segments out of the saved file. Per-channel overrides live in each channel's options.</div>
+    <div class="settings-row" style="margin-top:8px">
+      <label for="cf-comments">Fetch comments</label>
+      <input type="checkbox" id="cf-comments" ${cur.fetch_comments?'checked':''}>
+    </div>
+    <div class="settings-hint" style="margin-bottom:6px">Adds <code>--write-comments</code> to every download — fetches each video's comment tree into its info.json so the player's Comments tab works. Slow on popular videos. Per-channel overrides live in each channel's options.</div>
     <hr style="border-color:var(--border);margin:12px 0">
     <div style="font-weight:700;margin-bottom:8px">Subtitles (global defaults)</div>
     <div class="settings-hint" style="margin-bottom:6px">Applied to every download. Individual channels can override these in their Channel options dialog.</div>
@@ -2027,12 +2032,13 @@ async function saveSettings(btn){
   const subsFormat=document.getElementById('cf-subs-format')?.value||'';
   const playerClients=document.getElementById('cf-player-clients')?.value||'';
   const sponsorblock=document.getElementById('cf-sponsorblock')?.value||'mark';
+  const fetchComments=document.getElementById('cf-comments')?.checked||false;
   const convMode=document.getElementById('cf-convert-mode')?.value||'';
   const convCrf=parseInt(document.getElementById('cf-convert-crf')?.value||'23',10);
   const convPreset=document.getElementById('cf-convert-preset')?.value||'';
   const convAudio=document.getElementById('cf-convert-audio')?.value||'';
   const convKeep=document.getElementById('cf-convert-keep')?.checked||false;
-  const payload={transcode,scheduler_enabled:schedEnabled,scheduler_interval_hours:schedInterval,max_concurrent:maxConcurrent,use_bundled_ytdlp:useBundledYtdlp,use_pot_provider:usePotProvider,subtitles_enabled:subsEnabled,subtitles_auto:subsAuto,subtitles_embed:subsEmbed,subtitle_langs:subsLangs,subtitle_format:subsFormat,youtube_player_clients:playerClients,sponsorblock_mode:sponsorblock,convert_mode:convMode,convert_crf:convCrf,convert_preset:convPreset,convert_audio_format:convAudio,convert_keep_original:convKeep};
+  const payload={transcode,scheduler_enabled:schedEnabled,scheduler_interval_hours:schedInterval,max_concurrent:maxConcurrent,use_bundled_ytdlp:useBundledYtdlp,use_pot_provider:usePotProvider,subtitles_enabled:subsEnabled,subtitles_auto:subsAuto,subtitles_embed:subsEmbed,subtitle_langs:subsLangs,subtitle_format:subsFormat,youtube_player_clients:playerClients,sponsorblock_mode:sponsorblock,fetch_comments:fetchComments,convert_mode:convMode,convert_crf:convCrf,convert_preset:convPreset,convert_audio_format:convAudio,convert_keep_original:convKeep};
   if(bindMode)payload.bind_mode=bindMode;
   if(plexPath!==undefined)payload.plex_library_path=plexPath;
   if(sourceUrl!==undefined)payload.source_url=sourceUrl;