From d795b9a25eb14a33fa10c1059cd0360981489abb Mon Sep 17 00:00:00 2001
From: OorjitSethi <90506135+OorjitSethi@users.noreply.github.com>
Date: Sun, 11 Oct 2026 18:26:55 -0500
Subject: [PATCH] PERF-01: isolate editor snapshot and transcript redraw
 optimizations

Split from #728 on current main. Full validation is recorded in the draft PR; the trim and mask-modifier failures reproduce on unchanged main.

Co-Authored-By: Codex <noreply@openai.com>
---
 crates/ui-egui/src/lib.rs                     |   2 +
 crates/ui-egui/src/panels/effect_controls.rs  |   5 +-
 crates/ui-egui/src/panels/misc.rs             |   5 +-
 crates/ui-egui/src/panels/mixer.rs            |  14 ++-
 crates/ui-egui/src/panels/mod.rs              |   2 +
 crates/ui-egui/src/panels/text.rs             |  14 ++-
 crates/ui-egui/src/panels/timeline.rs         |  23 ++--
 crates/ui-egui/src/panels/transcript_cache.rs | 106 ++++++++++++++++++
 crates/ui-egui/tests/transcript_ui.rs         |  42 +++++++
 docs/performance.md                           |  21 ++++
 10 files changed, 212 insertions(+), 22 deletions(-)
 create mode 100644 crates/ui-egui/src/panels/transcript_cache.rs

diff --git a/crates/ui-egui/src/lib.rs b/crates/ui-egui/src/lib.rs
index 2e3a7beb..421a6329 100644
--- a/crates/ui-egui/src/lib.rs
+++ b/crates/ui-egui/src/lib.rs
@@ -344,6 +344,7 @@ pub struct FilmcraftApp {
     pub shortcut_editor: panels::shortcuts_dialog::EditorState,
     pub toast: Option<(String, f64)>,
     pub tl: panels::timeline::TlState,
+    pub(crate) transcript_cache: panels::transcript_cache::TranscriptCache,
     /// Commands from outside the UI (native menu bar), invoked on the UI thread.
     pub command_inbox: Option<Receiver<String>>,
     /// A V key press (or a Ctrl+V paste) reached the app and its release hasn't yet.
@@ -568,6 +569,7 @@ impl FilmcraftApp {
             toast: None,
             graphics_notice: None,
             tl: Default::default(),
+            transcript_cache: Default::default(),
             command_inbox: None,
             v_down: false,
             stale_clipboard_media: None,
diff --git a/crates/ui-egui/src/panels/effect_controls.rs b/crates/ui-egui/src/panels/effect_controls.rs
index d453e8e2..972b1004 100644
--- a/crates/ui-egui/src/panels/effect_controls.rs
+++ b/crates/ui-egui/src/panels/effect_controls.rs
@@ -99,7 +99,10 @@ pub fn show(app: &mut FilmcraftApp, ui: &mut egui::Ui, rect: Rect) {
         crate::dock::placeholder(ui, rect, &t, tl!("(no clip selected)"));
         return;
     };
-    let Some(seq) = app.session.active_sequence().cloned() else {
+    // Keep the immutable project snapshot alive while UI commands mutate the session.
+    // Cloning a sequence here copies every clip and effect on every redraw.
+    let project = app.session.project.clone();
+    let Some(seq) = app.session.state.active_sequence.and_then(|id| project.sequence(id)) else {
         crate::dock::placeholder(ui, rect, &t, tl!("(no sequences)"));
         return;
     };
diff --git a/crates/ui-egui/src/panels/misc.rs b/crates/ui-egui/src/panels/misc.rs
index 63e494b8..01a2aee2 100644
--- a/crates/ui-egui/src/panels/misc.rs
+++ b/crates/ui-egui/src/panels/misc.rs
@@ -61,7 +61,10 @@ pub const MARKER_FILTER: [filmcraft_project::Label; 7] = {
 /// `markers.filter.<label>`), then the sequence markers (`markers.row.<id>`; click = go to it).
 pub fn markers(app: &mut FilmcraftApp, ui: &mut egui::Ui, rect: Rect) {
     let t = app.tokens;
-    let Some(seq) = app.session.active_sequence().cloned() else {
+    // Keep the immutable project snapshot alive while UI commands mutate the session.
+    // Cloning a sequence here copies every clip and effect on every redraw.
+    let project = app.session.project.clone();
+    let Some(seq) = app.session.state.active_sequence.and_then(|id| project.sequence(id)) else {
         crate::dock::placeholder(ui, rect, &t, tl!("(no sequence)"));
         return;
     };
diff --git a/crates/ui-egui/src/panels/mixer.rs b/crates/ui-egui/src/panels/mixer.rs
index 565b171d..50083d5d 100644
--- a/crates/ui-egui/src/panels/mixer.rs
+++ b/crates/ui-egui/src/panels/mixer.rs
@@ -207,12 +207,15 @@ fn targets(seq: &Sequence, id: TrackId) -> Vec<&Track> {
 
 pub fn track_mixer(app: &mut FilmcraftApp, ui: &mut egui::Ui, rect: Rect) {
     let t = app.tokens;
-    let Some(seq) = app.session.active_sequence().cloned() else {
+    // Keep the immutable project snapshot alive while UI commands mutate the session.
+    // Cloning a sequence here copies every clip and effect on every redraw.
+    let project = app.session.project.clone();
+    let Some(seq) = app.session.state.active_sequence.and_then(|id| project.sequence(id)) else {
         crate::dock::placeholder(ui, rect, &t, tl!("(no sequence)"));
         return;
     };
     let meters = poll_meters(app, ui);
-    let mut cx = Ctx { t, seq: &seq, now: app.session.playhead(), recording: app.session.mixrec.active(), acts: Vec::new() };
+    let mut cx = Ctx { t, seq, now: app.session.playhead(), recording: app.session.mixrec.active(), acts: Vec::new() };
     let footer = Rect::from_min_max(pos2(rect.min.x, rect.max.y - 34.0), rect.max);
     let body = Rect::from_min_max(pos2(rect.min.x, rect.min.y + 2.0), pos2(rect.max.x, footer.min.y));
     // effects / sends disclosure at the left edge
@@ -250,7 +253,7 @@ pub fn track_mixer(app: &mut FilmcraftApp, ui: &mut egui::Ui, rect: Rect) {
             ui.ctx(),
             |ui| {
                 for id in seq.audio_tracks.iter().chain(&seq.submix_tracks).map(|t| t.id) {
-                    let label = strip_label(&seq, id);
+                    let label = strip_label(seq, id);
                     let name = seq.mix_track(id).map(|t| t.name.clone()).unwrap_or_default();
                     let mut shown = !app.ui.mixer_hidden.contains(&id.0);
                     let r = ui.checkbox(&mut shown, format!("{label}  {name}"));
@@ -888,7 +891,10 @@ fn transport(app: &mut FilmcraftApp, ui: &mut egui::Ui, row: Rect, cx: &mut Ctx)
 /// Volume and Panner). With the keyframe button on, changes add keyframes at the playhead.
 pub fn clip_mixer(app: &mut FilmcraftApp, ui: &mut egui::Ui, rect: Rect) {
     let t = app.tokens;
-    let Some(seq) = app.session.active_sequence().cloned() else {
+    // Keep the immutable project snapshot alive while UI commands mutate the session.
+    // Cloning a sequence here copies every clip and effect on every redraw.
+    let project = app.session.project.clone();
+    let Some(seq) = app.session.state.active_sequence.and_then(|id| project.sequence(id)) else {
         crate::dock::placeholder(ui, rect, &t, tl!("(no sequence)"));
         return;
     };
diff --git a/crates/ui-egui/src/panels/mod.rs b/crates/ui-egui/src/panels/mod.rs
index e1d8aef7..8c91e894 100644
--- a/crates/ui-egui/src/panels/mod.rs
+++ b/crates/ui-egui/src/panels/mod.rs
@@ -344,3 +344,5 @@ pub fn panel_menu_popup(app: &mut FilmcraftApp, ui: &mut egui::Ui) {
         ui.ctx().data_mut(|d| d.remove::<(PanelKind, egui::Pos2)>(id));
     }
 }
+
+pub(crate) mod transcript_cache;
diff --git a/crates/ui-egui/src/panels/text.rs b/crates/ui-egui/src/panels/text.rs
index 4b61ed28..42db9be4 100644
--- a/crates/ui-egui/src/panels/text.rs
+++ b/crates/ui-egui/src/panels/text.rs
@@ -74,7 +74,8 @@ fn tool_button(app: &mut FilmcraftApp, ui: &mut egui::Ui, r: Rect, icon: Icon, i
 
 fn captions(app: &mut FilmcraftApp, ui: &mut egui::Ui, rect: Rect) {
     let t = app.tokens;
-    let Some(seq) = app.session.active_sequence().cloned() else {
+    let project = app.session.project.clone();
+    let Some(seq) = app.session.state.active_sequence.and_then(|id| project.sequence(id)) else {
         crate::dock::placeholder(ui, rect, &t, tl!("Open a sequence to work with captions"));
         return;
     };
@@ -304,7 +305,8 @@ fn transcript(app: &mut FilmcraftApp, ui: &mut egui::Ui, rect: Rect) {
         return;
     }
     let mut actions: Vec<(String, Value)> = Vec::new();
-    let words = filmcraft_engine::transcript::sequence_words(&app.session);
+    let content = app.transcript_cache.content(&app.session);
+    let words = &content.words;
     if words.is_empty() {
         let c = rect.center();
         icons::paint(ui.painter(), Rect::from_center_size(c - vec2(0.0, 70.0), vec2(40.0, 40.0)), Icon::Captions, t.text_dim);
@@ -335,7 +337,7 @@ fn transcript(app: &mut FilmcraftApp, ui: &mut egui::Ui, rect: Rect) {
     let mut child = ui.new_child(egui::UiBuilder::new().max_rect(Rect::from_min_size(bar.min, vec2(sw, 24.0))));
     let sresp = crate::widgets::search_field(&mut child, &mut app.ui.transcript_search, tl!("Search"), sw, &t);
     app.auto.add("text.transcript.search", sresp.rect, "Search transcript");
-    let hits: Vec<std::ops::Range<usize>> = filmcraft_edit::transcript::search(&words, &app.ui.transcript_search);
+    let highlights = app.transcript_cache.highlights(&app.ui.transcript_search);
     let mut x = bar.min.x + sw + 10.0;
     let range = sel.map(|(a, b)| json!({"from": a.min(b), "to": a.max(b)}));
     let tools: [(Icon, &str, &str, bool, &str, Value); 4] = [
@@ -362,8 +364,8 @@ fn transcript(app: &mut FilmcraftApp, ui: &mut egui::Ui, rect: Rect) {
     let list = Rect::from_min_max(pos2(rect.min.x + 6.0, bar.max.y + 8.0), pos2(rect.max.x - 6.0, rect.max.y - 4.0));
     ui.painter().rect_filled(list, 3.0, t.app_bg);
     let ph = app.session.playhead();
-    let current = filmcraft_edit::transcript::word_at(&words, ph);
-    let paras = filmcraft_edit::transcript::paragraphs(&words, filmcraft_time::Tick::from_seconds_f64(1.5));
+    let current = filmcraft_edit::transcript::word_at(words, ph);
+    let paras = &content.paragraphs;
     let rate = app.session.sequence_rate();
     let df = app.session.active_sequence().is_some_and(|q| q.settings.drop_frame);
     let mut child = ui.new_child(egui::UiBuilder::new().max_rect(list.shrink(6.0)).id_salt("transcript-list"));
@@ -379,7 +381,7 @@ fn transcript(app: &mut FilmcraftApp, ui: &mut egui::Ui, rect: Rect) {
                 for i in pr.clone() {
                     let w = &words[i];
                     let in_sel = sa.is_some_and(|a| i >= a) && sb.is_some_and(|b| i <= b);
-                    let hit = hits.iter().any(|h| h.contains(&i));
+                    let hit = highlights.get(i).copied().unwrap_or(false);
                     let mut text = egui::RichText::new(&w.text).size(13.0).color(if Some(i) == current { t.hot_text } else { t.text });
                     if in_sel {
                         text = text.background_color(t.row_selected);
diff --git a/crates/ui-egui/src/panels/timeline.rs b/crates/ui-egui/src/panels/timeline.rs
index e2cee888..6930ec35 100644
--- a/crates/ui-egui/src/panels/timeline.rs
+++ b/crates/ui-egui/src/panels/timeline.rs
@@ -273,7 +273,10 @@ pub fn show(app: &mut FilmcraftApp, ui: &mut egui::Ui, rect: Rect) {
         return;
     };
     // A damaged project can name an active sequence that no longer exists.
-    let Some(seq) = app.session.active_sequence().cloned() else {
+    // Keep the immutable project snapshot alive while UI commands mutate the session.
+    // Cloning a sequence here copies every clip and effect on every redraw.
+    let project = app.session.project.clone();
+    let Some(seq) = app.session.state.active_sequence.and_then(|id| project.sequence(id)) else {
         empty_state(app, ui, rect);
         return;
     };
@@ -446,7 +449,7 @@ pub fn show(app: &mut FilmcraftApp, ui: &mut egui::Ui, rect: Rect) {
     // clips whose media changes in the preview (a trimmed or rolled edge): drawn from the edited
     // item, so the waveform and thumbnail stay where the media is instead of squeezing (#374)
     let mut edited: HashMap<ClipId, TrackItem> = HashMap::new();
-    preview_drag(app, &seq, &mut previews, &mut edited);
+    preview_drag(app, seq, &mut previews, &mut edited);
     // Option-drag copies: the clips stay where they are and their copies follow the pointer
     let copying = matches!(app.tl.drag, Some(Drag::Move { copy: true, .. }));
     let copies: Vec<(ClipId, (Tick, Tick, Option<TrackId>))> = if copying { previews.drain().collect() } else { Vec::new() };
@@ -535,7 +538,7 @@ pub fn show(app: &mut FilmcraftApp, ui: &mut egui::Ui, rect: Rect) {
             }
         }
         // selected edit points (trim mode): red brackets for ripple, yellow for roll / regular trim
-        super::trim_monitor::paint_edit_points(app, &p, &seq, tr, r, &layout);
+        super::trim_monitor::paint_edit_points(app, &p, seq, tr, r, &layout);
         // transitions (one being dragged where the drag puts it; the selected ones outlined)
         for trn in &tr.transitions {
             let (start, end) = match &app.tl.drag {
@@ -556,7 +559,7 @@ pub fn show(app: &mut FilmcraftApp, ui: &mut egui::Ui, rect: Rect) {
     }
 
     // ---- track headers
-    draw_headers(app, ui, &seq, &rows, rect, vclip, aclip, &t);
+    draw_headers(app, ui, seq, &rows, rect, vclip, aclip, &t);
     // divider between video and audio
     let div = Rect::from_min_max(pos2(rect.min.x, split_y), pos2(content.max.x, split_y + DIVIDER_H));
     painter.rect_filled(div, 0.0, t.app_bg);
@@ -583,11 +586,11 @@ pub fn show(app: &mut FilmcraftApp, ui: &mut egui::Ui, rect: Rect) {
     // ---- caption tracks
     if cap_n > 0 {
         painter.rect_filled(Rect::from_min_max(pos2(rect.min.x, caption_area.max.y), pos2(content.max.x, caption_area.max.y + DIVIDER_H)), 0.0, t.app_bg);
-        super::timeline_captions::paint(app, ui, &seq, caption_area, &layout, &t);
+        super::timeline_captions::paint(app, ui, seq, caption_area, &layout, &t);
     }
 
     // ---- top block: timecode + toggles, ruler
-    draw_top(app, ui, rect, &seq, &layout, &t, seq_id);
+    draw_top(app, ui, rect, seq, &layout, &t, seq_id);
 
     // ---- playhead
     let ph = app.session.playhead();
@@ -636,12 +639,12 @@ pub fn show(app: &mut FilmcraftApp, ui: &mut egui::Ui, rect: Rect) {
     );
 
     // ---- interaction
-    interact(app, ui, &seq, &layout, rect);
-    super::timeline_volume::interact(app, ui, &seq, &layout, aclip);
+    interact(app, ui, seq, &layout, rect);
+    super::timeline_volume::interact(app, ui, seq, &layout, aclip);
     // track keyframes (drawn and edited on top of the clips)
-    super::timeline_automation::show(app, ui, &seq, &layout, aclip);
+    super::timeline_automation::show(app, ui, seq, &layout, aclip);
     if cap_n > 0 {
-        super::timeline_captions::interact(app, ui, &seq, caption_area, &layout);
+        super::timeline_captions::interact(app, ui, seq, caption_area, &layout);
     }
     let _ = (visible, TICKS_PER_SECOND);
 }
diff --git a/crates/ui-egui/src/panels/transcript_cache.rs b/crates/ui-egui/src/panels/transcript_cache.rs
new file mode 100644
index 00000000..f51e9395
--- /dev/null
+++ b/crates/ui-egui/src/panels/transcript_cache.rs
@@ -0,0 +1,106 @@
+//! Derived transcript data survives playhead movement and UI redraws, but never project edits.
+
+use std::ops::Range;
+use std::sync::{Arc, Weak};
+
+use filmcraft_edit::transcript::{SeqWord, paragraphs, search};
+use filmcraft_engine::Session;
+use filmcraft_project::{ItemId, Project};
+use filmcraft_time::Tick;
+
+#[derive(Default)]
+pub(crate) struct Content {
+    pub words: Vec<SeqWord>,
+    pub paragraphs: Vec<Range<usize>>,
+}
+
+#[derive(Default)]
+pub(crate) struct TranscriptCache {
+    // A weak key neither retains a closed project nor allows an allocation address to be reused.
+    project: Weak<Project>,
+    view: Option<ItemId>,
+    content: Arc<Content>,
+    query: Option<String>,
+    highlights: Arc<Vec<bool>>,
+}
+
+impl TranscriptCache {
+    pub fn content(&mut self, session: &Session) -> Arc<Content> {
+        let view = session.state.active_sequence;
+        let project = Arc::downgrade(&session.project);
+        if !self.project.ptr_eq(&project) || self.view != view {
+            let words = filmcraft_engine::transcript::sequence_words(session);
+            let paragraphs = paragraphs(&words, Tick::from_seconds_f64(1.5));
+            self.content = Arc::new(Content { words, paragraphs });
+            self.project = project;
+            self.view = view;
+            self.query = None;
+        }
+        self.content.clone()
+    }
+
+    pub fn highlights(&mut self, query: &str) -> Arc<Vec<bool>> {
+        if self.query.as_deref() != Some(query) {
+            let mut flags = vec![false; self.content.words.len()];
+            // Search ranges can overlap. Mark their union in linear time rather than checking
+            // every match for every word on every redraw.
+            let mut marked = 0;
+            for hit in search(&self.content.words, query) {
+                let start = hit.start.max(marked);
+                if let Some(slice) = flags.get_mut(start..hit.end) {
+                    slice.fill(true);
+                }
+                marked = marked.max(hit.end);
+            }
+            self.highlights = Arc::new(flags);
+            self.query = Some(query.to_owned());
+        }
+        self.highlights.clone()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use serde_json::json;
+
+    #[test]
+    fn cached_sequence_refreshes_after_edit_undo_and_project_replacement() {
+        let mut s = Session::default();
+        s.execute("file.openDemoProject", json!({})).unwrap();
+        let clip = s.active_sequence().unwrap().audio_tracks[0].items[0].clone();
+        let transcript = |middle: &str| {
+            json!({"words": [
+            {"text": "hello", "start": clip.source_in.0, "end": clip.source_in.0 + 100},
+            {"text": middle, "start": clip.source_in.0 + 100, "end": clip.source_in.0 + 200},
+            {"text": "hello", "start": clip.source_in.0 + 200, "end": clip.source_in.0 + 300}]})
+        };
+        s.execute("transcript.set", json!({"item": clip.item.0, "transcript": transcript("hello")})).unwrap();
+        let mut cache = TranscriptCache::default();
+        let first = cache.content(&s);
+        assert_eq!(&*cache.highlights("hello hello"), &[true, true, true]);
+        assert_eq!(&*cache.highlights("missing"), &[false, false, false]);
+        s.set_playhead(Tick(150));
+        assert!(Arc::ptr_eq(&first, &cache.content(&s)));
+        s.execute("transcript.set", json!({"item": clip.item.0, "transcript": transcript("world")})).unwrap();
+        assert_eq!(cache.content(&s).words[1].text, "world");
+        assert_eq!(&*cache.highlights("hello hello"), &[false, false, false]);
+        s.undo();
+        assert_eq!(cache.content(&s).words[1].text, "hello");
+        s.execute("file.openDemoProject", json!({})).unwrap();
+        assert!(cache.content(&s).words.is_empty());
+        assert!(cache.highlights("hello").is_empty());
+    }
+
+    #[test]
+    fn changing_sequence_invalidates_content_without_a_project_edit() {
+        let mut s = Session::default();
+        s.execute("file.openDemoProject", json!({})).unwrap();
+        let mut cache = TranscriptCache::default();
+        let first = cache.content(&s);
+        s.state.active_sequence = None;
+        let empty = cache.content(&s);
+        assert!(!Arc::ptr_eq(&first, &empty));
+        assert!(empty.words.is_empty());
+    }
+}
diff --git a/crates/ui-egui/tests/transcript_ui.rs b/crates/ui-egui/tests/transcript_ui.rs
index 105236cc..a7774281 100644
--- a/crates/ui-egui/tests/transcript_ui.rs
+++ b/crates/ui-egui/tests/transcript_ui.rs
@@ -12,6 +12,48 @@ use filmcraft_ui_egui::FilmcraftApp;
 use filmcraft_ui_egui::control::ControlRequest;
 use serde_json::{Value, json};
 
+/// Reproducible redraw cost for a long sequence transcript, with and without many search hits.
+/// Run alone with `--ignored --nocapture`; timings are observations, not CI thresholds.
+#[test]
+#[ignore = "manual transcript performance measurement"]
+fn long_transcript_redraw_benchmark() {
+    let mut d = Driver::demo();
+    let item = d.harness.state().session.active_sequence().unwrap().audio_tracks[0].items[0].item;
+    let words: Vec<Value> = (0..10_000)
+        .map(|i| {
+            json!({"text": "hello", "start": filmcraft_time::Tick::from_seconds_f64(i as f64 * 0.4).0,
+            "end": filmcraft_time::Tick::from_seconds_f64(i as f64 * 0.4 + 0.3).0, "speaker": (i / 40) % 2})
+        })
+        .collect();
+    d.ok(
+        "engine.execute",
+        json!({"command": "transcript.set", "params": {"item": item.0,
+        "transcript": {"language": "en", "speakers": [{"name": "First"}, {"name": "Second"}], "words": words}}}),
+    );
+    d.ok("ui.set", json!({"workspace": "Captions and Graphics"}));
+    d.harness.state_mut().ui.text_tab = "Transcript".into();
+    // Extend only the synthetic fixture clip so all 10,000 words are actually laid out.
+    let session = &mut d.harness.state_mut().session;
+    let id = session.state.active_sequence.unwrap();
+    let seq = std::sync::Arc::make_mut(&mut session.project).sequence_mut(id).unwrap();
+    seq.audio_tracks[0].items.truncate(1);
+    seq.audio_tracks[0].items[0].source_in = filmcraft_time::Tick::ZERO;
+    seq.audio_tracks[0].items[0].duration = filmcraft_time::Tick::from_seconds_f64(4000.0);
+    assert_eq!(filmcraft_engine::transcript::sequence_words(session).len(), 10_000);
+    for query in ["", "hello"] {
+        d.harness.state_mut().ui.transcript_search = query.into();
+        d.frames(5);
+        let mut times = Vec::new();
+        for _ in 0..40 {
+            let start = std::time::Instant::now();
+            d.frames(1);
+            times.push(start.elapsed().as_secs_f64() * 1000.0);
+        }
+        times.sort_by(f64::total_cmp);
+        println!("transcript words=10000 query={query:?} p50_ms={:.3} p95_ms={:.3}", times[20], times[38]);
+    }
+}
+
 struct Driver {
     harness: Harness<'static, FilmcraftApp>,
     tx: Sender<ControlRequest>,
diff --git a/docs/performance.md b/docs/performance.md
index 3549cbcb..c2bed4d9 100644
--- a/docs/performance.md
+++ b/docs/performance.md
@@ -4,6 +4,27 @@
 and `.md` (options and sections: [testing.md](testing.md) §5). Agents read live counters with
 `perf.stats` ([control-protocol.md](control-protocol.md)).
 
+## Interactive builds and redraw work
+
+For interactive editing, build with `cargo build --release -p filmcraft --features whisper` and
+launch `target/release/filmcraft`. The development profile optimizes dependencies and codec hot
+loops, but still builds the editor at optimization level 1; it is not the release performance
+baseline. Keep the same build profile when comparing code changes.
+
+Timeline, Effect Controls, Markers, Captions and mixer drawing borrow a sequence from a shared
+immutable project snapshot instead of deep-copying its clips and effects on every redraw. Commands
+can still replace the session's project during the pass; the snapshot stays valid until drawing
+finishes. Transcript words and paragraphs are cached by project snapshot and active sequence, and
+search highlights by query. Playhead movement reuses them; project edits, undo/redo and view changes
+refresh them. Highlight lookup is constant work per word, including searches with many matches.
+
+To measure a long transcript, run
+`cargo test -p filmcraft-ui-egui --test transcript_ui long_transcript_redraw_benchmark -- --ignored --nocapture`
+alone. It reports median and p95 redraw time for 10,000 words with an empty query and a query matching
+every word. It measures UI update work, not native display presentation, and deliberately has no
+machine-dependent timing assertion. The transcript still lays out every word; list virtualization
+is future work.
+
 **Read the numbers with the load average.** These results come from a shared Apple M4 Pro
 (14 cores, 48 GB, macOS 15 / Darwin 24.6) with parallel agent builds running: load average
 **75–200** during every run below. Wall-clock columns (fps, shown/dropped frames, seek latency)