diff --git a/crates/engine/src/cmd/file.rs b/crates/engine/src/cmd/file.rs
index d7356b23..aa0b4719 100644
--- a/crates/engine/src/cmd/file.rs
+++ b/crates/engine/src/cmd/file.rs
@@ -162,17 +162,42 @@ pub fn add_opened(s: &mut Session, name: &str, path: Option<String>, p: Presenta
     s.add_document(st)
 }
 
+/// A presentation read from disk, not yet added to a session: [`read_presentation_file`] needs no
+/// session, so a host can run it on a background thread and add the result with
+/// [`add_presentation_file`].
+#[derive(Debug)]
+pub struct LoadedFile {
+    pub path: String,
+    pub doc: Presentation,
+    /// A native or PPTX file (saving goes back to it); an outline import has no file to save to.
+    pub native: bool,
+    /// Pictures and media a `.deckcraft` file had lost or damaged.
+    pub damaged: Vec<String>,
+}
+
+/// Read and parse the presentation at `path`.
 #[cfg(not(target_arch = "wasm32"))]
-fn open(s: &mut Session, p: &Value) -> Result<Value> {
-    let path = str_param(p, "path").ok_or_else(|| bad("file.open", "missing `path`"))?;
+pub fn read_presentation_file(path: &str) -> Result<LoadedFile> {
     let bytes = std::fs::read(path).map_err(|e| EngineError::Other(format!("{path}: {e}")))?;
     let (doc, damaged) = open_checked(path, &bytes)?;
     let native = deckcraft_format::sniff(&bytes) || deckcraft_pptx::sniff(&bytes);
-    // A damaged file opens as an untitled repaired copy, so Save can't overwrite the original
-    // with the empty media.
+    Ok(LoadedFile { path: path.to_string(), doc, native, damaged })
+}
+
+/// Add a presentation read by [`read_presentation_file`] as the active document. A damaged file
+/// opens as an untitled repaired copy, so Save can't overwrite the original with the empty media.
+pub fn add_presentation_file(s: &mut Session, f: LoadedFile) -> Result<(usize, Vec<String>)> {
+    let LoadedFile { path, doc, native, damaged } = f;
     let keep_path = native && damaged.is_empty();
-    let i = add_opened(s, path, keep_path.then(|| path.to_string()), doc);
+    let i = add_opened(s, &path, keep_path.then(|| path.clone()), doc);
     report_damage(s, &damaged)?;
+    Ok((i, damaged))
+}
+
+#[cfg(not(target_arch = "wasm32"))]
+fn open(s: &mut Session, p: &Value) -> Result<Value> {
+    let path = str_param(p, "path").ok_or_else(|| bad("file.open", "missing `path`"))?;
+    let (i, damaged) = add_presentation_file(s, read_presentation_file(path)?)?;
     let st = s.doc()?;
     Ok(json!({"document": i, "slides": st.doc.slides.len(), "missingFonts": st.missing_fonts, "damagedMedia": damaged}))
 }
diff --git a/crates/ui-egui/src/backstage.rs b/crates/ui-egui/src/backstage.rs
index dccb10ff..59f06aa6 100644
--- a/crates/ui-egui/src/backstage.rs
+++ b/crates/ui-egui/src/backstage.rs
@@ -593,10 +593,8 @@ fn recent_list(app: &mut SlideApp, ui: &mut Ui, query: &str, limit: usize) {
         icons::paint(p, Rect::from_center_size(pos2(rect.left() + 20.0, rect.center().y), vec2(22.0, 22.0)), Icon::Slide, t.text, false);
         p.text(pos2(rect.left() + 44.0, rect.center().y - 8.0), Align2::LEFT_CENTER, &name, theme::font(13.0), t.text);
         p.text(pos2(rect.left() + 44.0, rect.center().y + 9.0), Align2::LEFT_CENTER, &folder, theme::font(11.0), t.text_dim);
-        if resp.on_hover_text(&path).clicked()
-            && let Err(e) = app.open_path(&path)
-        {
-            app.notify(e);
+        if resp.on_hover_text(&path).clicked() {
+            app.open_path_in_background(&path);
         }
     }
 }
diff --git a/crates/ui-egui/src/backstage_tests.rs b/crates/ui-egui/src/backstage_tests.rs
index 015262f8..e0f4e6d2 100644
--- a/crates/ui-egui/src/backstage_tests.rs
+++ b/crates/ui-egui/src/backstage_tests.rs
@@ -232,6 +232,15 @@ fn open_lists_recent_files_and_opens_one() {
     h.run_steps(2);
     assert!(h.query_by_label("Other.pptx").is_none(), "the search box filters by name");
     h.get_by_label("Quarterly review.pptx").click();
+    h.run_steps(1);
+    // The file is read in the background (#350); its progress window shows meanwhile.
+    for _ in 0..300 {
+        if h.state().opening.is_none() {
+            break;
+        }
+        std::thread::sleep(std::time::Duration::from_millis(10));
+        h.run_steps(1);
+    }
     h.run_steps(3);
     assert_eq!(h.state().session.documents().len(), 2);
     assert_eq!(h.state().session.active().unwrap().path.as_deref(), Some(path.as_str()));
diff --git a/crates/ui-egui/src/lib.rs b/crates/ui-egui/src/lib.rs
index 2d83074c..d1caa59e 100644
--- a/crates/ui-egui/src/lib.rs
+++ b/crates/ui-egui/src/lib.rs
@@ -18,6 +18,7 @@ pub mod fillui;
 pub mod icons;
 pub mod media;
 pub mod menus;
+pub mod opening;
 pub mod panes;
 mod preview;
 pub mod qat;
@@ -441,6 +442,8 @@ pub struct SlideApp {
     /// Audio/video playback (editor control bar, slide show, `media.*` commands).
     pub media: media::MediaHost,
     pub status: Option<(String, f64)>,
+    /// A presentation being read in the background, with its progress window ([`opening`]).
+    pub opening: Option<opening::Opening>,
     /// A brief notice over the bottom of the window (missing fonts on opening a file).
     pub toast: Option<toast::Toast>,
     /// Documents (by uid) whose missing fonts have been mentioned.
@@ -527,6 +530,7 @@ impl SlideApp {
             anim_preview: None,
             media: media::MediaHost::new(audio_out),
             status: None,
+            opening: None,
             toast: None,
             fonts_noted: Default::default(),
             perf: Perf { frame_ms: 0.0, render_ms: 0.0, fps: 0.0 },
@@ -1091,10 +1095,8 @@ impl SlideApp {
 
     fn pick_and_open(&mut self) {
         if let Some(pick) = self.services.pick_open.as_mut() {
-            if let Some(path) = pick("open")
-                && let Err(e) = self.open_path(&path)
-            {
-                self.notify(e);
+            if let Some(path) = pick("open") {
+                self.open_path_in_background(&path);
             }
         } else if self.services.open_async.is_some() {
             let id = self.begin_external_file("open");
@@ -1106,11 +1108,16 @@ impl SlideApp {
 
     pub fn open_path(&mut self, path: &str) -> Result<(), String> {
         self.session.execute("file.open", &json!({"path": path})).map_err(|e| e.to_string())?;
+        self.note_recent(path);
+        Ok(())
+    }
+
+    /// A presentation was opened from `path`: first in the recent list, shown in Normal view.
+    pub(crate) fn note_recent(&mut self, path: &str) {
         self.ui.recent.retain(|p| p != path);
         self.ui.recent.insert(0, path.to_string());
         self.ui.recent.truncate(RECENT_MAX);
         self.ui.view = ViewMode::Normal;
-        Ok(())
     }
 
     /// Transitions › Sound › Other Sound…: pick an audio file for the selected slides.
@@ -1449,6 +1456,7 @@ impl SlideApp {
         }
         self.last_time = now;
         self.drain_control(ctx);
+        self.poll_opening();
         if self.fonts_ready {
             self.drain_inbox();
         }
@@ -1665,6 +1673,7 @@ impl SlideApp {
         let t = theme::Tokens::get(&ctx);
         self.tick_media(&ctx);
         eyedropper::tick(self, &ctx);
+        self.opening_ui(&ctx);
         if self.show.is_some() {
             self.anim_preview = None;
             show::ui(self, ui);
diff --git a/crates/ui-egui/src/opening.rs b/crates/ui-egui/src/opening.rs
new file mode 100644
index 00000000..d48734ec
--- /dev/null
+++ b/crates/ui-egui/src/opening.rs
@@ -0,0 +1,195 @@
+//! Opening a presentation in the background (#350): reading and parsing a large PPTX no longer
+//! freezes the window. A modal shows the file being opened and the time so far, and Cancel (or
+//! Escape) gives the window back at once; the abandoned read finishes on its thread and is dropped.
+
+use crate::SlideApp;
+use deckcraft_engine::cmd::file::LoadedFile;
+use std::sync::mpsc::{Receiver, TryRecvError};
+
+/// A presentation being read on a background thread.
+pub struct Opening {
+    pub path: String,
+    pub started_ms: f64,
+    rx: Receiver<Result<LoadedFile, String>>,
+}
+
+impl Opening {
+    /// The file name shown in the window.
+    pub fn name(&self) -> String {
+        std::path::Path::new(&self.path).file_name().map(|n| n.to_string_lossy().to_string()).unwrap_or_else(|| self.path.clone())
+    }
+}
+
+impl SlideApp {
+    /// Open `path` without blocking the UI: the file is read on a thread and the progress window
+    /// shows until it arrives. Without threads (the web build) it opens at once.
+    pub fn open_path_in_background(&mut self, path: &str) {
+        if let Some(o) = &self.opening {
+            let name = o.name();
+            self.set_status(format!("Still opening {name}"));
+            return;
+        }
+        #[cfg(target_arch = "wasm32")]
+        {
+            if let Err(e) = self.open_path(path) {
+                self.set_status(e);
+            }
+        }
+        #[cfg(not(target_arch = "wasm32"))]
+        {
+            let (tx, rx) = std::sync::mpsc::channel();
+            let p = path.to_string();
+            let spawned = std::thread::Builder::new().name("deckcraft-open".into()).spawn(move || {
+                let r = deckcraft_engine::cmd::file::read_presentation_file(&p).map_err(|e| e.to_string());
+                let _ = tx.send(r);
+            });
+            match spawned {
+                Ok(_) => self.opening = Some(Opening { path: path.to_string(), started_ms: crate::now_ms(), rx }),
+                Err(_) => {
+                    if let Err(e) = self.open_path(path) {
+                        self.set_status(e);
+                    }
+                }
+            }
+        }
+    }
+
+    /// Stop waiting for the file being opened; the presentation stays as it is.
+    pub fn cancel_opening(&mut self) {
+        if let Some(o) = self.opening.take() {
+            self.set_status(format!("Stopped opening {}", o.name()));
+        }
+    }
+
+    /// Take the background read's result when it has arrived.
+    pub(crate) fn poll_opening(&mut self) {
+        let Some(o) = &self.opening else { return };
+        let r = match o.rx.try_recv() {
+            Ok(r) => r,
+            Err(TryRecvError::Empty) => return,
+            Err(TryRecvError::Disconnected) => Err(format!("{}: reading the file stopped unexpectedly", o.path)),
+        };
+        let Some(o) = self.opening.take() else { return };
+        match r {
+            Ok(f) => match deckcraft_engine::cmd::file::add_presentation_file(&mut self.session, f) {
+                Ok(_) => self.note_recent(&o.path),
+                Err(e) => self.set_status(e.to_string()),
+            },
+            Err(e) => self.set_status(e),
+        }
+    }
+
+    /// The progress window over everything while a file opens.
+    pub(crate) fn opening_ui(&mut self, ctx: &egui::Context) {
+        let Some(o) = &self.opening else { return };
+        let name = o.name();
+        let secs = ((crate::now_ms() - o.started_ms) / 1000.0).max(0.0);
+        let mut cancel = false;
+        egui::Modal::new(egui::Id::new("deckcraft-opening")).show(ctx, |ui| {
+            ui.set_width(320.0);
+            ui.horizontal(|ui| {
+                ui.add(egui::Spinner::new().size(18.0));
+                ui.label(format!("Opening {name}…"));
+            });
+            ui.add_space(6.0);
+            ui.weak(format!("Reading the file: {secs:.0} s"));
+            ui.add_space(6.0);
+            ui.with_layout(egui::Layout::right_to_left(egui::Align::Center), |ui| {
+                cancel = ui.button("Cancel").clicked();
+            });
+        });
+        if cancel || ctx.input_mut(|i| i.consume_key(egui::Modifiers::NONE, egui::Key::Escape)) {
+            self.cancel_opening();
+        }
+        ctx.request_repaint_after(std::time::Duration::from_millis(100));
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    #![allow(clippy::unwrap_used, clippy::expect_used)]
+
+    use super::*;
+    use deckcraft_engine::Session;
+
+    fn saved_deck(name: &str, slides: usize) -> String {
+        let mut s = Session::with_new();
+        for _ in 1..slides {
+            s.execute("slide.new", &serde_json::json!({})).unwrap();
+        }
+        let path = std::env::temp_dir().join(format!("deckcraft-opening-{}-{name}.deckcraft", std::process::id()));
+        let path = path.to_string_lossy().to_string();
+        s.execute("file.saveAs", &serde_json::json!({"path": path})).unwrap();
+        path
+    }
+
+    fn wait(app: &mut SlideApp) {
+        for _ in 0..500 {
+            app.poll_opening();
+            if app.opening.is_none() {
+                return;
+            }
+            std::thread::sleep(std::time::Duration::from_millis(10));
+        }
+        panic!("opening did not finish");
+    }
+
+    #[test]
+    fn opens_in_the_background_and_adds_the_document() {
+        let path = saved_deck("ok", 3);
+        let mut app = SlideApp::new(Session::new(), crate::Services::default());
+        app.open_path_in_background(&path);
+        assert!(app.opening.is_some(), "the read runs on a thread");
+        wait(&mut app);
+        assert_eq!(app.session.documents().len(), 1);
+        assert_eq!(app.session.active().unwrap().doc.slides.len(), 3);
+        assert_eq!(app.ui.recent.first(), Some(&path));
+        std::fs::remove_file(&path).ok();
+    }
+
+    #[test]
+    fn cancel_drops_the_result() {
+        let path = saved_deck("cancel", 2);
+        let mut app = SlideApp::new(Session::with_new(), crate::Services::default());
+        app.open_path_in_background(&path);
+        app.cancel_opening();
+        assert!(app.opening.is_none());
+        std::thread::sleep(std::time::Duration::from_millis(200));
+        app.poll_opening();
+        assert_eq!(app.session.documents().len(), 1, "nothing is added after Cancel");
+        std::fs::remove_file(&path).ok();
+    }
+
+    #[test]
+    fn errors_show_in_the_status_bar_and_a_second_open_waits() {
+        let mut app = SlideApp::new(Session::new(), crate::Services::default());
+        app.open_path_in_background("/nonexistent/deckcraft/missing.pptx");
+        app.open_path_in_background("/nonexistent/deckcraft/other.pptx");
+        assert!(app.status.as_ref().is_some_and(|(m, _)| m.starts_with("Still opening missing.pptx")));
+        wait(&mut app);
+        assert!(app.session.documents().is_empty());
+        assert!(app.status.as_ref().is_some_and(|(m, _)| m.contains("missing.pptx")), "{:?}", app.status);
+    }
+
+    #[test]
+    fn the_progress_window_shows_and_cancel_closes_it() {
+        use egui_kittest::kittest::Queryable;
+        let mut app = SlideApp::new(Session::with_new(), crate::Services::default());
+        // A read that never answers: the window stays until Cancel.
+        let (_tx, rx) = std::sync::mpsc::channel();
+        app.opening = Some(Opening { path: "/decks/Big deck.pptx".into(), started_ms: crate::now_ms(), rx });
+        let mut harness = egui_kittest::Harness::builder().with_size(egui::vec2(1000.0, 700.0)).build_ui_state(
+            |ui, app: &mut SlideApp| {
+                app.logic(ui.ctx());
+                app.ui(ui);
+            },
+            app,
+        );
+        harness.run_steps(3);
+        harness.get_by_label("Opening Big deck.pptx…");
+        harness.get_by_label("Cancel").click();
+        harness.run_steps(2);
+        assert!(harness.state().opening.is_none());
+        assert_eq!(harness.state().session.documents().len(), 1);
+    }
+}