diff --git a/crates/ui-egui/src/file_ui.rs b/crates/ui-egui/src/file_ui.rs
index db3769ec9a..bebfd6df12 100644
--- a/crates/ui-egui/src/file_ui.rs
+++ b/crates/ui-egui/src/file_ui.rs
@@ -731,6 +731,10 @@ fn unit_pt(units: &str) -> f64 {
 
 /// A number field in `units` for a value kept in points; true when edited.
 fn length_field(ui: &mut egui::Ui, label: &str, pt: &mut f64, units: &str) -> bool {
+    length_field_prec(ui, label, pt, units, 0)
+}
+
+fn length_field_prec(ui: &mut egui::Ui, label: &str, pt: &mut f64, units: &str, decimals: u32) -> bool {
     let t = Tokens::get(ui.ctx());
     let k = unit_pt(units);
     let mut v = (*pt / k) as f32;
@@ -738,7 +742,7 @@ fn length_field(ui: &mut egui::Ui, label: &str, pt: &mut f64, units: &str) -> bo
     ui.horizontal(|ui| {
         ui.label(egui::RichText::new(label).color(t.text_dim));
         let before = v;
-        crate::widgets::value_field(ui, &mut v, 0.0..=10000.0, units, 70.0);
+        crate::widgets::value_field_prec(ui, &mut v, 0.0..=10000.0, units, 70.0, decimals);
         if (v - before).abs() > 1e-4 && v.is_finite() {
             *pt = f64::from(v) * k;
             changed = true;
@@ -747,6 +751,53 @@ fn length_field(ui: &mut egui::Ui, label: &str, pt: &mut f64, units: &str) -> bo
     changed
 }
 
+/// Unrotated paper dimensions in points, for either the current preset or a custom array.
+fn print_paper_size(value: Option<&Value>) -> (f64, f64) {
+    let default = (612.0, 792.0);
+    match value {
+        Some(Value::Array(a)) => (a.first().and_then(Value::as_f64).unwrap_or(default.0), a.get(1).and_then(Value::as_f64).unwrap_or(default.1)),
+        Some(Value::String(name)) => photocraft_engine::print_cmds::PAPERS.iter().find(|p| p.0.eq_ignore_ascii_case(name)).map_or(default, |p| (p.1, p.2)),
+        _ => default,
+    }
+}
+
+fn print_paper_controls(ui: &mut egui::Ui, f: &mut Map<String, Value>) {
+    let previous = f.get("paper").cloned();
+    let mut selected = if previous.as_ref().is_some_and(Value::is_array) { "custom".to_string() } else { s(f, "paper", "letter") };
+    ui.horizontal(|ui| {
+        let mut papers: Vec<_> = photocraft_engine::print_cmds::PAPERS.iter().map(|p| (p.0.to_string(), p.0)).collect();
+        papers.push(("custom".to_string(), tl!("Custom")));
+        if crate::widgets::dropdown(ui, "print-paper", &mut selected, &papers, 110.0) {
+            if selected == "custom" {
+                let (w, h) = print_paper_size(f.get("__customPaper").or(previous.as_ref()));
+                f.insert("paper".into(), json!([w, h]));
+            } else {
+                if let Some(paper @ Value::Array(_)) = previous {
+                    f.insert("__customPaper".into(), paper);
+                }
+                f.insert("paper".into(), json!(selected));
+            }
+        }
+        dropdown_str(ui, "print-orient", f, "orientation", &[("portrait", tl!("Portrait")), ("landscape", tl!("Landscape"))], 110.0);
+    });
+    if f.get("paper").is_some_and(Value::is_array) {
+        let (mut w, mut h) = print_paper_size(f.get("paper"));
+        let units = s(f, "units", "cm");
+        let changed = ui
+            .horizontal(|ui| {
+                let wc = length_field_prec(ui, tl!("Paper Width"), &mut w, &units, 2);
+                let hc = length_field_prec(ui, tl!("Paper Height"), &mut h, &units, 2);
+                wc || hc
+            })
+            .inner;
+        if changed {
+            // Keep points in the command, and let the engine apply orientation and validation.
+            f.insert("paper".into(), json!([w, h]));
+            f.insert("__customPaper".into(), json!([w, h]));
+        }
+    }
+}
+
 fn print_body(app: &mut PhotocraftApp, ui: &mut egui::Ui, f: &mut Map<String, Value>) {
     let t = Tokens::get(ui.ctx());
     let Some(doc) = app.session.active().map(|d| d.doc.clone()) else { return };
@@ -834,11 +885,7 @@ fn print_body(app: &mut PhotocraftApp, ui: &mut egui::Ui, f: &mut Map<String, Va
                 }
             });
             number(ui, f, "copies", tl!("Copies"), 1.0..=999.0, "", 1.0);
-            ui.horizontal(|ui| {
-                let papers: Vec<(&str, &str)> = photocraft_engine::print_cmds::PAPERS.iter().map(|p| (p.0, p.0)).collect();
-                dropdown_str(ui, "print-paper", f, "paper", &papers, 110.0);
-                dropdown_str(ui, "print-orient", f, "orientation", &[("portrait", tl!("Portrait")), ("landscape", tl!("Landscape"))], 110.0);
-            });
+            print_paper_controls(ui, f);
             head(ui, tl!("Color Management"));
             dropdown_str(
                 ui,
diff --git a/crates/ui-egui/src/i18n/cs.tsv b/crates/ui-egui/src/i18n/cs.tsv
index 17630d87b8..ea65085c36 100644
--- a/crates/ui-egui/src/i18n/cs.tsv
+++ b/crates/ui-egui/src/i18n/cs.tsv
@@ -20,6 +20,10 @@
 # dialog commands is kept.
 
 # Guide layout dialog
+
+# Custom print paper (#3820)
+	Paper Width	Šířka papíru
+	Paper Height	Výška papíru
 	Columns	Sloupce
 	Rows	Řádky
 	Number	Počet
diff --git a/crates/ui-egui/src/i18n/de.tsv b/crates/ui-egui/src/i18n/de.tsv
index b2882792ac..b1d22f83fe 100644
--- a/crates/ui-egui/src/i18n/de.tsv
+++ b/crates/ui-egui/src/i18n/de.tsv
@@ -16,6 +16,10 @@
 # letters; the trailing "…" of dialog commands is kept. Plural rule: one|other (n != 1 -> other).
 
 # Guide layout dialog
+
+# Custom print paper (#3820)
+	Paper Width	Papierbreite
+	Paper Height	Papierhöhe
 	Columns	Spalten
 	Rows	Zeilen
 	Number	Anzahl
diff --git a/crates/ui-egui/src/i18n/el.tsv b/crates/ui-egui/src/i18n/el.tsv
index e697582a61..0b5bdb81cd 100644
--- a/crates/ui-egui/src/i18n/el.tsv
+++ b/crates/ui-egui/src/i18n/el.tsv
@@ -4,6 +4,10 @@
 # Covers every source label the i18n coverage check lists.
 
 # Guide layout dialog
+
+# Custom print paper (#3820)
+	Paper Width	Πλάτος χαρτιού
+	Paper Height	Ύψος χαρτιού
 	Rows	Γραμμές
 	Number	Πλήθος
 	Top:	Επάνω:
diff --git a/crates/ui-egui/src/i18n/es.tsv b/crates/ui-egui/src/i18n/es.tsv
index a27a6478b9..78c4087f56 100644
--- a/crates/ui-egui/src/i18n/es.tsv
+++ b/crates/ui-egui/src/i18n/es.tsv
@@ -13,6 +13,10 @@
 # letters; the trailing "…" of dialog commands is kept.
 
 # Guide layout dialog
+
+# Custom print paper (#3820)
+	Paper Width	Ancho del papel
+	Paper Height	Altura del papel
 	Columns	Columnas
 	Rows	Filas
 	Number	Número
diff --git a/crates/ui-egui/src/i18n/fr.tsv b/crates/ui-egui/src/i18n/fr.tsv
index 1f23a0be36..f5fecc2a95 100644
--- a/crates/ui-egui/src/i18n/fr.tsv
+++ b/crates/ui-egui/src/i18n/fr.tsv
@@ -20,6 +20,10 @@
 # dialog commands is kept.
 
 # Guide layout dialog
+
+# Custom print paper (#3820)
+	Paper Width	Largeur du papier
+	Paper Height	Hauteur du papier
 	Rows	Lignes
 	Number	Nombre
 	Margin	Marge
diff --git a/crates/ui-egui/src/i18n/hu.tsv b/crates/ui-egui/src/i18n/hu.tsv
index c8fd0724ba..2a0eace598 100644
--- a/crates/ui-egui/src/i18n/hu.tsv
+++ b/crates/ui-egui/src/i18n/hu.tsv
@@ -17,6 +17,10 @@
 # singular noun in Hungarian, so every @plural entry has a single form.
 
 # Plural messages and command id overrides
+# Custom paper dimensions in Print Settings.
+	Paper Width	Papír szélessége
+	Paper Height	Papír magassága
+
 @id	edit.purge.undo	Visszavonási adatok
 @id	edit.stroke	Körvonalazás…
 @id	edit.transform.scale	Méretezés
diff --git a/crates/ui-egui/src/i18n/id.tsv b/crates/ui-egui/src/i18n/id.tsv
index 311cd13ffe..deb42f1bd7 100644
--- a/crates/ui-egui/src/i18n/id.tsv
+++ b/crates/ui-egui/src/i18n/id.tsv
@@ -13,6 +13,10 @@
 # letters; the trailing "…" of dialog commands is kept.
 
 # Guide layout dialog
+
+# Custom print paper (#3820)
+	Paper Width	Lebar kertas
+	Paper Height	Tinggi kertas
 	Rows	Baris
 	Number	Jumlah
 	Margin	Margin
diff --git a/crates/ui-egui/src/i18n/it.tsv b/crates/ui-egui/src/i18n/it.tsv
index db7a98944d..9f10a8fc30 100644
--- a/crates/ui-egui/src/i18n/it.tsv
+++ b/crates/ui-egui/src/i18n/it.tsv
@@ -13,6 +13,10 @@
 # letters; the trailing … of dialog commands is kept.
 
 # Guide layout dialog
+
+# Custom print paper (#3820)
+	Paper Width	Larghezza della carta
+	Paper Height	Altezza della carta
 	Columns	Colonne
 	Rows	Righe
 	Number	Numero
diff --git a/crates/ui-egui/src/i18n/ja.tsv b/crates/ui-egui/src/i18n/ja.tsv
index 9cf72823bb..94ec6fad24 100644
--- a/crates/ui-egui/src/i18n/ja.tsv
+++ b/crates/ui-egui/src/i18n/ja.tsv
@@ -16,6 +16,10 @@
 # letters; the trailing "…" of dialog commands is kept.
 
 # Guide layout dialog
+
+# Custom print paper (#3820)
+	Paper Width	用紙の幅
+	Paper Height	用紙の高さ
 	Rows	行
 	Number	数
 	Top:	上:
diff --git a/crates/ui-egui/src/i18n/ko.tsv b/crates/ui-egui/src/i18n/ko.tsv
index 8e80eb5bce..815a54c17a 100644
--- a/crates/ui-egui/src/i18n/ko.tsv
+++ b/crates/ui-egui/src/i18n/ko.tsv
@@ -8,6 +8,10 @@
 # Product names and technology names retain their spelling.
 
 # Guide layout dialog
+
+# Custom print paper (#3820)
+	Paper Width	용지 너비
+	Paper Height	용지 높이
 	Columns	열
 	Rows	행
 	Number	개수
diff --git a/crates/ui-egui/src/i18n/nl.tsv b/crates/ui-egui/src/i18n/nl.tsv
index 89d2e90539..55163dd7c6 100644
--- a/crates/ui-egui/src/i18n/nl.tsv
+++ b/crates/ui-egui/src/i18n/nl.tsv
@@ -9,6 +9,10 @@
 # informally ("je") and uses sentence case.
 
 # Guide layout dialog
+
+# Custom print paper (#3820)
+	Paper Width	Papierbreedte
+	Paper Height	Papierhoogte
 	Rows	Rijen
 	Number	Aantal
 	Top:	Boven:
diff --git a/crates/ui-egui/src/i18n/pl.tsv b/crates/ui-egui/src/i18n/pl.tsv
index 95099d354c..cd4a9b5616 100644
--- a/crates/ui-egui/src/i18n/pl.tsv
+++ b/crates/ui-egui/src/i18n/pl.tsv
@@ -19,6 +19,10 @@
 # Zapisz jako…), sentence case as in the English source. Quotes: „…”.
 
 # Guide layout dialog
+
+# Custom print paper (#3820)
+	Paper Width	Szerokość papieru
+	Paper Height	Wysokość papieru
 	Rows	Wiersze
 	Number	Liczba
 	Top:	Góra:
diff --git a/crates/ui-egui/src/i18n/pt-br.tsv b/crates/ui-egui/src/i18n/pt-br.tsv
index f5f69f9f44..75af646558 100644
--- a/crates/ui-egui/src/i18n/pt-br.tsv
+++ b/crates/ui-egui/src/i18n/pt-br.tsv
@@ -16,6 +16,10 @@
 # stay as they are; the trailing "…" of dialog commands is kept.
 
 # Guide layout dialog
+
+# Custom print paper (#3820)
+	Paper Width	Largura do papel
+	Paper Height	Altura do papel
 	Rows	Linhas
 	Number	Número
 	Margin	Margem
diff --git a/crates/ui-egui/src/i18n/ru.tsv b/crates/ui-egui/src/i18n/ru.tsv
index 9ee6e311fd..7b43459580 100644
--- a/crates/ui-egui/src/i18n/ru.tsv
+++ b/crates/ui-egui/src/i18n/ru.tsv
@@ -20,6 +20,10 @@
 # Menus
 
 # Guide layout dialog
+
+# Custom print paper (#3820)
+	Paper Width	Ширина бумаги
+	Paper Height	Высота бумаги
 	Rows	Строки
 	Number	Количество
 	Margin	Поля
diff --git a/crates/ui-egui/src/i18n/sr.tsv b/crates/ui-egui/src/i18n/sr.tsv
index 75ceb71673..11604ef8b9 100644
--- a/crates/ui-egui/src/i18n/sr.tsv
+++ b/crates/ui-egui/src/i18n/sr.tsv
@@ -4185,3 +4185,7 @@ shape	Appearance	Izgled
 	Import .GRD	Uvezi .GRD
 	Choose a gradient file (.grd).	Izaberite datoteku gradijenta (.grd).
 	Auto Add/Delete	Automatsko dodavanje/brisanje
+
+# Custom print paper (#3820)
+	Paper Width	Širina papira
+	Paper Height	Visina papira
diff --git a/crates/ui-egui/src/i18n/tr.tsv b/crates/ui-egui/src/i18n/tr.tsv
index e912f26c27..0d25e02043 100644
--- a/crates/ui-egui/src/i18n/tr.tsv
+++ b/crates/ui-egui/src/i18n/tr.tsv
@@ -7,6 +7,10 @@
 # entries the same way. Product and technology names (PhotoCraft, ArtCraft, OpenType, CMYK, Lab ...)
 # stay as they are; the trailing "…" of dialog commands is kept.
 #
+# Custom paper dimensions in Print Settings.
+	Paper Width	Kağıt Genişliği
+	Paper Height	Kağıt Yüksekliği
+
 	Rows	Satırlar
 	Number	Sayı
 	Top:	Üst:
diff --git a/crates/ui-egui/src/i18n/uk.tsv b/crates/ui-egui/src/i18n/uk.tsv
index 0af7c871da..d3d227a1aa 100644
--- a/crates/ui-egui/src/i18n/uk.tsv
+++ b/crates/ui-egui/src/i18n/uk.tsv
@@ -11,6 +11,10 @@
 # Menus
 
 # Guide layout dialog
+
+# Custom print paper (#3820)
+	Paper Width	Ширина паперу
+	Paper Height	Висота паперу
 	Rows	Рядки
 	Number	Кількість
 	Top:	Зверху:
diff --git a/crates/ui-egui/src/i18n/vi.tsv b/crates/ui-egui/src/i18n/vi.tsv
index f01c25ce7f..3192be995d 100644
--- a/crates/ui-egui/src/i18n/vi.tsv
+++ b/crates/ui-egui/src/i18n/vi.tsv
@@ -21,6 +21,10 @@
 
 
 # Phase 3.1 — top-level menus, File menu, Edit menu
+# Custom paper dimensions in Print Settings.
+	Paper Width	Chiều rộng giấy
+	Paper Height	Chiều cao giấy
+
 	Again	Lặp lại
 	AI Integrations…	Tích hợp AI…
 	All	Tất cả
diff --git a/crates/ui-egui/src/i18n/zh-hans.tsv b/crates/ui-egui/src/i18n/zh-hans.tsv
index 31ecc7b5a0..b8c4e8a6b3 100644
--- a/crates/ui-egui/src/i18n/zh-hans.tsv
+++ b/crates/ui-egui/src/i18n/zh-hans.tsv
@@ -12,6 +12,10 @@
 # Product and technology names retain their spelling. See docs/localization-zh-hans.md.
 
 # Guide layout dialog
+
+# Custom print paper (#3820)
+	Paper Width	纸张宽度
+	Paper Height	纸张高度
 	Columns	列
 	Rows	行
 	Number	数量
diff --git a/crates/ui-egui/src/i18n/zh-hant.tsv b/crates/ui-egui/src/i18n/zh-hant.tsv
index 3b43fff17a..877e666c4e 100644
--- a/crates/ui-egui/src/i18n/zh-hant.tsv
+++ b/crates/ui-egui/src/i18n/zh-hant.tsv
@@ -20,6 +20,10 @@
 # separators as in the English.
 
 # Guide layout dialog
+
+# Custom print paper (#3820)
+	Paper Width	紙張寬度
+	Paper Height	紙張高度
 	Columns	欄
 	Rows	列
 	Number	數量
diff --git a/crates/ui-egui/tests/custom_print_paper.rs b/crates/ui-egui/tests/custom_print_paper.rs
new file mode 100644
index 0000000000..7361468bbc
--- /dev/null
+++ b/crates/ui-egui/tests/custom_print_paper.rs
@@ -0,0 +1,142 @@
+//! #3820: edit custom paper through the real Print dialog, preview it, and print safely to PDF.
+use egui::{Event, Key, Modifiers, PointerButton, vec2};
+use egui_kittest::{Harness, kittest::Queryable};
+use photocraft_ui_egui::{PhotocraftApp, file_ui::open_print};
+use serde_json::{Map, Value, json};
+
+fn app() -> (PhotocraftApp, u64) {
+    let mut app = PhotocraftApp::new(photocraft_engine::Session::new(), photocraft_ui_egui::Services::default());
+    app.run("file.new", json!({"width": 120, "height": 80, "name": "Custom paper"})).unwrap();
+    let id = open_print(&mut app);
+    // Every UI path in these fixtures is safe even if an unexpected Enter confirms the dialog.
+    app.ui.dialog_mut(id).unwrap().fields.insert("dryRun".into(), json!(true));
+    (app, id)
+}
+
+fn show(ui: &mut egui::Ui, app: &mut PhotocraftApp) {
+    if ui.ctx().fonts(|fonts| fonts.families().contains(&egui::FontFamily::Name("medium".into()))) {
+        photocraft_ui_egui::dialogs::show(app, ui.ctx());
+    }
+}
+
+fn harness(app: PhotocraftApp) -> Harness<'static, PhotocraftApp> {
+    let mut h = Harness::builder().with_size(vec2(1000.0, 900.0)).with_max_steps(64).build_ui_state(show, app);
+    PhotocraftApp::setup_context(&h.ctx, Default::default());
+    h.run_steps(4);
+    h
+}
+
+fn click(h: &mut Harness<'_, PhotocraftApp>, label: &str) {
+    if let Some(node) = h.query_by_label(label) {
+        node.click();
+    } else {
+        h.get_by_value(label).click();
+    }
+    h.run_steps(3);
+}
+
+fn shown(h: &Harness<'_, PhotocraftApp>, text: &str) -> bool {
+    h.query_by_label(text).is_some() || h.query_by_value(text).is_some()
+}
+
+fn enter_length(h: &mut Harness<'_, PhotocraftApp>, label: &str, value: &str) {
+    // Anchor the pointer to the real label; the existing length widget immediately follows it.
+    let pos = h.get_by_label(label).rect().right_center() + vec2(35.0, 0.0);
+    h.event(Event::PointerMoved(pos));
+    h.event(Event::PointerButton { pos, button: PointerButton::Primary, pressed: true, modifiers: Modifiers::NONE });
+    h.run_steps(1);
+    h.event(Event::PointerButton { pos, button: PointerButton::Primary, pressed: false, modifiers: Modifiers::NONE });
+    h.run_steps(1);
+    h.key_press_modifiers(Modifiers::COMMAND, Key::A);
+    h.event(Event::Text(value.into()));
+    h.run_steps(1);
+    h.key_press(Key::Tab);
+    h.run_steps(3);
+}
+
+fn fields(h: &Harness<'_, PhotocraftApp>, id: u64) -> Map<String, Value> {
+    h.state().ui.dialogs.iter().find(|d| d.id == id).unwrap().fields.clone()
+}
+
+#[test]
+fn custom_paper_ui_edits_reach_preview_and_pdf_and_restore_when_reopened() {
+    let (mut app, id) = app();
+    let output = std::env::temp_dir().join(format!("pc-custom-paper-{}.pdf", std::process::id()));
+    let dialog = app.ui.dialog_mut(id).unwrap();
+    dialog.fields.insert("output".into(), json!(output.to_string_lossy()));
+    dialog.fields.insert("dryRun".into(), json!(true));
+    let mut h = harness(app);
+    click(&mut h, "letter");
+    assert!(shown(&h, "a4"), "the real preset menu must be open");
+    assert!(shown(&h, "Custom"), "the paper menu must offer editable custom dimensions");
+    click(&mut h, "Custom");
+    assert_eq!(fields(&h, id)["paper"], json!([612.0, 792.0]), "Custom starts at the current preset dimensions");
+    click(&mut h, "Centimeters");
+    click(&mut h, "Inches");
+    enter_length(&mut h, "Paper Width", "7.25");
+    enter_length(&mut h, "Paper Height", "9.25");
+    let edited = fields(&h, id);
+    assert_eq!(edited["paper"], json!([522.0, 666.0]), "the engine takes points, not the displayed units");
+    click(&mut h, "Custom");
+    click(&mut h, "a4");
+    assert_eq!(fields(&h, id)["paper"], "a4");
+    assert!(!shown(&h, "Paper Width"));
+    click(&mut h, "a4");
+    click(&mut h, "Custom");
+    assert_eq!(fields(&h, id)["paper"], edited["paper"], "returning from a preset restores the custom dimensions");
+    click(&mut h, "Inches");
+    click(&mut h, "Millimeters");
+    assert_eq!(fields(&h, id)["paper"], edited["paper"], "changing units must not change the physical paper");
+    click(&mut h, "Millimeters");
+    click(&mut h, "Inches");
+    assert_eq!(fields(&h, id)["paper"], edited["paper"]);
+    let preview = photocraft_engine::print_cmds::layout(&h.state().session.active().unwrap().doc, &Value::Object(edited.clone()), "file.print").unwrap();
+    assert_eq!(preview.paper, (522.0, 666.0));
+    click(&mut h, "Portrait");
+    click(&mut h, "Landscape");
+    let edited = fields(&h, id);
+    assert_eq!(edited["paper"], json!([522.0, 666.0]), "orientation must not rewrite the unrotated custom dimensions");
+    let preview = photocraft_engine::print_cmds::layout(&h.state().session.active().unwrap().doc, &Value::Object(edited), "file.print").unwrap();
+    assert_eq!(preview.paper, (666.0, 522.0));
+    click(&mut h, "Print");
+    let pdf = std::fs::read(&output).unwrap();
+    assert!(String::from_utf8_lossy(&pdf).contains("/MediaBox [0 0 666.00 522.00]"));
+    assert_eq!(h.state().session.file_menu.last_print.as_ref().unwrap()["paper"], json!([522.0, 666.0]));
+    let id = open_print(h.state_mut());
+    h.state_mut().ui.dialog_mut(id).unwrap().fields.insert("dryRun".into(), json!(true));
+    h.run_steps(3);
+    assert!(shown(&h, "Custom"), "reopening array-valued settings must show Custom");
+    assert_eq!(fields(&h, id)["paper"], json!([522.0, 666.0]));
+    std::fs::remove_file(output).unwrap();
+}
+
+#[test]
+fn remembered_custom_paper_is_shown_without_becoming_a_preset() {
+    let (mut app, id) = app();
+    app.ui.dialog_mut(id).unwrap().fields.insert("paper".into(), json!([432.0, 576.0]));
+    let h = harness(app);
+    assert!(shown(&h, "Custom"), "an array paper size must not display as letter");
+    assert!(shown(&h, "Paper Width"));
+    assert!(shown(&h, "Paper Height"));
+    assert_eq!(fields(&h, id)["paper"], json!([432.0, 576.0]));
+}
+
+#[test]
+fn invalid_custom_paper_is_rejected_without_printing_or_changing_the_document() {
+    let (mut app, id) = app();
+    let output = std::env::temp_dir().join(format!("pc-invalid-custom-paper-{}.pdf", std::process::id()));
+    app.ui.dialog_mut(id).unwrap().fields.insert("output".into(), json!(output.to_string_lossy()));
+    let before = app.session.active().unwrap().doc.clone();
+    let mut h = harness(app);
+    click(&mut h, "letter");
+    click(&mut h, "Custom");
+    enter_length(&mut h, "Paper Width", "0");
+    let edited = fields(&h, id);
+    assert!(photocraft_engine::print_cmds::layout(&h.state().session.active().unwrap().doc, &Value::Object(edited), "file.print").is_err());
+    click(&mut h, "Print");
+    let error = &h.state().ui.status;
+    assert!(error.contains("paper is too small"), "{error}");
+    assert_eq!(h.state().session.active().unwrap().doc, before);
+    assert!(h.state().session.file_menu.last_print.is_none());
+    assert!(!output.exists());
+}