diff --git a/crates/engine/src/cmd.rs b/crates/engine/src/cmd.rs
--- a/crates/engine/src/cmd.rs
+++ b/crates/engine/src/cmd.rs
@@ -2,7 +2,7 @@
 
 use photocraft_color::{BlendMode, Color, ColorMode, SampleType};
 use photocraft_doc::{Adjustment, Document, Fill, Layer, LayerContent, LayerId, LayerMask, Size};
-use photocraft_geom::Rect;
+use photocraft_geom::{Rect, TILE_SIZE};
 use photocraft_raster::Surface;
 use serde_json::{Map, Value, json};
 
@@ -417,15 +417,34 @@ fn build() -> Vec<CommandSpec> {
                     }
                     let old = doc.selection.take();
                     let combined = match (mode.as_str(), old) {
-                        ("add", Some(o)) => sel::combine_surfaces(&o, &shape, area, |a, b| a.max(b)),
-                        ("subtract", Some(o)) => sel::combine_surfaces(&o, &shape, area, |a, b| a * (1.0 - b)),
+                        ("add", Some(o)) => sel::combine_sparse(&o, &shape, area, |a, b| a.max(b)),
+                        ("subtract", Some(o)) => sel::combine_sparse(&o, &shape, area, |a, b| a * (1.0 - b)),
                         ("intersect", Some(o)) => sel::combine_surfaces(&o, &shape, area, |a, b| a.min(b)),
                         // With no existing selection, neither operation can select new pixels.
                         ("subtract" | "intersect", None) => Surface::new(photocraft_color::PixelFormat::GRAY8),
                         _ => shape,
                     };
-                    // Nothing selected on the canvas (e.g. dragged entirely outside it) deselects.
-                    doc.selection = (!combined.content_bounds().is_empty()).then_some(combined);
+                    // Sparse Add/Subtract keeps untouched pixels outside the canvas. Only pixels
+                    // inside the document count when deciding whether a selection remains.
+                    let has_on_canvas_pixels = (combined.default_bytes().first().copied().unwrap_or(0) != 0 && {
+                        let canvas_tiles = u64::from(area.width().div_ceil(TILE_SIZE as u32)) * u64::from(area.height().div_ceil(TILE_SIZE as u32));
+                        let stored = combined.tiles().filter(|(coord, _)| !coord.rect().intersect(&area).is_empty()).count() as u64;
+                        stored < canvas_tiles
+                    }) || combined.tiles().any(|(coord, tile)| {
+                        let tile_rect = coord.rect();
+                        let overlap = tile_rect.intersect(&area);
+                        if overlap.is_empty() {
+                            return false;
+                        }
+                        let x0 = (overlap.x0 - tile_rect.x0) as usize;
+                        let x1 = (overlap.x1 - tile_rect.x0) as usize;
+                        (overlap.y0 - tile_rect.y0..overlap.y1 - tile_rect.y0).any(|y| {
+                            let row_start = y as usize * TILE_SIZE as usize;
+                            let start = row_start + x0;
+                            tile.bytes().get(start..row_start + x1).is_some_and(|row| row.iter().any(|&v| v != 0))
+                        })
+                    });
+                    doc.selection = has_on_canvas_pixels.then_some(combined);
                     Ok(())
                 })?;
                 Ok(Value::Null)
diff --git a/crates/engine/src/tests.rs b/crates/engine/src/tests.rs
--- a/crates/engine/src/tests.rs
+++ b/crates/engine/src/tests.rs
@@ -433,6 +433,34 @@ fn marquee_dragged_past_the_canvas_stops_at_its_edge() {
     assert_eq!(bounds(&mut s), Value::Null);
 }
 
+#[test]
+fn subtract_clears_selection_when_only_disjoint_off_canvas_pixels_remain() {
+    use photocraft_geom::Rect;
+    use photocraft_raster::Surface;
+    let mut s = session_with_doc(); // 64 × 48
+    let mut selection = Surface::new(photocraft_color::PixelFormat::GRAY8);
+    // Simulate a moved sparse selection: one remnant on either side of the canvas and one
+    // in-canvas region that the subtract marquee will remove. The off-canvas bounds straddle
+    // the canvas, so testing only their bounding-box intersection would incorrectly keep it.
+    selection.fill_rect(Rect::new(-4, 10, 0, 14), &[1.0]);
+    selection.fill_rect(Rect::new(20, 10, 24, 14), &[1.0]);
+    selection.fill_rect(Rect::new(64, 10, 68, 14), &[1.0]);
+    Arc::make_mut(&mut s.active_mut().unwrap().doc).selection = Some(selection);
+
+    s.execute("select.rect", json!({"x": 0, "y": 0, "width": 64, "height": 48, "mode": "subtract"})).unwrap();
+
+    assert!(s.active().unwrap().doc.selection.is_none());
+}
+
+#[test]
+fn subtract_clears_selection_with_nonzero_default_when_canvas_is_empty() {
+    use photocraft_raster::Surface;
+    let mut s = session_with_doc();
+    Arc::make_mut(&mut s.active_mut().unwrap().doc).selection = Some(Surface::with_default(photocraft_color::PixelFormat::GRAY8, &[1.0]));
+    s.execute("select.rect", json!({"x": 0, "y": 0, "width": 64, "height": 48, "mode": "subtract"})).unwrap();
+    assert!(s.active().unwrap().doc.selection.is_none());
+}
+
 #[test]
 fn marquee_steps_are_named_after_their_tool() {
     // #513: the elliptical marquee shares `select.rect` but records its own step name.
diff --git a/crates/engine/tests/it/empty_selection_combine.rs b/crates/engine/tests/it/empty_selection_combine.rs
--- a/crates/engine/tests/it/empty_selection_combine.rs
+++ b/crates/engine/tests/it/empty_selection_combine.rs
@@ -91,3 +91,42 @@ fn subtracting_from_an_existing_selection_preserves_other_pixels_and_history() {
         assert_eq!(coverage(&session, 12, 12), removed);
     }
 }
+
+#[test]
+fn marquee_add_subtract_match_dense_combination_across_tiles() {
+    use photocraft_algo::selection as sel;
+    use photocraft_color::PixelFormat;
+    use photocraft_geom::Rect;
+    use photocraft_raster::Surface;
+
+    // A partial edge tile, a shape crossing a tile boundary, and an untouched distant tile.
+    let area = Rect::from_xywh(0, 0, 777, 263);
+    let r = Rect::from_xywh(247, -5, 24, 32);
+    for depth in [8, 16, 32] {
+        for ellipse in [false, true] {
+            for mode in ["add", "subtract"] {
+                let mut session = Session::new();
+                session.execute("file.new", json!({"width": 777, "height": 263, "depth": depth})).unwrap();
+                let mut old = Surface::new(PixelFormat::GRAY8);
+                old.fill_rect(Rect::from_xywh(240, 0, 40, 40), &[0.6]);
+                old.fill_rect(Rect::from_xywh(768, 256, 9, 7), &[0.4]);
+                std::sync::Arc::make_mut(&mut session.active_mut().unwrap().doc).selection = Some(old.clone());
+                let shape = if ellipse {
+                    sel::ellipse_surface(r, area, true)
+                } else {
+                    let mut shape = Surface::new(PixelFormat::GRAY8);
+                    shape.fill_rect(r.intersect(&area), &[1.0]);
+                    shape
+                };
+                let expected = sel::combine_surfaces(&old, &shape, area, |a, b| if mode == "add" { a.max(b) } else { a * (1.0 - b) });
+                session.execute("select.rect", json!({"x": 247, "y": -5, "width": 24, "height": 32, "ellipse": ellipse, "mode": mode})).unwrap();
+                let pixels = |session: &Session| session.active().unwrap().doc.selection.as_ref().unwrap().read_region(area);
+                assert_eq!(pixels(&session), expected.read_region(area), "depth={depth}, ellipse={ellipse}, mode={mode}");
+                assert!(session.undo());
+                assert_eq!(pixels(&session), old.read_region(area));
+                assert!(session.redo());
+                assert_eq!(pixels(&session), expected.read_region(area));
+            }
+        }
+    }
+}