diff --git a/crates/compose/src/masks/tests.rs b/crates/compose/src/masks/tests.rs
index b6aa42fe1f..9076770913 100644
--- a/crates/compose/src/masks/tests.rs
+++ b/crates/compose/src/masks/tests.rs
@@ -137,6 +137,36 @@ fn parallel_feathered_pixel_mask_returns_the_same_tiles() {
     assert!(outputs.iter().all(|surface| Arc::ptr_eq(surface.tiles().next().unwrap().1, outputs[0].tiles().next().unwrap().1)));
 }
 
+/// A feather reaching the canvas edge sees the mask continue past it with its edge values, as
+/// in Photoshop (#9475): a rectangle near the edge feathers the same on a taller canvas holding
+/// the same mask (clamping each blur pass at the edge brightened it by up to 12 levels at
+/// 32 px), and a mask white up to the edge stays white along it.
+#[test]
+fn feather_at_the_canvas_edge_continues_the_mask_past_it() {
+    let mut layer = Layer::raster("edge", PixelFormat::RGBA8);
+    let mut mask = photocraft_doc::LayerMask::hide_all();
+    mask.surface.fill_rect(Rect::new(40, 20, 120, 60), &[1.0]);
+    mask.feather = 32.0;
+    layer.mask = Some(mask);
+    let canvas = Rect::new(0, 0, 160, 80);
+    let tight = combined_mask(&layer, canvas).unwrap().read_region(canvas);
+    let tall = combined_mask(&layer, Rect::new(0, -200, 160, 280)).unwrap().read_region(canvas);
+    let worst = tight.iter().zip(&tall).map(|(a, b)| (a - b).abs()).fold(0.0_f32, f32::max);
+    assert!(worst < 1e-4, "the canvas edge changed the feather by {worst}");
+
+    let mut half = photocraft_doc::LayerMask::hide_all();
+    half.surface.fill_rect(Rect::new(0, 0, 80, 80), &[1.0]);
+    half.feather = 32.0;
+    layer.mask = Some(half);
+    let m = combined_mask(&layer, canvas).unwrap().read_region(canvas);
+    // Photoshop: every row the same, no darkening towards the top or bottom edge.
+    let middle = &m[40 * 160..41 * 160];
+    for (y, row) in m.as_chunks::<160>().0.iter().enumerate() {
+        assert!(row.iter().zip(middle).all(|(a, b)| (a - b).abs() < 1e-5), "row {y} differs from row 40");
+    }
+    assert!(middle[0] > 0.99, "the left edge stays white: {}", middle[0]);
+}
+
 #[cfg(not(target_arch = "wasm32"))]
 #[test]
 fn vector_mask_build_inside_a_small_rayon_pool_completes() {
diff --git a/crates/compose/src/soft_shape_tests.rs b/crates/compose/src/soft_shape_tests.rs
index 222b3f3e9a..37575beef5 100644
--- a/crates/compose/src/soft_shape_tests.rs
+++ b/crates/compose/src/soft_shape_tests.rs
@@ -91,7 +91,8 @@ fn density_shows_the_fill_beyond_the_path() {
 /// mask break differently: 127 and 128, both what Photoshop's "127/128" allows.)
 #[test]
 fn soft_shape_matches_an_equal_vector_mask() {
-    for (density, feather) in [(1.0, 10.0), (0.45, 0.0), (0.3, 6.5), (0.0, 0.0), (1.0, 0.0), (0.5, 3.0)] {
+    // 40 px reaches past the canvas edge: both feather it the same way there (#9475).
+    for (density, feather) in [(1.0, 10.0), (0.45, 0.0), (0.3, 6.5), (0.0, 0.0), (1.0, 0.0), (0.5, 3.0), (1.0, 40.0)] {
         for depth in [SampleType::U8, SampleType::U16] {
             let (s, f) = (shape_doc(depth, density, feather, None), fill_doc(depth, density, feather));
             for y in [0, 45, 50, 100, 151, 199] {
diff --git a/crates/vector/src/feather.rs b/crates/vector/src/feather.rs
index 5048916d49..2e3cd10d08 100644
--- a/crates/vector/src/feather.rs
+++ b/crates/vector/src/feather.rs
@@ -42,6 +42,21 @@ pub fn path_area(path: &Path, sigma: f32) -> Rect {
     }
 }
 
+/// The largest buffer (in `f32`s) the blur has allocated, for tests of its working set.
+#[cfg(test)]
+static LARGEST_BUFFER: std::sync::atomic::AtomicUsize = std::sync::atomic::AtomicUsize::new(0);
+
+/// Tests make [`line_buffers`] fail for lines of this many samples (0: never).
+#[cfg(test)]
+static FAIL_LINE_BUFFERS: std::sync::atomic::AtomicUsize = std::sync::atomic::AtomicUsize::new(0);
+
+/// Notes a blur buffer of `len` floats ([`LARGEST_BUFFER`]); nothing outside tests.
+fn note_buffer(len: usize) {
+    #[cfg(test)]
+    LARGEST_BUFFER.fetch_max(len, std::sync::atomic::Ordering::Relaxed);
+    let _ = len;
+}
+
 /// One box pass: weight 1 on `±r` around each sample and `end` (0..1) on the two samples just
 /// beyond, so the pass's variance is any real number, not only the `r(r+1)/3` of whole boxes.
 #[derive(Clone, Copy, Debug, PartialEq)]
@@ -99,34 +114,77 @@ fn blur_line(line: &mut [f32], scratch: &mut [f32], pass: BoxPass) {
 /// Apply three clamped box passes to independent contiguous rows. Scoped OS threads are used
 /// instead of Rayon because mask construction can happen inside a Rayon tile job; nested Rayon
 /// work here can deadlock when every worker waits on the same OnceLock (#276).
-fn blur_rows_chunk(v: &mut [f32], w: usize, pass: BoxPass) {
-    let mut scratch = vec![0.0; w];
+fn blur_rows_chunk(v: &mut [f32], w: usize, pass: BoxPass, halo: usize, buffers: &mut (Vec<f32>, Vec<f32>)) {
+    let (padded, scratch) = buffers;
     for row in v.chunks_mut(w) {
-        for _ in 0..3 {
-            blur_line(row, &mut scratch, pass);
-        }
+        blur_padded_line(row, padded, scratch, pass, halo);
     }
 }
 
-fn blur_rows(v: &mut [f32], w: usize, pass: BoxPass) {
-    if w == 0 || v.is_empty() {
+/// A padded line and its scratch for lines of `len` samples with `halo` more on each side; `None`
+/// when they don't fit in memory.
+fn line_buffers(len: usize, halo: usize) -> Option<(Vec<f32>, Vec<f32>)> {
+    #[cfg(test)]
+    if FAIL_LINE_BUFFERS.load(std::sync::atomic::Ordering::Relaxed) == len {
+        return None;
+    }
+    let n = halo.checked_mul(2)?.checked_add(len)?;
+    let (mut padded, mut scratch) = (Vec::new(), Vec::new());
+    padded.try_reserve_exact(n).ok()?;
+    scratch.try_reserve_exact(n).ok()?;
+    padded.resize(n, 0.0);
+    scratch.resize(n, 0.0);
+    note_buffer(n);
+    Some((padded, scratch))
+}
+
+/// The three box passes on `line` continued `halo` samples past each end with its end values, as
+/// Photoshop continues a mask past the canvas edge (#9475). Padding each line, row pass then
+/// column pass, gives exactly what padding the whole plane would: the plane's padded corners
+/// repeat its corner values, which is what the padded rows' own row pass leaves there. `padded`
+/// and `scratch` hold `line.len() + 2 * halo` samples.
+fn blur_padded_line(line: &mut [f32], padded: &mut [f32], scratch: &mut [f32], pass: BoxPass, halo: usize) {
+    let (Some(&a), Some(&b)) = (line.first(), line.last()) else { return };
+    let n = line.len() + 2 * halo;
+    let (Some(padded), Some(scratch)) = (padded.get_mut(..n), scratch.get_mut(..n)) else { return };
+    padded[..halo].fill(a);
+    padded[halo..halo + line.len()].copy_from_slice(line);
+    padded[halo + line.len()..].fill(b);
+    for _ in 0..3 {
+        blur_line(padded, scratch, pass);
+    }
+    line.copy_from_slice(&padded[halo..halo + line.len()]);
+}
+
+/// How many workers blur `lines` lines of `len` samples.
+fn row_workers(lines: usize, len: usize) -> usize {
+    if cfg!(target_arch = "wasm32") || lines.saturating_mul(len) < 1 << 16 {
+        1
+    } else {
+        std::thread::available_parallelism().map_or(1, |n| n.get()).clamp(1, 16).min(lines.max(1))
+    }
+}
+
+/// Line buffers for each of [`row_workers`]; `None` unless every one fits in memory.
+fn worker_buffers(lines: usize, len: usize, halo: usize) -> Option<Vec<(Vec<f32>, Vec<f32>)>> {
+    (0..row_workers(lines, len)).map(|_| line_buffers(len, halo)).collect()
+}
+
+/// Blurs the rows of `v` on one worker per entry of `buffers` (from [`worker_buffers`]).
+fn blur_rows(v: &mut [f32], w: usize, pass: BoxPass, halo: usize, buffers: &mut [(Vec<f32>, Vec<f32>)]) {
+    if w == 0 || v.is_empty() || buffers.is_empty() {
         return;
     }
     let h = v.len() / w;
-    let threads = if cfg!(target_arch = "wasm32") || v.len() < 1 << 16 {
-        1
-    } else {
-        std::thread::available_parallelism().map_or(1, |n| n.get()).clamp(1, 16).min(h.max(1))
-    };
-    if threads == 1 {
-        blur_rows_chunk(v, w, pass);
+    if let [only] = buffers {
+        blur_rows_chunk(v, w, pass, halo, only);
         return;
     }
-    let rows_per_worker = h.div_ceil(threads);
+    let rows_per_worker = h.div_ceil(buffers.len());
     let chunk_len = rows_per_worker.saturating_mul(w);
     std::thread::scope(|scope| {
-        for chunk in v.chunks_mut(chunk_len) {
-            scope.spawn(move || blur_rows_chunk(chunk, w, pass));
+        for (chunk, buffers) in v.chunks_mut(chunk_len).zip(buffers.iter_mut()) {
+            scope.spawn(move || blur_rows_chunk(chunk, w, pass, halo, buffers));
         }
     });
 }
@@ -147,50 +205,62 @@ fn transpose_into(v: &[f32], out: &mut [f32], w: usize, h: usize) {
 }
 
 /// Approximate Gaussian blur of a `w`×`h` plane: three box passes per axis whose variance is
-/// exactly `sigma²` ([`BoxPass`]; edges clamp, which is exact here since the planes extend into
-/// constant mask regions). Whole-pixel boxes rounded the variance (a 6 px feather blurred like
-/// 6.48 px, a 3 px one like 3.46) and missed Photoshop's feathered edges by up to 4 levels
-/// (psd-tools mask-parameters-no-real-channel.psd); the exact variance is within 1.5. The axes
-/// are processed in parallel on bounded OS threads, preserving each line's floating-point
+/// exactly `sigma²` ([`BoxPass`]). Beyond its edges the plane continues with its edge values, as
+/// Photoshop feathers a mask at the canvas edge: a mask white up to the edge stays white there,
+/// one black there stays black (#9475). Whole-pixel boxes rounded the variance (a 6 px feather
+/// blurred like 6.48 px, a 3 px one like 3.46) and missed Photoshop's feathered edges by up to 4
+/// levels (psd-tools mask-parameters-no-real-channel.psd); the exact variance is within 1.5. The
+/// axes are processed in parallel on bounded OS threads, preserving each line's floating-point
 /// operation order.
 pub fn gaussian(v: &mut [f32], w: usize, h: usize, sigma: f32) {
     if w == 0 || h == 0 || v.len() != w.saturating_mul(h) {
         return;
     }
     let Some(pass) = BoxPass::for_sigma(sigma) else { return };
-    // Reserve before modifying the input so allocation failure can use the sequential fallback
-    // without leaving a half-blurred mask.
+    // Each pass reads `r + 1` samples past a pixel, so three see `3(r + 1)` past the edge. Clamping
+    // every pass at the edge instead repeated the blurred values there, which brightened a 32 px
+    // feather by up to 12 levels where the canvas cut it off.
+    let m = pass.r.saturating_add(1).saturating_mul(3);
+    blur_plane(v, w, h, pass, m);
+}
+
+/// Three box passes along each axis of the `w`×`h` plane, every line continued `halo` samples
+/// past its ends ([`blur_padded_line`]).
+fn blur_plane(v: &mut [f32], w: usize, h: usize, pass: BoxPass, halo: usize) {
+    // Reserve everything before modifying the input, so an allocation failure can use the
+    // sequential fallback (which also reserves first) and never leaves a half-blurred mask.
     let mut transposed = Vec::new();
-    if transposed.try_reserve_exact(v.len()).is_err() {
-        gaussian_sequential(v, w, h, pass);
+    let buffers = if transposed.try_reserve_exact(v.len()).is_ok() { worker_buffers(h, w, halo).zip(worker_buffers(w, h, halo)) } else { None };
+    let Some((mut rows, mut columns)) = buffers else {
+        gaussian_sequential(v, w, h, pass, halo);
         return;
-    }
+    };
     transposed.resize(v.len(), 0.0);
-    blur_rows(v, w, pass);
+    note_buffer(v.len());
+    blur_rows(v, w, pass, halo, &mut rows);
     transpose_into(v, &mut transposed, w, h);
-    blur_rows(&mut transposed, h, pass);
+    blur_rows(&mut transposed, h, pass, halo, &mut columns);
     transpose_into(&transposed, v, h, w);
 }
 
-/// Allocation-light reference path used if parallel transpose storage cannot be reserved.
-fn gaussian_sequential(v: &mut [f32], w: usize, h: usize, pass: BoxPass) {
+/// Allocation-light reference path used if parallel transpose storage cannot be reserved. It
+/// reserves its lines before touching `v`: without them the plane is left as it was, never half
+/// blurred.
+fn gaussian_sequential(v: &mut [f32], w: usize, h: usize, pass: BoxPass, halo: usize) {
     let mut line = Vec::new();
-    let mut scratch = Vec::new();
     let len = w.max(h);
-    if line.try_reserve_exact(len).is_err() || scratch.try_reserve_exact(len).is_err() {
+    if line.try_reserve_exact(len).is_err() {
         return;
     }
     line.resize(len, 0.0);
-    scratch.resize(len, 0.0);
+    let Some((mut padded, mut scratch)) = line_buffers(len, halo) else { return };
     for axis in [false, true] {
         let (lines, line_len) = if axis { (w, h) } else { (h, w) };
         for k in 0..lines {
             for i in 0..line_len {
                 line[i] = if axis { v[i * w + k] } else { v[k * w + i] };
             }
-            for _ in 0..3 {
-                blur_line(&mut line[..line_len], &mut scratch[..line_len], pass);
-            }
+            blur_padded_line(&mut line[..line_len], &mut padded, &mut scratch, pass, halo);
             for i in 0..line_len {
                 if axis {
                     v[i * w + k] = line[i];
@@ -254,9 +324,31 @@ mod tests {
         let mut parallel: Vec<_> = (0..w * h).map(|i| ((i * 37 % 1009) as f32) / 1008.0).collect();
         let mut sequential = parallel.clone();
         let pass = BoxPass::for_sigma(sigma).unwrap();
-        gaussian_sequential(&mut sequential, w, h, pass);
-        gaussian(&mut parallel, w, h, sigma);
-        assert_eq!(parallel, sequential, "parallel blur must retain the exact per-line arithmetic");
+        let source = parallel.clone();
+        for halo in [0, 3 * (pass.r + 1)] {
+            parallel.copy_from_slice(&source);
+            sequential.copy_from_slice(&source);
+            gaussian_sequential(&mut sequential, w, h, pass, halo);
+            blur_plane(&mut parallel, w, h, pass, halo);
+            assert_eq!(parallel, sequential, "parallel blur must retain the exact per-line arithmetic (halo {halo})");
+        }
+    }
+
+    /// Padding each line (#9493) blurs exactly as padding the whole plane with its edge values
+    /// and cropping it afterwards does, without the padded plane.
+    #[test]
+    fn padding_each_line_matches_padding_the_plane() {
+        let (w, h, sigma): (usize, usize, f32) = (90, 70, 9.0);
+        let source: Vec<f32> = (0..w * h).map(|i| ((i * 37 % 1009) as f32) / 1008.0).collect();
+        let pass = BoxPass::for_sigma(sigma).unwrap();
+        let m = 3 * (pass.r + 1);
+        let (pw, ph) = (w + 2 * m, h + 2 * m);
+        let mut plane: Vec<f32> = (0..pw * ph).map(|i| source[((i / pw).clamp(m, m + h - 1) - m) * w + (i % pw).clamp(m, m + w - 1) - m]).collect();
+        blur_plane(&mut plane, pw, ph, pass, 0);
+        let want: Vec<f32> = (0..w * h).map(|i| plane[(i / w + m) * pw + i % w + m]).collect();
+        let mut got = source;
+        gaussian(&mut got, w, h, sigma);
+        assert_eq!(got, want);
     }
 
     #[test]
@@ -269,7 +361,7 @@ mod tests {
         let source: Vec<_> = (0..w * h).map(|i| ((i * 37 % 1009) as f32) / 1008.0).collect();
         let mut sequential = source.clone();
         let start = Instant::now();
-        gaussian_sequential(&mut sequential, w, h, pass);
+        gaussian_sequential(&mut sequential, w, h, pass, 3 * (pass.r + 1));
         let sequential_time = start.elapsed();
         let mut parallel = source;
         let start = Instant::now();
@@ -278,4 +370,36 @@ mod tests {
         assert_eq!(parallel, sequential);
         eprintln!("50 MP mask blur: sequential={sequential_time:?}, parallel={parallel_time:?}");
     }
+
+    /// CodeRabbit on #9493: continuing the mask past the plane's edge must not build a padded copy
+    /// of the whole plane. At the 1000 px maximum feather the halo is about 3000 px a side, so a
+    /// padded 400 × 400 plane would be 6400 × 6400 floats; the blur needs only the plane's
+    /// transpose and one padded line per worker.
+    #[test]
+    fn a_maximum_feather_does_not_pad_the_whole_plane() {
+        let (w, h) = (400usize, 400usize);
+        let mut v: Vec<f32> = (0..w * h).map(|i| if (i % w) < w / 2 { 1.0 } else { 0.0 }).collect();
+        gaussian(&mut v, w, h, feather_sigma(1000.0));
+        let largest = LARGEST_BUFFER.load(std::sync::atomic::Ordering::Relaxed);
+        assert!(largest <= w * h, "the blur allocated a buffer of {largest} floats for a {w} × {h} plane");
+        assert!(v.iter().all(|x| x.is_finite()));
+    }
+
+    /// CodeRabbit on #9493: when a line buffer can't be had, the mask is never left half blurred
+    /// (rows untouched but columns blurred, or some workers' rows only): the plane is either fully
+    /// blurred or untouched.
+    #[test]
+    fn a_failed_line_buffer_never_leaves_a_half_blurred_mask() {
+        let (w, h, sigma): (usize, usize, f32) = (433, 211, 7.0);
+        let source: Vec<f32> = (0..w * h).map(|i| ((i * 37 % 1009) as f32) / 1008.0).collect();
+        let mut want = source.clone();
+        gaussian(&mut want, w, h, sigma);
+        for fail in [w, h] {
+            FAIL_LINE_BUFFERS.store(fail, std::sync::atomic::Ordering::Relaxed);
+            let mut got = source.clone();
+            gaussian(&mut got, w, h, sigma);
+            FAIL_LINE_BUFFERS.store(0, std::sync::atomic::Ordering::Relaxed);
+            assert!(got == want || got == source, "lines of {fail} failing left a partly blurred plane");
+        }
+    }
 }