diff --git a/crates/ui-egui/src/canvas/filter_preview.rs b/crates/ui-egui/src/canvas/filter_preview.rs
index 0144390b46..3a6ee54d97 100644
--- a/crates/ui-egui/src/canvas/filter_preview.rs
+++ b/crates/ui-egui/src/canvas/filter_preview.rs
@@ -84,11 +84,14 @@ pub(crate) fn ensure_filter_preview(app: &mut PhotocraftApp, idx: usize, ctx: &e
                 Ok(computed) if finished == request => publish_filter_preview(app, idx, finished, computed.result, computed.ms),
                 // Mid-drag, a frame for the value the pointer has just left is what makes the
                 // canvas follow the drag. Three things have to hold for it: it belongs to this
-                // preview, it has a frame to show (an empty one would blank a good preview), and
-                // it must not displace a frame that is already exact for the dialog, which is
-                // what a slider dragged out and back would otherwise make the canvas do.
+                // preview (at any proxy factor: it is drawn at its own, and a radius drag can move
+                // the factor, #10474), it has a frame to show (an empty one would blank a good
+                // preview), and it must not displace a frame that is already exact for the dialog,
+                // which is what a slider dragged out and back would otherwise make the canvas do.
                 Ok(computed)
-                    if finished.same_target(&request) && computed.result.is_some() && !app.filter_preview.as_ref().is_some_and(|p| p.key == request) =>
+                    if finished.same_target_at_any_scale(&request)
+                        && computed.result.is_some()
+                        && !app.filter_preview.as_ref().is_some_and(|p| p.key == request) =>
                 {
                     publish_filter_preview(app, idx, finished, computed.result, computed.ms);
                 }
diff --git a/crates/ui-egui/src/canvas/tests.rs b/crates/ui-egui/src/canvas/tests.rs
index 6bce89cc0d..73ef696976 100644
--- a/crates/ui-egui/src/canvas/tests.rs
+++ b/crates/ui-egui/src/canvas/tests.rs
@@ -371,6 +371,31 @@ fn drive_until(app: &mut PhotocraftApp, ctx: &egui::Context, what: &str, done: i
     }
 }
 
+#[cfg(not(target_arch = "wasm32"))]
+#[test]
+fn a_held_preview_frame_stays_on_the_canvas_when_the_proxy_factor_changes() {
+    // #10474: Minimum and Maximum take their proxy factor from the radius, so a radius drag across
+    // a whole pixel changes `FilterPreviewKey::k`. The frame for the old factor stays on the
+    // canvas, drawn at its own factor, until the one for the new factor lands.
+    let mut app = PhotocraftApp::new(photocraft_engine::Session::new(), Default::default());
+    app.run("file.new", json!({"width": 2400, "height": 2200})).unwrap();
+    app.background_jobs = true;
+    app.ui.views.get_mut(0).expect("the new document's view").zoom = 0.25;
+    crate::filter_dialog::open(&mut app, "filter.other.maximum").unwrap();
+    app.ui.dialogs[0].fields.insert("radius".into(), json!(6.0));
+    let ctx = egui::Context::default();
+    drive_until(&mut app, &ctx, "the first frame", |a| a.filter_preview.as_ref().is_some_and(|p| p.result.is_some()));
+    assert_eq!(app.filter_preview.as_ref().map(|p| p.key.k), Some(2), "6 px on a reduced document previews on the proxy");
+
+    // 1.5 px previews at full resolution: a new request at a new factor, computed on the worker.
+    app.ui.dialogs[0].fields.insert("radius".into(), json!(1.5));
+    let (k, _, size) = ensure_filter_preview(&mut app, 0, &ctx).expect("the held frame stays on the canvas");
+    assert_eq!((k, size), (2, [1200, 1100]), "drawn at its own factor");
+    drive_until(&mut app, &ctx, "the full-resolution frame", |a| a.filter_preview.as_ref().is_some_and(|p| p.key.k == 1 && p.result.is_some()));
+    let (k, _, size) = ensure_filter_preview(&mut app, 0, &ctx).expect("the new frame");
+    assert_eq!((k, size), (1, [2400, 2200]));
+}
+
 #[cfg(not(target_arch = "wasm32"))]
 #[test]
 fn a_preview_computed_mid_drag_reaches_the_canvas_and_then_converges() {
diff --git a/crates/ui-egui/src/dialogs/filter_dialog.rs b/crates/ui-egui/src/dialogs/filter_dialog.rs
index 08ce0f6ab1..f2e97a80e8 100644
--- a/crates/ui-egui/src/dialogs/filter_dialog.rs
+++ b/crates/ui-egui/src/dialogs/filter_dialog.rs
@@ -279,16 +279,17 @@ pub fn preview_factor(command: &str, params: &Value, k: u32) -> u32 {
     // Minimum and Maximum measure the radius in whole pixels (Squareness rounds it, Roundness
     // floors it, and 0 is the identity), so at k = 4 a typed 1.5 px became 0.375 = 0 proxy px and
     // the preview showed the untouched document while OK applied a real effect (#10474). Keep at
-    // least one proxy pixel of radius, and for Squareness the largest k whose rounded proxy radius,
-    // scaled back, is within 1 px of the one OK applies (6 px at k = 4 rounded to 2 proxy px, 8 px).
+    // least one proxy pixel of radius, and the largest k whose whole-pixel proxy radius, scaled
+    // back, is within 1 px of the one OK applies: Squareness at 6 px and k = 4 rounds 1.5 to 2 proxy
+    // px (8 px), and Roundness's disc reaches floor(1.5) = 1 proxy px along an axis (4 px).
     // A large radius still previews on a proxy: at full size Roundness costs O(radius) per pixel.
     if matches!(command, "filter.other.minimum" | "filter.other.maximum") {
         let Some(r) = params.get("radius").and_then(Value::as_f64).filter(|r| r.is_finite()) else { return k };
         let r = r.clamp(0.2, 500.0);
-        let square = params.get("preserve").and_then(Value::as_str) != Some("roundness");
+        let whole: fn(f64) -> f64 = if params.get("preserve").and_then(Value::as_str) == Some("roundness") { f64::floor } else { f64::round };
         let fits = |d: u32| {
             let d = f64::from(d);
-            d <= r.floor() && (!square || ((r / d).round() * d - r.round()).abs() <= 1.0)
+            d <= r.floor() && (whole(r / d) * d - whole(r)).abs() <= 1.0
         };
         return (1..=k).rev().find(|&d| fits(d)).unwrap_or(1);
     }
diff --git a/crates/ui-egui/src/dialogs/filter_dialog/tests.rs b/crates/ui-egui/src/dialogs/filter_dialog/tests.rs
index c5013adc25..dcaa32ddbd 100644
--- a/crates/ui-egui/src/dialogs/filter_dialog/tests.rs
+++ b/crates/ui-egui/src/dialogs/filter_dialog/tests.rs
@@ -1047,16 +1047,29 @@ fn proxy_previews_scale_only_lengths() {
     assert_eq!(preview_factor("filter.pixelate.mosaic", &json!({"cellSize": "x"}), 4), 4);
     let (k, diff) = error("filter.pixelate.mosaic", json!({"cellSize": 10.0}));
     assert!(k == 2 && diff < 0.01, "Mosaic: {diff}");
+    // Roundness at a fractional radius: at 6.5 px, k = 3 reaches floor(2.17) = 2 proxy px along an
+    // axis (6 px), where k = 4 reached floor(1.63) = 1 (4 px), so the chosen factor previews closer
+    // to what OK applies than k = 4 did (#10474).
+    for cmd in ["filter.other.minimum", "filter.other.maximum"] {
+        let p = json!({"radius": 6.5, "preserve": "roundness"});
+        let (k, diff) = error(cmd, p.clone());
+        let applied = preview_document(&doc, Some(bg), cmd, &p, 1).unwrap();
+        let at_four = preview_document(&doc, Some(bg), cmd, &p, 4).unwrap();
+        let four = mean_abs(&every_kth(&applied, 4), &every_kth(&at_four, 1));
+        assert!(k == 3 && diff < four, "{cmd}: k = {k} differs by {diff}, k = 4 by {four}");
+    }
     // Minimum and Maximum keep at least one proxy pixel of radius (#10474), and a large radius
-    // keeps a proxy instead of a full-size preview that costs minutes at radius 500. Squareness
-    // also keeps its rounded radius within 1 px of OK's: 6 px is 2 proxy px at k = 3 (6 px), not
-    // at k = 4 (8 px); 50 px is 17 at k = 3 (51 px), not 13 at k = 4 (52 px).
+    // keeps a proxy instead of a full-size preview that costs minutes at radius 500. Both keep
+    // their whole-pixel radius within 1 px of OK's. Squareness rounds: 6 px is 2 proxy px at k = 3
+    // (6 px), not at k = 4 (8 px); 50 px is 17 at k = 3 (51 px), not 13 at k = 4 (52 px). Roundness's
+    // disc reaches floor(r) along an axis: 6 px is 2 at k = 3 (6 px), not 1 at k = 4 (4 px); 50 px
+    // is 25 at k = 2, not 12 at k = 4 or 16 at k = 3 (48 px).
     let factors = |cmd: &str, preserve: &str| {
         [0.2, 1.5, 2.5, 3.9, 6.0, 50.0, 500.0, 9999.0].map(|radius| preview_factor(cmd, &json!({"radius": radius, "preserve": preserve}), 4))
     };
     for cmd in ["filter.other.minimum", "filter.other.maximum"] {
         assert_eq!(factors(cmd, "squareness"), [1, 1, 2, 3, 3, 3, 4, 4], "{cmd} squareness");
-        assert_eq!(factors(cmd, "roundness"), [1, 1, 2, 3, 4, 4, 4, 4], "{cmd} roundness");
+        assert_eq!(factors(cmd, "roundness"), [1, 1, 2, 3, 3, 2, 4, 4], "{cmd} roundness");
         assert_eq!(preview_factor(cmd, &json!({"radius": 6.0}), 4), 3, "{cmd}: Squareness is the default");
         assert_eq!(preview_factor(cmd, &json!({"radius": "x"}), 4), 4, "{cmd}: a junk radius keeps the proxy");
     }