diff --git a/crates/pipeline/src/tests_window.rs b/crates/pipeline/src/tests_window.rs
index 1d5b15c65..b0fc3de8f 100644
--- a/crates/pipeline/src/tests_window.rs
+++ b/crates/pipeline/src/tests_window.rs
@@ -238,6 +238,38 @@ fn a_mask_overlay_window_matches_the_whole_render() {
     }
 }
 
+// Given the Show All Masks view (issue #1712), a window of it matches the same pixels of the
+// whole render with that overlay: the union is composed from the same mask alphas, so the zoomed
+// loupe can show it as a window like a single mask's overlay (issue #547)
+#[test]
+fn an_all_masks_overlay_window_matches_the_whole_render() {
+    use crate::{MaskView, Overlay};
+    use lightcraft_develop::{LocalAdjustments, Mask, MaskComponent, MaskOp, MaskShape};
+    use lightcraft_geom::Point;
+    let shapes = [
+        (3, MaskShape::Radial { center: Point::new(0.45, 0.5), rx: 0.2, ry: 0.15, angle: 20.0, feather: 50.0, invert: false }, 0.0),
+        (4, MaskShape::Linear { start: Point::new(0.4, 0.55), end: Point::new(0.6, 0.5) }, 100.0),
+    ];
+    let mut s = DevelopSettings::default();
+    for (id, shape, refine) in shapes {
+        s.masks.push(Mask {
+            id,
+            components: vec![MaskComponent { name: None, op: MaskOp::Add, invert: false, shape }],
+            adjust: LocalAdjustments { exposure: 1.0, ..Default::default() },
+            refine,
+            ..Default::default()
+        });
+    }
+    for view in [MaskView::Color, MaskView::WhiteOnBlack, MaskView::ImageOnBlack] {
+        let overlay = Overlay::AllMasks { view, color: [255, 0, 0], opacity: 70 };
+        let f = render(&noisy_scene(), &SourceInfo::default(), &s, &RenderRequest { overlay, ..RenderRequest::fit(W, H) }).image;
+        let w = render(&noisy_scene(), &SourceInfo::default(), &s, &RenderRequest { overlay, window: Some(MID), ..RenderRequest::fit(W, H) }).image;
+        let (max, mean) = compare(&f, &w, MID);
+        // (refine edges differs by a rounding step, as in `mask_edge_refinement_does_not_depend_on_the_window`)
+        assert!(max <= 3 && mean <= 0.25, "{view:?}: max {max} (≤ 3), mean {mean:.3} (≤ 0.25)");
+    }
+}
+
 // ---- Deep zoom: the wide stages' kernels are far wider than any margin -------------------------
 
 /// Hard-edged blocks of very different brightness at several scales: the wide stages' edge-aware
diff --git a/crates/ui-egui/src/panels/detail.rs b/crates/ui-egui/src/panels/detail.rs
index 151d4a57a..d35147628 100644
--- a/crates/ui-egui/src/panels/detail.rs
+++ b/crates/ui-egui/src/panels/detail.rs
@@ -250,10 +250,12 @@ fn animated_rect(ctx: &egui::Context, anim: &mut bool, target: Rect) -> Rect {
 }
 
 /// Whether an overlay can be rendered on a region tile without changing what the tile means.
-/// Mask overlays are evaluated from the same mask alpha as the full render; the other diagnostic
-/// overlays either need whole-frame context or deliberately replace the loupe.
+/// Mask overlays (one mask, or every visible mask at once) are evaluated from the same mask
+/// alphas as the full render; the other diagnostic overlays either need whole-frame context or
+/// deliberately replace the loupe.
 pub(crate) fn overlay_supports_window(overlay: lightcraft_pipeline::Overlay) -> bool {
-    matches!(overlay, lightcraft_pipeline::Overlay::None | lightcraft_pipeline::Overlay::Mask { .. })
+    use lightcraft_pipeline::Overlay;
+    matches!(overlay, Overlay::None | Overlay::Mask { .. } | Overlay::AllMasks { .. })
 }
 
 /// What a window render of the loupe needs to know about the view it belongs to.
diff --git a/crates/ui-egui/src/tests_preview_limit.rs b/crates/ui-egui/src/tests_preview_limit.rs
index 4f058f536..76d3ea29c 100644
--- a/crates/ui-egui/src/tests_preview_limit.rs
+++ b/crates/ui-egui/src/tests_preview_limit.rs
@@ -227,6 +227,40 @@ mod in_the_loupe {
         assert_eq!(h.app.region_view.map(|r| r.settings), Some(plain.settings), "back to the plain window");
     }
 
+    // Given the Show All Masks view (issue #1712) at 1:1, the zoomed loupe is a canvas-sized frame
+    // plus a native window carrying the overlay, as a selected mask's is: the union is evaluated
+    // from the same mask alphas, so it must not fall back to a whole-frame render at the zoomed
+    // size (slower, and its detail stops at the frame render's resolution)
+    #[test]
+    fn an_all_masks_overlay_at_one_to_one_is_a_window() {
+        let (mut h, native) = detail();
+        for (command, params) in [
+            ("panel.masking", json!({})),
+            ("mask.add", json!({"kind": "radial", "center": [0.4, 0.5], "rx": 0.2, "ry": 0.2})),
+            ("mask.add", json!({"kind": "linear", "start": [0.2, 0.3], "end": [0.8, 0.7]})),
+        ] {
+            let r = h.request("engine.execute", json!({"command": command, "params": params}), T);
+            assert_eq!(r["ok"], true, "{command}: {r}");
+        }
+        let r = h.request("engine.execute", json!({"command": "view.maskOverlayAll", "params": {}}), T);
+        assert_eq!(r["ok"], true, "{r}");
+        assert!(h.app.ui.mask_overlay_all, "the all-masks overlay is on");
+        h.request("engine.execute", json!({"command": "view.zoom100"}), T);
+        h.settle(SETTLE);
+        let id = h.app.session.active().expect("a photo");
+        let d = h.app.session.develop_of(id).expect("settings");
+        assert!(
+            matches!(crate::panels::detail::view_overlay(&h.app, &d), lightcraft_pipeline::Overlay::AllMasks { .. }),
+            "the all-masks overlay is shown"
+        );
+        let main = rendered_long_edge(&h);
+        assert!(main <= 2560 && main < native, "native {native}: the whole-frame render is {main}");
+        let region = h.app.region_view.expect("a window render with the all-masks overlay");
+        assert_eq!(region.full.0.max(region.full.1), native, "cut from the photo's own pixels");
+        let (w, hh) = region_tile(&h).expect("its texture");
+        assert!(w >= 1300 && hh >= 700, "it covers the canvas: {w}×{hh}");
+    }
+
     // Given the user capped the preview at 1600 px, then 1:1 is rendered no larger than that
     #[test]
     fn an_explicit_limit_still_caps_one_to_one() {