M3.86: Alt-click the mask thumbnail to view only the mask

transparency.viewOpacityMask {on?, id?} shows only the opacity mask on the
canvas as greyscale coverage (luminance, clip and invert applied; the rest of
the art hidden) and enters mask editing; called again it shows the artwork and
keeps editing. The Transparency panel's mask thumbnail runs it on Alt-click and
is outlined in the accent colour while the mask is viewed. The state is
DocState.mask_view (shown while that mask is edited: DocState::shown_mask) and
reaches the renderer as RenderOptions.mask_view.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/maskedit.rs

@@ -40,6 +40,15 @@ pub fn specs() -> Vec<CommandSpec> {
             has_doc,
             leave
         ),
+        cmd!(
+            "transparency.viewOpacityMask",
+            "View Opacity Mask",
+            ["Window", "Transparency"],
+            None,
+            "{on?: bool (default: toggle), id?} show only the opacity mask of `id` (default: the object whose mask is edited, else the first selected object with a mask) on the canvas, as greyscale coverage, and edit it, as Alt-clicking the mask thumbnail does; off shows the artwork again (still editing) → {on, id}",
+            has_doc,
+            view_mask
+        ),
     ]
 }
 
@@ -74,12 +83,45 @@ fn enter(s: &mut Session, p: &Value) -> Result<Value> {
     if st.doc.mask_edit.is_some() {
         return Err(bad(C, "already editing an opacity mask"));
     }
-    let id = id_param(p, "id")
-        .or_else(|| st.selection.subjects().iter().copied().find(|i| st.doc.node(*i).is_some_and(|n| n.mask.is_some())))
-        .ok_or_else(|| bad(C, "select an object with an opacity mask"))?;
+    let id = id_param(p, "id").or_else(|| first_masked(st)).ok_or_else(|| bad(C, "select an object with an opacity mask"))?;
+    let layer = start(s, id)?;
+    Ok(json!({ "layer": layer.0 }))
+}
+
+/// The first selected object with an opacity mask.
+fn first_masked(st: &crate::DocState) -> Option<NodeId> {
+    st.selection.subjects().iter().copied().find(|i| st.doc.node(*i).is_some_and(|n| n.mask.is_some()))
+}
+
+/// Enter mask editing for `id` (one undo step) and isolate the editing layer. Returns the layer.
+fn start(s: &mut Session, id: NodeId) -> Result<NodeId> {
     let layer = s.edit("Edit Opacity Mask", |d, sel| begin(d, sel, id))?;
     show_editing(s, Some(layer))?;
-    Ok(json!({ "layer": layer.0 }))
+    Ok(layer)
+}
+
+fn view_mask(s: &mut Session, p: &Value) -> Result<Value> {
+    const C: &str = "transparency.viewOpacityMask";
+    let st = s.doc()?;
+    let editing = st.doc.mask_edit.map(|m| m.object);
+    if !p.get("on").and_then(Value::as_bool).unwrap_or(st.shown_mask().is_none()) {
+        let st = s.doc_mut()?;
+        let id = st.mask_view.take();
+        st.revision += 1;
+        return Ok(json!({ "on": false, "id": id.map(|i| i.0) }));
+    }
+    let id = id_param(p, "id").or(editing).or_else(|| first_masked(st)).ok_or_else(|| bad(C, "select an object with an opacity mask"))?;
+    match editing {
+        Some(e) if e != id => return Err(bad(C, "already editing another object's opacity mask")),
+        Some(_) => {}
+        None => {
+            start(s, id)?;
+        }
+    }
+    let st = s.doc_mut()?;
+    st.mask_view = Some(id);
+    st.revision += 1;
+    Ok(json!({ "on": true, "id": id.0 }))
 }
 
 /// Enter mask editing for `id` inside an edit: copy its mask art onto a new editing layer and
@@ -123,29 +165,35 @@ pub(crate) fn finish(d: &mut Document, sel: &mut Selection) {
     }
 }
 
-/// Isolate the editing layer and draw into it (`Some`), or return to the document (`None`).
+/// Isolate the editing layer and draw into it (`Some`), or return to the document (`None`, which
+/// also ends View Opacity Mask).
 pub(crate) fn show_editing(s: &mut Session, layer: Option<NodeId>) -> Result<()> {
     let st = s.doc_mut()?;
     st.isolation = layer;
     st.active_layer = layer.or_else(|| st.doc.default_layer());
+    if layer.is_none() {
+        st.mask_view = None;
+    }
     st.revision += 1;
     Ok(())
 }
 
 /// After undo or redo, isolation follows the restored document into or out of mask editing
 /// (`was` is the editing layer before, if any), so it never names a layer that is gone (whose id
-/// a later object could reuse).
+/// a later object could reuse). Out of mask editing, View Opacity Mask ends.
 pub(crate) fn follow_history(st: &mut crate::DocState, was: Option<NodeId>) {
     match st.doc.mask_edit {
         Some(me) => {
             st.isolation = Some(me.layer);
             st.active_layer = Some(me.layer);
         }
-        None if was.is_some() && st.isolation == was => {
-            st.isolation = None;
-            st.active_layer = st.doc.default_layer();
+        None => {
+            st.mask_view = None;
+            if was.is_some() && st.isolation == was {
+                st.isolation = None;
+                st.active_layer = st.doc.default_layer();
+            }
         }
-        None => {}
     }
 }
 

crates/engine/src/lib.rs

@@ -101,6 +101,9 @@ pub struct DocState {
     pub last_selection_cmd: Option<(String, Value)>,
     /// Process-unique id of this open document (tab indices shift when tabs close).
     pub uid: u64,
+    /// View Opacity Mask (Alt-click the mask thumbnail): the masked object whose mask the canvas
+    /// shows alone, in greyscale, while that mask is edited (see [`DocState::shown_mask`]).
+    pub mask_view: Option<NodeId>,
 }
 
 static NEXT_DOC_UID: std::sync::atomic::AtomicU64 = std::sync::atomic::AtomicU64::new(1);
@@ -122,6 +125,7 @@ impl DocState {
             last_transform: None,
             last_selection_cmd: None,
             uid: NEXT_DOC_UID.fetch_add(1, std::sync::atomic::Ordering::Relaxed),
+            mask_view: None,
         }
     }
     /// Unsaved changes: the document differs from the saved one (selection changes don't count).
@@ -154,6 +158,11 @@ impl DocState {
         }
         self.active_layer.filter(|l| self.doc.node(*l).is_some_and(|n| n.is_layer() && !n.locked)).or_else(|| self.doc.default_layer())
     }
+    /// The object whose opacity mask View Opacity Mask shows ([`DocState::mask_view`]): only while
+    /// its mask is being edited, so leaving editing by any route (undo, deleting the object) ends it.
+    pub fn shown_mask(&self) -> Option<NodeId> {
+        self.mask_view.filter(|id| self.doc.mask_edit.is_some_and(|m| m.object == *id))
+    }
 }
 
 /// Coordinates beyond this (points) are rejected: ~1,400 m, far past Illustrator's large canvas.
@@ -827,6 +836,8 @@ mod tests_linked_stops;
 #[cfg(test)]
 mod tests_live;
 #[cfg(test)]
+mod tests_maskview;
+#[cfg(test)]
 mod tests_menucmds;
 #[cfg(test)]
 mod tests_opacitymask;

crates/engine/src/tests_maskview.rs

@@ -0,0 +1,71 @@
+//! View Opacity Mask (M3.86): the canvas shows only the mask, as greyscale coverage, while it is
+//! edited; toggling it off shows the artwork again, and leaving editing by any route ends it.
+
+use serde_json::json;
+use vectorcraft_geom::Affine;
+use vectorcraft_render::{RenderOptions, Renderer};
+
+use super::*;
+
+fn rect(s: &mut Session, x: f64, w: f64, fill: &str) -> NodeId {
+    let id = NodeId(s.execute("shape.rectangle", &json!({"x": x, "y": 0, "width": w, "height": 200})).unwrap()["id"].as_u64().unwrap());
+    s.execute("paint.setFill", &json!({"color": fill, "ids": [id.0]})).unwrap();
+    id
+}
+
+/// The canvas pixel at (x, 100), rendered as the canvas renders the document.
+fn canvas_pixel(s: &Session, x: u32) -> [u8; 4] {
+    let st = s.doc().unwrap();
+    let opts = RenderOptions { mask_view: st.shown_mask(), ..Default::default() };
+    Renderer::new().render(&st.doc, 300, 300, Affine::IDENTITY, &opts).pixel(x, 100)
+}
+
+#[test]
+fn view_opacity_mask_shows_its_luminance_and_hides_the_artwork() {
+    let mut s = Session::new();
+    s.execute("file.new", &json!({"width": 300, "height": 300})).unwrap();
+    let obj = rect(&mut s, 0.0, 200.0, "#ff0000");
+    let other = rect(&mut s, 220.0, 60.0, "#0000ff");
+    // Green mask art over the left half: luminance 0.7152.
+    let m = rect(&mut s, 0.0, 100.0, "#00ff00");
+    s.execute("select.set", &json!({"ids": [obj.0, m.0]})).unwrap();
+    s.execute("transparency.makeOpacityMask", &json!({})).unwrap();
+    assert_eq!(canvas_pixel(&s, 250)[..3], [0, 0, 255], "the artwork, before");
+    let undo = s.doc().unwrap().history.undo.len();
+    // Viewing enters mask editing (one undo step) and shows only the mask.
+    let r = s.execute("transparency.viewOpacityMask", &json!({})).unwrap();
+    assert_eq!(r, json!({"on": true, "id": obj.0}));
+    let st = s.doc().unwrap();
+    assert_eq!((st.doc.mask_edit.map(|e| e.object), st.shown_mask()), (Some(obj), Some(obj)));
+    assert_eq!(st.history.undo.len(), undo + 1);
+    assert_eq!(canvas_pixel(&s, 50), [182, 182, 182, 255], "the mask art's luminance");
+    assert_eq!(canvas_pixel(&s, 150), [0, 0, 0, 255], "clipping: black outside the mask art (the red object is hidden)");
+    assert_eq!(canvas_pixel(&s, 250), [0, 0, 0, 255], "other art is hidden");
+    // The view follows the mask's options and edits of its art live.
+    s.execute("transparency.setOpacityMask", &json!({"invert": true, "clip": false})).unwrap();
+    assert_eq!(canvas_pixel(&s, 50), [73, 73, 73, 255]);
+    assert_eq!(canvas_pixel(&s, 250), [0, 0, 0, 255], "not clipping, inverted: white outside, inverted");
+    s.execute("transparency.setOpacityMask", &json!({"invert": false, "clip": true})).unwrap();
+    s.execute("object.move", &json!({"dx": 50, "dy": 0})).unwrap();
+    assert_eq!(canvas_pixel(&s, 120), [182, 182, 182, 255]);
+    // Toggling again shows the artwork, still editing the mask.
+    assert_eq!(s.execute("transparency.viewOpacityMask", &json!({})).unwrap(), json!({"on": false, "id": obj.0}));
+    let st = s.doc().unwrap();
+    assert!(st.doc.mask_edit.is_some() && st.shown_mask().is_none());
+    assert_eq!(canvas_pixel(&s, 250)[..3], [0, 0, 255]);
+    // Leaving editing ends the view; so does undoing back out of editing.
+    s.execute("transparency.viewOpacityMask", &json!({"on": true})).unwrap();
+    s.execute("transparency.stopEditingOpacityMask", &json!({})).unwrap();
+    assert_eq!(s.doc().unwrap().mask_view, None);
+    s.execute("transparency.viewOpacityMask", &json!({"id": obj.0})).unwrap();
+    assert!(s.doc().unwrap().shown_mask().is_some());
+    s.execute("edit.undo", &json!({})).unwrap();
+    let st = s.doc().unwrap();
+    assert!(st.doc.mask_edit.is_none() && st.mask_view.is_none());
+    assert_eq!(canvas_pixel(&s, 250)[..3], [0, 0, 255]);
+    // Nothing to view: an error, and the view state is untouched.
+    s.execute("select.set", &json!({"ids": [other.0]})).unwrap();
+    assert!(s.execute("transparency.viewOpacityMask", &json!({})).is_err());
+    assert!(s.execute("transparency.viewOpacityMask", &json!({"on": true, "id": other.0})).is_err());
+    assert!(s.doc().unwrap().mask_view.is_none());
+}

crates/render/src/lib.rs

@@ -82,6 +82,9 @@ pub struct RenderOptions {
     /// Pattern editing mode's tile edge and swatch bounds colour (Object → Pattern → Tile Edge
     /// Color), RGB.
     pub tile_edge: [u8; 3],
+    /// View Opacity Mask (Alt-click the mask thumbnail): instead of the artwork, show this object's
+    /// opacity mask alone as its coverage in greyscale (white = opaque, black = transparent).
+    pub mask_view: Option<NodeId>,
 }
 
 impl Default for RenderOptions {
@@ -97,6 +100,7 @@ impl Default for RenderOptions {
             trim: false,
             skip_templates: false,
             tile_edge: vectorcraft_doc::LAYER_COLORS[0].1,
+            mask_view: None,
         }
     }
 }
@@ -323,6 +327,18 @@ impl Renderer {
         // Raster filters (drop shadow, glows, blur) are rendered offscreen on this thread when the
         // main context is multithreaded (see `fx::with_filters`).
         let threads = self.threads;
+        self.stats = FrameStats::default();
+        let inv = view.inverse();
+        let visible = inv.transform_rect_bbox(Rect::new(0.0, 0.0, w as f64, h as f64));
+        let px = 1.0 / view.determinant().abs().sqrt().max(1e-12);
+        let frame = Frame { mt: threads > 0, doc, view, visible, px, opts };
+        // View Opacity Mask: the mask's coverage in greyscale instead of the artwork.
+        if let Some(m) = opts.mask_view.and_then(|id| doc.node(id)).and_then(|n| n.mask.as_deref()) {
+            self.stamp += 1;
+            let pixels = self.mask_values(&frame, m, w, h).into_iter().flat_map(|v| [v, v, v, 255]).collect();
+            self.stats.micros = now().saturating_sub(start);
+            return Rendered { width: w as u32, height: h as u32, pixels };
+        }
         let slot = if threads == 0 { self.ctx_st.take() } else { self.ctx.take() };
         let mut ctx = match slot {
             Some(mut c) if c.width() == w && c.height() == h => {
@@ -331,12 +347,6 @@ impl Renderer {
             }
             _ => RenderContext::new_with(w, h, vello_cpu::RenderSettings { num_threads: threads, ..Default::default() }),
         };
-        self.stats = FrameStats::default();
-        let inv = view.inverse();
-        let visible = inv.transform_rect_bbox(Rect::new(0.0, 0.0, w as f64, h as f64));
-        let px = 1.0 / view.determinant().abs().sqrt().max(1e-12);
-        let frame = Frame { mt: threads > 0, doc, view, visible, px, opts };
-
         if let Some(bg) = opts.background {
             ctx.set_transform(Affine::IDENTITY);
             ctx.set_paint(peniko::Color::from_rgba8(bg[0], bg[1], bg[2], bg[3]));
@@ -548,6 +558,12 @@ impl Renderer {
 
     /// Render an opacity mask's art offscreen and turn its luminance into a coverage mask.
     fn opacity_mask(&mut self, f: &Frame, m: &vectorcraft_doc::OpacityMask, w: u16, h: u16) -> vello_cpu::Mask {
+        vello_cpu::Mask::from_parts(self.mask_values(f, m, w, h), w, h)
+    }
+
+    /// The coverage of opacity mask `m` per output pixel (row-major): its art rendered offscreen,
+    /// as luminance ([`mask_value`]).
+    fn mask_values(&mut self, f: &Frame, m: &vectorcraft_doc::OpacityMask, w: u16, h: u16) -> Vec<u8> {
         let mut mctx = single_threaded_context(w, h);
         // Mask art is a picture of its own: it takes no part in a knockout group around the object.
         let outer =
@@ -557,8 +573,7 @@ impl Renderer {
         mctx.flush();
         let mut pm = Pixmap::new(w, h);
         mctx.render(&mut pm, &mut self.resources);
-        let data = pm.data().iter().map(|p| mask_value(p.r, p.g, p.b, p.a, m.clip, m.invert)).collect();
-        vello_cpu::Mask::from_parts(data, w, h)
+        pm.data().iter().map(|p| mask_value(p.r, p.g, p.b, p.a, m.clip, m.invert)).collect()
     }
 
     /// Everything [`Self::draw_arc`] does after culling.

crates/ui-egui/src/canvas.rs

@@ -145,6 +145,7 @@ pub fn show(app: &mut VectorcraftApp, ui: &mut Ui) {
     let painter = ui.painter_at(rect);
     let Some(st) = app.session.active() else { return };
     let doc = st.doc.clone();
+    let mask_view = st.shown_mask();
 
     // Pasteboard, artboard shadows and paper.
     painter.rect_filled(rect, 0.0, t.pasteboard);
@@ -217,6 +218,7 @@ pub fn show(app: &mut VectorcraftApp, ui: &mut Ui) {
                 let [r, g, b, _] = c.to_rgba8(1.0);
                 [r, g, b]
             }),
+            mask_view,
             ..opts
         };
         // Light documents render synchronously (no lag vs overlays); heavy ones go to the worker.

crates/ui-egui/src/panels/mod.rs

@@ -439,6 +439,8 @@ mod tests_effectedit;
 #[cfg(test)]
 mod tests_freeform;
 #[cfg(test)]
+mod tests_maskview;
+#[cfg(test)]
 mod tests_stroke;
 #[cfg(test)]
 mod tests_strokedepth;

crates/ui-egui/src/panels/tests_maskview.rs

@@ -0,0 +1,47 @@
+//! Headless frames of the Transparency panel's mask thumbnail (M3.86): Alt-click views only the
+//! mask (and edits it), Alt-click again shows the artwork.
+
+use serde_json::json;
+use vectorcraft_engine::Session;
+
+use super::*;
+
+/// One frame of the Transparency panel with `events`, `modifiers` held.
+fn frame(ctx: &egui::Context, app: &mut VectorcraftApp, events: Vec<egui::Event>, modifiers: egui::Modifiers) {
+    let events = std::iter::once(egui::Event::ModifiersChanged(modifiers)).chain(events).collect();
+    let mut out = ctx.run_ui(egui::RawInput { events, ..Default::default() }, |ui| transparency::show(app, ui));
+    out.textures_delta.clear();
+}
+
+/// Click the mask thumbnail with `modifiers` held.
+fn click_mask(ctx: &egui::Context, app: &mut VectorcraftApp, modifiers: egui::Modifiers) {
+    frame(ctx, app, vec![], egui::Modifiers::NONE);
+    let at = ctx.read_response(egui::Id::new(transparency::MASK_THUMB)).expect("the mask thumbnail is drawn").rect.center();
+    let button = |pressed| egui::Event::PointerButton { pos: at, button: egui::PointerButton::Primary, pressed, modifiers };
+    frame(ctx, app, vec![egui::Event::PointerMoved(at), button(true), button(false)], modifiers);
+    frame(ctx, app, vec![], egui::Modifiers::NONE);
+}
+
+#[test]
+fn alt_clicking_the_mask_thumbnail_views_only_the_mask() {
+    let mut app = VectorcraftApp::new(Session::new(), Default::default());
+    let run = |app: &mut VectorcraftApp, id: &str, p: serde_json::Value| app.session.execute(id, &p).unwrap();
+    run(&mut app, "file.new", json!({"width": 100, "height": 100}));
+    let a = run(&mut app, "shape.rectangle", json!({"x": 0, "y": 0, "width": 50, "height": 50}))["id"].as_u64().unwrap();
+    let b = run(&mut app, "shape.rectangle", json!({"x": 10, "y": 10, "width": 20, "height": 20}))["id"].clone();
+    run(&mut app, "select.set", json!({"ids": [a, b]}));
+    run(&mut app, "transparency.makeOpacityMask", json!({}));
+    let ctx = egui::Context::default();
+    let viewed = |app: &VectorcraftApp| app.session.active().unwrap().shown_mask().map(|n| n.0);
+    click_mask(&ctx, &mut app, egui::Modifiers::ALT);
+    assert_eq!(viewed(&app), Some(a));
+    assert!(app.session.active().unwrap().doc.mask_edit.is_some(), "viewing the mask edits it");
+    click_mask(&ctx, &mut app, egui::Modifiers::ALT);
+    assert_eq!(viewed(&app), None);
+    assert!(app.session.active().unwrap().doc.mask_edit.is_some(), "still editing");
+    // A plain click while editing does nothing; Alt views again.
+    click_mask(&ctx, &mut app, egui::Modifiers::NONE);
+    assert_eq!(viewed(&app), None);
+    click_mask(&ctx, &mut app, egui::Modifiers::ALT);
+    assert_eq!(viewed(&app), Some(a));
+}

crates/ui-egui/src/panels/transparency.rs

@@ -1,7 +1,8 @@
 //! Transparency panel: blend mode, opacity (field + slider popup), object/mask thumbnails with the
-//! opacity-mask controls (make/release, link, clip, invert; Shift-click the mask to disable it),
-//! Isolate Blending, the three-state Knockout Group (on → neutral → off) and Opacity & Mask Define
-//! Knockout Shape; the menu's Page Isolated Blending and Page Knockout Group.
+//! opacity-mask controls (make/release, link, clip, invert; Shift-click the mask to disable it,
+//! Alt-click it to view only the mask), Isolate Blending, the three-state Knockout Group (on →
+//! neutral → off) and Opacity & Mask Define Knockout Shape; the menu's Page Isolated Blending and
+//! Page Knockout Group.
 
 use egui::{Sense, Stroke, StrokeKind, Ui, vec2};
 use serde_json::json;
@@ -14,6 +15,9 @@ use crate::theme::Tokens;
 use crate::widgets::{self, TransparencyEdit, menu_item};
 use crate::{VectorcraftApp, icons};
 
+/// Interaction id of the mask thumbnail.
+pub(super) const MASK_THUMB: &str = "tr-mask-thumb";
+
 /// The panel's values ([`vectorcraft_engine::Session::transparency_info`]) and its first target
 /// (the selection, or the object whose mask is being edited), recomputed only when the document
 /// or the selection changes.
@@ -100,25 +104,31 @@ pub fn show(app: &mut VectorcraftApp, ui: &mut Ui) {
                     if lresp.on_hover_text(if m.linked { "Unlink the mask" } else { "Link the mask" }).clicked() {
                         app.run("transparency.setOpacityMask", json!({"linked": !m.linked})).ok();
                     }
-                    let (mr, mresp) = ui.allocate_exact_size(vec2(50.0, 50.0), Sense::click());
+                    let (mr, _) = ui.allocate_exact_size(vec2(50.0, 50.0), Sense::hover());
+                    let mresp = ui.interact(mr, egui::Id::new(MASK_THUMB), Sense::click());
                     let bg = if m.clip != m.invert { [0, 0, 0, 255] } else { [255, 255, 255, 255] };
                     ui.painter().rect_filled(mr, 0.0, egui::Color32::from_rgb(bg[0], bg[1], bg[2]));
                     node_thumb(app, ui, "tr-mask", &m.art, mr.shrink(2.0), Some(bg));
-                    ui.painter().rect_stroke(
-                        mr,
-                        0.0,
-                        Stroke::new(mask_w, if editing.is_some() { t.border } else { t.input_border }),
-                        StrokeKind::Inside,
-                    );
+                    // Viewing the mask alone (Alt-click) highlights its thumbnail.
+                    let viewing = app.session.active().is_some_and(|d| d.shown_mask().is_some());
+                    let border = match (viewing, editing.is_some()) {
+                        (true, _) => Stroke::new(2.0, t.accent),
+                        (false, true) => Stroke::new(mask_w, t.border),
+                        (false, false) => Stroke::new(mask_w, t.input_border),
+                    };
+                    ui.painter().rect_stroke(mr, 0.0, border, StrokeKind::Inside);
                     if m.disabled {
                         let red = Stroke::new(2.0, egui::Color32::from_rgb(220, 40, 40));
                         ui.painter().line_segment([mr.left_top(), mr.right_bottom()], red);
                         ui.painter().line_segment([mr.right_top(), mr.left_bottom()], red);
                     }
-                    let shift = ui.input(|i| i.modifiers.shift);
-                    if mresp.on_hover_text("Click to edit the mask; Shift-click to disable or enable it").clicked() {
+                    let (shift, alt) = ui.input(|i| (i.modifiers.shift, i.modifiers.alt));
+                    let tip = "Click to edit the mask; Alt-click to view only the mask; Shift-click to disable or enable it";
+                    if mresp.on_hover_text(tip).clicked() {
                         if shift {
                             app.run(if m.disabled { "transparency.enableOpacityMask" } else { "transparency.disableOpacityMask" }, json!({})).ok();
+                        } else if alt {
+                            app.run("transparency.viewOpacityMask", json!({"id": id.0})).ok();
                         } else if editing.is_none() {
                             app.run("transparency.editOpacityMask", json!({"id": id.0})).ok();
                         }

docs/mcp.md

@@ -225,6 +225,11 @@ Groups are not isolated unless Isolate Blending is on: a blend mode inside a gro
 opacity mask, knockout or a clip reaches the art below the group, on screen and in PDF (a knockout group's elements
 then composite against the art below it). SVG has no non-isolated groups, so there such groups stay isolated.
 
+`transparency.viewOpacityMask {on?, id?}` is Alt-clicking the mask thumbnail: the canvas shows only the mask of `id`
+(default: the mask being edited, else the first selected masked object) as greyscale coverage (white = opaque) and the
+mask is edited; called again it shows the artwork, still editing. It is view state of the open document: the canvas
+(and `ui.screenshot`) shows it, exports never do, and it ends when mask editing ends.
+
 ## Clipping masks
 
 `object.clippingMask.make` clips the selected objects by the topmost one, which may be a path, a compound path or a