Offer intersection in the mask panel: an Intersect button and a third chip state

The pipeline could now keep only where two selections agree, but the panel
had no way to ask for it: the part row's chip flipped between + and -, and
the buttons under the parts joined an added or a subtracted correction.

An "∩ Intersect" button joins a painted part that intersects, and the chip
on a part row cycles + -> - -> ∩ and round, so an existing part can be
turned into an intersection without being repainted. Both go through the
same session calls as before, indexing Join::ALL, whose first two entries
kept their places. The chip is now a tagged gesture, so it is in the
gesture book.

ui/dr-ui/src/develop/masks.rs

@@ -543,7 +543,9 @@ impl DevelopSession {
             .record(&self.graph, Edit::Action(labels::step::MASK_PART_REMOVED));
     }
 
-    /// Change how a part joins: added to the mask, or taken out of it.
+    /// TRACES: FR-DEV-19a
+    /// Change how a part joins: added to the mask, taken out of it, or kept
+    /// only where the mask already was. `join` indexes `Join::ALL`.
     pub fn set_mask_part_join(&mut self, id: &str, index: usize, join: usize) {
         use dr_pipeline::mask::Join;
         let Some(&join) = Join::ALL.get(join) else {

ui/dr-ui/src/masks_ui.rs

@@ -1534,6 +1534,31 @@ mod tests {
         assert_eq!(s.mask_feather(&id), 0.03, "and the base was left alone");
     }
 
+    /// TRACES: FR-DEV-19a
+    /// The panel's third join reaches the layer: the "∩ Intersect" button
+    /// joins an intersecting part, the part row reports it as the chip's third
+    /// state, and cycling the chip on from there comes back round to a union.
+    #[test]
+    fn the_intersect_button_joins_a_part_that_intersects() {
+        use dr_pipeline::mask::Join;
+        let Some(mut s) = session() else {
+            eprintln!("no adapter; skipping");
+            return;
+        };
+        let id = s.add_gradient_mask(true).expect("radial");
+        s.set_active_mask(Some(&id));
+
+        let index = s.add_mask_part(&id, 2).expect("a part was joined");
+        let join = |s: &DevelopSession| {
+            s.masks().get(&id).expect("layer").part(index).expect("part").join
+        };
+        assert_eq!(join(&s), Join::Intersect);
+        assert_eq!(s.mask_parts(&id)[index].2, 2, "the chip shows ∩");
+
+        s.set_mask_part_join(&id, index, (2 + 1) % 3);
+        assert_eq!(join(&s), Join::Union, "and the chip cycles back to +");
+    }
+
     /// TRACES: FR-DEV-3 | FR-UI-1
     /// The fault this pass exists for, stated as a test.
     ///

ui/dr-ui/ui/masks.slint

@@ -89,7 +89,8 @@ export struct PartRow {
     id: string,
     /// What this part selects — "brush", "subject", "radial".
     label: string,
-    /// 0 adds to the mask, 1 takes away from it. Meaningless on the first.
+    /// 0 adds to the mask, 1 takes away from it, 2 keeps only where both
+    /// are (the order of `Join::ALL`). Meaningless on the first.
     join: int,
     /// Whether the brush and the edge controls point at this part.
     selected: bool,
@@ -178,7 +179,7 @@ component MaskEntry inherits Rectangle {
     callback part-join-picked(int, int);
     callback part-hidden-toggled(int, bool);
     callback part-removed(int);
-    /// Join a fresh painted correction: 0 adds, 1 subtracts.
+    /// Join a fresh painted correction: 0 adds, 1 subtracts, 2 intersects.
     callback part-added(int);
     /// TRACES: FR-DEV-19c
     callback shown-toggled(bool);
@@ -362,20 +363,30 @@ component MaskEntry inherits Rectangle {
                     padding-right: Theme.gap-sm;
                     spacing: Theme.gap-sm;
 
-                    // One chip, two states, because there are two ways to
-                    // join and a photographer flips between them while
-                    // looking at the picture rather than at a menu.
+                    // GESTURE: Change how a part joins its mask
+                    //   where:   Develop
+                    //   touch:   Tap the + / − / ∩ chip on the part's row
+                    //   pointer: Click the + / − / ∩ chip on the part's row
+                    //   why:     A chip that cycles rather than a menu, because
+                    //            a photographer flips a join while looking at
+                    //            the picture, not at the panel: add, take
+                    //            away, keep only where both agree, and round.
+                    //
+                    // TRACES: FR-DEV-19a
                     join := TouchArea {
                         width: Theme.touch-target;
                         mouse-cursor: part.base ? MouseCursor.default : MouseCursor.pointer;
                         enabled: root.enabled && !part.base;
-                        clicked => { root.part-join-picked(i, part.join == 0 ? 1 : 0); }
+                        clicked => { root.part-join-picked(i, Math.mod(part.join + 1, 3)); }
 
                         Label {
                             // The first part joins nothing — a chip saying
                             // "adds" on it would be a control that moves and
                             // changes no pixel.
-                            text: part.base ? "◆" : (part.join == 0 ? "+" : "−");
+                            text: part.base ? "◆"
+                                : part.join == 0 ? "+"
+                                : part.join == 1 ? "−"
+                                : "∩";
                             emphasised: join.has-hover || part.selected;
                             horizontal-alignment: center;
                             vertical-alignment: center;
@@ -448,6 +459,16 @@ component MaskEntry inherits Rectangle {
                     enabled: root.enabled;
                     clicked => { root.part-added(1); }
                 }
+
+                // TRACES: FR-DEV-19a
+                // Keep only what this part also covers: paint where the mask
+                // should survive and everything else falls away — the sky
+                // that is also bright, the subject that is also skin.
+                Button {
+                    text: "∩ Intersect";
+                    enabled: root.enabled;
+                    clicked => { root.part-added(2); }
+                }
             }
 
             // TRACES: FR-DEV-19b