Show a mask as soon as it is made

Choosing a category is asking what it selected, and for a subject or a
category that question had no other answer on screen: the model's outline is
not derivable from anything visible, a fresh layer carries no adjustment to
judge it by, and the list it was chosen from says "architecture 23%" without
saying which 23%. The control that draws the mask existed but had to be found
and pressed, a panel's height away from the list the choice was made in.

So a new layer arrives with its mask showing, from the resting position only.
Somebody who has chosen the alpha or the outline keeps it, and nothing re-arms
in the background — every caller is a press that asked for a new mask.

That makes the canvas depend on how a layer arrived, which is correct and
worth stating: a session that has just made a mask draws a frame that a
session which read the same mask out of a sidecar does not. Viewing state is
not edit state and does not travel in a file, and
`a_stored_mask_renders_exactly_what_the_model_rendered` now says so at both
ends.

ui/dr-ui/src/develop.rs

@@ -2523,6 +2523,28 @@ impl DevelopSession {
             .and_then(|i| RevealStyle::ALL.get(i).copied());
     }
 
+    /// TRACES: FR-DEV-19c
+    /// Show the mask of a layer that has just been made.
+    ///
+    /// **Making a mask is asking what it selected**, and for a subject or a
+    /// category that question has no other answer: the model's outline is not
+    /// derivable from anything on screen, the layer carries no adjustment yet,
+    /// and the list it was chosen from says "architecture 23%" and nothing
+    /// about *which* 23%. So the mask appears with the layer rather than
+    /// waiting to be asked for a second time.
+    ///
+    /// Only from the resting position, and only on an explicit action — the
+    /// same rule arming a brush follows. Somebody who has switched to the
+    /// alpha or the outline keeps it, and this never re-arms in the background;
+    /// the trap `Masking.overlay-hidden` documents is an automatic reveal that
+    /// undoes a switch a photographer turned off, and every caller of this is
+    /// a press that asked for a new mask.
+    fn show_new_mask(&mut self) {
+        if self.reveal_style.is_none() {
+            self.reveal_style = Some(dr_pipeline::mask::RevealStyle::Tint);
+        }
+    }
+
     // ----------------------------------------------------------------------
     // The region overlay
     // ----------------------------------------------------------------------
@@ -3356,6 +3378,7 @@ impl DevelopSession {
             return None;
         }
         self.active_masks = vec![id.clone()];
+        self.show_new_mask();
         self.history
             .record(&self.graph, Edit::Action(labels::step::MASK_ADDED));
         Some(id)
@@ -3399,6 +3422,7 @@ impl DevelopSession {
             return None;
         }
         self.active_masks = vec![id.clone()];
+        self.show_new_mask();
         self.history
             .record(&self.graph, Edit::Action(labels::step::MASK_ADDED));
         Some(id)
@@ -3429,6 +3453,7 @@ impl DevelopSession {
             return None;
         }
         self.active_masks = vec![id.clone()];
+        self.show_new_mask();
         self.history
             .record(&self.graph, Edit::Action(labels::step::MASK_ADDED));
         Some(id)
@@ -3460,6 +3485,7 @@ impl DevelopSession {
         }
         self.active_masks = vec![id.clone()];
         self.active_part = 0;
+        self.show_new_mask();
         self.history
             .record(&self.graph, Edit::Action(labels::step::MASK_ADDED));
         Some(id)
@@ -3486,6 +3512,7 @@ impl DevelopSession {
             return None;
         }
         self.active_masks = vec![id.clone()];
+        self.show_new_mask();
         self.history
             .record(&self.graph, Edit::Action(labels::step::MASK_ADDED));
         Some(id)
@@ -6543,6 +6570,16 @@ mod tests {
 
         let mut live = session_with_a_left_half_subject(&ctx);
         let id = live.add_subject_mask(0).expect("a subject layer");
+        // TRACES: FR-DEV-19c
+        // Making a mask shows it (`show_new_mask`), and this test is about the
+        // pixels the *edit* produces. Put away here rather than left on, and
+        // the asymmetry is the point rather than an inconvenience: the reveal
+        // is how somebody is looking at a photograph, so a session that has
+        // just made a layer legitimately draws a frame that a session which
+        // read the same layer out of a file does not. Both of those are
+        // correct, and only one of them is what a stored raster has to
+        // reproduce.
+        live.set_mask_view(0);
         live.graph
             .masks_mut()
             .get_mut(&id)
@@ -6569,6 +6606,16 @@ mod tests {
             "the point: nothing has run a model in this session"
         );
         reopened.apply_version(parsed.default_version().expect("a version"));
+        // TRACES: FR-DEV-19c
+        // And a sidecar carries no viewing state: a photograph reopened is not
+        // reopened with its masks tinted red. Checked here because this is the
+        // one test that puts an edit through a file and renders both ends, so
+        // it is where the property would first go wrong.
+        assert_eq!(
+            reopened.mask_view(),
+            0,
+            "restoring an edit must not turn an overlay on"
+        );
         let from_the_file = read_back(&ctx, &reopened.render(64, 64).expect("render"));
 
         assert_eq!(