Show a mask's tint only while masking

The eyes are per layer and outlive the mode, so a photographer coming
back finds the layers they were looking at still lit. But the tint is a
way of looking at a mask, and outside Local there is no mask being looked
at: the sky stayed red through Repair and back in Photo, a mode that had
been left leaving its overlay behind — the fault ui-navigation.md D-N1
exists to prevent.

ui/dr-ui/src/develop.rs

@@ -2552,6 +2552,15 @@ impl DevelopSession {
     /// or reordered.
     pub(crate) fn reveal(&self) -> Option<dr_pipeline::mask::Reveal> {
         use dr_pipeline::mask::{Reveal, RevealedLayer};
+        // Only while masking. The eyes are per layer and outlive the mode,
+        // so a photographer coming back finds the layers they were looking
+        // at still lit — but a tint is a way of looking at a *mask*, and
+        // outside Local there is no mask being looked at. Without this the
+        // sky stayed red through Repair and back in Photo, a mode that had
+        // been left leaving its overlay behind (ui-navigation.md D-N1).
+        if !self.show_overlay {
+            return None;
+        }
         let layers: Vec<RevealedLayer> = self
             .graph
             .masks()
@@ -6986,6 +6995,27 @@ mod tests {
     /// — that the coverage is present, that it round-trips as bytes — would
     /// still pass if the raster came back at the wrong scale, upside down, or
     /// a threshold out.
+    #[test]
+    fn a_shown_mask_is_only_shown_while_masking() {
+        let Some(ctx) = headless() else { return };
+        let mut s = session_with_a_left_half_subject(&ctx);
+        let id = s.add_subject_mask(0).expect("a subject layer");
+        s.set_overlay(true);
+        s.set_mask_shown(&id, true);
+        assert!(s.any_mask_shown(), "lit, in Local mode");
+
+        // Leaving the mode — what `on_mode_picked` does for Photo and Spots.
+        s.set_overlay(false);
+        assert!(
+            !s.any_mask_shown(),
+            "the tint belongs to the mode, not to the photograph"
+        );
+        assert!(s.mask_shown(&id), "the eye itself is remembered");
+
+        s.set_overlay(true);
+        assert!(s.any_mask_shown(), "and is lit again on return");
+    }
+
     #[test]
     fn a_stored_mask_renders_exactly_what_the_model_rendered() {
         let Some(ctx) = headless() else { return };