Draw the view orthographic while a sketch is open (#1758)

Thanks @pablopeza.

crates/ui-egui/src/lib.rs

@@ -976,6 +976,12 @@ impl SolveApp {
         self.dialog.as_ref().is_some_and(|d| matches!(d.kind, dialogs::Kind::Sketch))
     }
 
+    /// Is the view drawn in perspective? The perspective preference, except while a sketch is
+    /// open: a sketch is drawn and measured on its plane, so the view is parallel (no foreshortening).
+    pub fn perspective_view(&self) -> bool {
+        self.ui.perspective && self.session.active_sketch.is_none()
+    }
+
     pub fn set_status(&mut self, s: impl Into<String>, error: bool) {
         self.status = Some((s.into(), now_ms(), error));
     }
@@ -1077,7 +1083,7 @@ impl SolveApp {
 
     fn fitted_bounds(&self, mut cam: Camera, bounds: &solidcraft_engine::geom::Aabb3) -> Camera {
         if let Some(layout) = &self.viewport.layout {
-            return viewport_layout::fit_bounds(cam, bounds, layout, self.ui.perspective).unwrap_or(cam);
+            return viewport_layout::fit_bounds(cam, bounds, layout, self.perspective_view()).unwrap_or(cam);
         }
         let previous = cam;
         cam.fit(bounds);

crates/ui-egui/src/viewport.rs

@@ -267,7 +267,7 @@ pub fn scene_radius(app: &SolveApp) -> f64 {
 
 pub fn projection(app: &SolveApp, rect: Rect) -> Proj {
     let mut cam = app.cam;
-    cam.fov = if app.ui.perspective { 0.6 } else { 0.0 };
+    cam.fov = if app.perspective_view() { 0.6 } else { 0.0 };
     let aspect = rect.width() as f64 / rect.height().max(1.0) as f64;
     Proj { cam, vp: cam.view_proj(aspect, scene_radius(app)), rect }
 }
@@ -1102,7 +1102,7 @@ pub(crate) fn pick_cached(app: &mut SolveApp, proj: &Proj, pos: Pos2) -> Vec<Hit
     app.dialog.as_ref().and_then(|d| d.active_input()).map(|i| i.accept).hash(&mut h);
     app.session.visibility.hidden_bodies.hash(&mut h);
     app.session.visibility.hidden_occurrences.hash(&mut h);
-    app.ui.perspective.hash(&mut h);
+    app.perspective_view().hash(&mut h);
     (proj.rect.width().to_bits(), proj.rect.height().to_bits()).hash(&mut h);
     crate::sketch_display::projection_key(proj, app.viewport.pixels_per_point()).hash(&mut h);
     let key = h.finish();
@@ -5950,6 +5950,22 @@ mod grid_plane_tests {
         }
     }
 
+    /// A sketch is drawn in a parallel view even when the preference asks for perspective; the
+    /// preference comes back when the sketch is finished.
+    #[test]
+    fn an_open_sketch_is_drawn_orthographic() {
+        let rect = Rect::from_min_size(Pos2::ZERO, egui::vec2(800.0, 600.0));
+        let mut app = SolveApp::new(solidcraft_engine::Session::default(), crate::Services::default());
+        app.ui.perspective = true;
+        assert!(projection(&app, rect).cam.fov > 0.0);
+        app.run("sketch.create", json!({"plane": "XZ"})).unwrap();
+        assert!(!app.perspective_view());
+        assert_eq!(projection(&app, rect).cam.fov, 0.0);
+        app.finish_sketch();
+        assert!(app.ui.perspective, "the preference is left alone");
+        assert!(projection(&app, rect).cam.fov > 0.0);
+    }
+
     /// Create Sketch shows the Origin even when the browser hides it, and draws its planes
     /// through the model so a plane inside a body can be seen and picked; closing it restores
     /// the hidden, depth-tested Origin.