View twist: review fixes

- Every engine twist (DVIEW, view.set, opening a drawing, the untiled view) goes through norm_angle, so a tiny negative twist is 0, not a full turn.
- VIEW Window in a twisted view stores the window's size along the view's axes (View::twisted_box), so a restore shows all of it.
- A deferred tangent or perpendicular snap on an arc in a twisted layout viewport turns the arc's angles with its centre.
- The active viewport's cached drawing is rebuilt when its twist changes; moving and scaling it can't turn it.

crates/engine/src/cmd/view.rs

@@ -293,7 +293,7 @@ fn run_view_set(s: &mut Session, p: &Value) -> Result<Value> {
             dview::set_twist(s, t, twist)?;
             s.state()?.view().twist
         }
-        Some(dview::Target::Model) => twist.rem_euclid(std::f64::consts::TAU),
+        Some(dview::Target::Model) => cadcraft_geom::norm_angle(twist),
         None => 0.0,
     };
     if !s.state_mut()?.set_view(View { center: c, height: h, twist }) {

crates/engine/src/cmd/view/dview.rs

@@ -13,7 +13,6 @@
 //! ZOOM (view history, not undo). A layout viewport's twist belongs to the viewport entity, so a
 //! DVIEW that changed it is one undo step.
 
-use std::f64::consts::TAU;
 use std::sync::Arc;
 
 use cadcraft_doc::{Entity, EntityKind, Handle, Insert, Line, Space};
@@ -69,7 +68,7 @@ pub(super) fn target(s: &Session) -> Result<Target> {
 
 /// A twist kept as files hold it: [0, 2π).
 fn normal(twist: f64) -> f64 {
-    twist.rem_euclid(TAU)
+    cadcraft_geom::norm_angle(twist)
 }
 
 /// The active viewport's twist, if it is `h`.

crates/engine/src/cmd/view/named_views.rs

@@ -119,7 +119,9 @@ fn save(s: &mut Session, name: &str, window: Option<Bounds2>, snapshot: bool) ->
             if !(b.width() > 1e-12 && b.height() > 1e-12 && b.min.is_finite() && b.max.is_finite()) {
                 return Err(EngineError::Other("The window has no area.".into()));
             }
-            (b.center(), b.height(), b.width())
+            // In a twisted view the window's box on screen is larger than its world box.
+            let (center, width, height) = crate::View::twisted_box(b, twist);
+            (center, height, width)
         }
         None => {
             let f = frame;
@@ -184,7 +186,7 @@ fn restore(s: &mut Session, name: &str) -> Result<Value> {
             // In an unlocked MSPACE viewport the saved twist turns the viewport (an edit of the
             // viewport, recorded as the command's undo step).
             let aspect = st.zoom_aspect(w / h.max(1.0)).max(1e-9);
-            let twist = if v.twist.is_finite() { v.twist.rem_euclid(std::f64::consts::TAU) } else { 0.0 };
+            let twist = cadcraft_geom::norm_angle(v.twist);
             st.set_zoom_frame(View { center: v.center, height: v.height.max(v.width / aspect), twist });
         }
     }

crates/engine/src/doc_state.rs

@@ -203,7 +203,7 @@ impl DocState {
     }
     fn keep_untiled_twist(&mut self, v: View) {
         use cadcraft_doc::{VportConfig, VportTile};
-        let twist = if v.twist.is_finite() { v.twist.rem_euclid(std::f64::consts::TAU) } else { 0.0 };
+        let twist = cadcraft_geom::norm_angle(v.twist);
         let active = self.doc.vports.iter().find(|c| c.is_active());
         if active.is_some_and(|c| c.tiles.len() > 1) {
             return;

crates/engine/src/session/files.rs

@@ -19,9 +19,7 @@ impl Session {
         let mut st = DocState::new(d, title, path);
         let ext = st.doc.extents(&Space::Model);
         // The current tile (the first), or the lone untiled view, keeps its saved twist.
-        let twist =
-            st.doc.vports.iter().find(|c| c.is_active()).and_then(|c| c.tiles.first()).map_or(0.0, |t| t.twist).rem_euclid(std::f64::consts::TAU);
-        let twist = if twist.is_finite() { twist } else { 0.0 };
+        let twist = cadcraft_geom::norm_angle(st.doc.vports.iter().find(|c| c.is_active()).and_then(|c| c.tiles.first()).map_or(0.0, |t| t.twist));
         if ext.is_empty() {
             st.set_view_quiet(View { twist, ..st.view() });
         } else {

crates/engine/src/snap/viewport.rs

@@ -77,7 +77,9 @@ impl<'a> Projection<'a> {
                     if !(center.is_finite() && radius.is_finite() && radius > 0.0) {
                         return None;
                     }
-                    SnapCurve::Arc { center, radius, start, end }
+                    // The arc turns on the sheet with the viewport's twist.
+                    let twist = self.viewport.view_twist();
+                    SnapCurve::Arc { center, radius, start: start + twist, end: end + twist }
                 }
                 SnapCurve::Line { a, b } => {
                     let (a, b) = (self.to_paper(a), self.to_paper(b));

crates/ui-egui/src/canvas/viewport.rs

@@ -35,6 +35,8 @@ pub(super) struct VpContent {
     pub(super) key: Option<(u64, u64, usize, Handle, i32, bool, bool)>,
     /// The viewport's view (centre, height) the list was built for, and the paper area it covers.
     view: (Vec2, f64),
+    /// The view twist it was built for: [`vp_motion`] moves and scales a list, it can't turn it.
+    twist: f64,
     area: Bounds2,
     /// The view on the previous frame and when it last changed (ms).
     last_view: (Vec2, f64),
@@ -106,7 +108,7 @@ pub(super) fn ensure_vp_content(app: &mut CadApp, xf_paper: &Xf, ctx: &egui::Con
         c.last_view = view;
         c.moved_at = now;
     }
-    let rebuild = match vp_motion(&vp, c.view).filter(|_| c.key == Some(key)) {
+    let rebuild = match vp_motion(&vp, c.view).filter(|_| c.key == Some(key) && c.twist == vp.view_twist()) {
         Some((k, b)) => {
             // The viewport in the list's own coordinates.
             let visible = Bounds2::new((rect.min - b) / k, (rect.max - b) / k);
@@ -141,6 +143,7 @@ pub(super) fn ensure_vp_content(app: &mut CadApp, xf_paper: &Xf, ctx: &egui::Con
         c.list = cadcraft_render::viewport_content(&st.doc, &st.space, h, area, &opts);
         c.key = Some(key);
         c.view = view;
+        c.twist = vp.view_twist();
         c.area = area;
         c.builds = c.builds.wrapping_add(1);
         app.canvas.build_ms = crate::now_ms() - t0;