feat: inline MSPACE viewport overlay on 2D paper canvas

- viewport_screen_rect now mirrors PaperCanvas::draw()'s camera-based
  to_px transform instead of using a fixed paper-limits scale, so the
  3D overlay lands exactly over the drawn viewport border at any zoom/pan

- blue border frame (stack layer above the shader) makes the active
  viewport boundary always visible even when it fills the canvas

- space-mode pill in MSPACE now dispatches LayoutSwitch("Model") so
  clicking it jumps to full Model Space instead of just exiting to PSPACE

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/view.rs

@@ -481,10 +481,11 @@ fn paper_canvas_view<'a>(tab: &'a super::document::DocumentTab) -> Element<'a, M
     if let Some(vp_handle) = scene.active_viewport {
         let (canvas_w, canvas_h) = scene.selection.borrow().vp_size;
         if let Some(rect) = scene.viewport_screen_rect(vp_handle, (canvas_w, canvas_h)) {
-            let w = rect.width.max(1.0);
-            let h = rect.height.max(1.0);
-            let x = rect.x.max(0.0);
-            let y = rect.y.max(0.0);
+            // Clamp to canvas bounds so Space widgets never get negative size.
+            let x = rect.x.max(0.0).min(canvas_w);
+            let y = rect.y.max(0.0).min(canvas_h);
+            let w = rect.width.clamp(1.0, canvas_w - x);
+            let h = rect.height.clamp(1.0, canvas_h - y);
 
             let vp_widget = shader(PaperViewportPane::new(scene, vp_handle))
                 .width(iced::Length::Fixed(w))
@@ -500,7 +501,34 @@ fn paper_canvas_view<'a>(tab: &'a super::document::DocumentTab) -> Element<'a, M
             .width(Fill)
             .height(Fill);
 
-            return stack![paper_sheet, positioned]
+            // Blue border drawn on top of the 3-D overlay so the viewport
+            // boundary is always visible even when the shader fills the area.
+            const VP_BORDER: Color = Color { r: 0.18, g: 0.52, b: 0.95, a: 1.0 };
+            let border_frame = container(
+                Space::new()
+                    .width(iced::Length::Fixed(w))
+                    .height(iced::Length::Fixed(h)),
+            )
+            .style(move |_: &Theme| container::Style {
+                border: iced::Border {
+                    color: VP_BORDER,
+                    width: 2.0,
+                    radius: 0.0.into(),
+                },
+                ..Default::default()
+            });
+
+            let border_layer = column![
+                Space::new().height(iced::Length::Fixed(y)),
+                row![
+                    Space::new().width(iced::Length::Fixed(x)),
+                    border_frame,
+                ],
+            ]
+            .width(Fill)
+            .height(Fill);
+
+            return stack![paper_sheet, positioned, border_layer]
                 .width(Fill)
                 .height(Fill)
                 .into();

src/scene/mod.rs

@@ -2169,45 +2169,51 @@ impl Scene {
     /// Convert a paper-space Viewport entity's position/size into a pixel
     /// `Rectangle` relative to the top-left of the paper canvas.
     ///
-    /// `canvas_px`  — the pixel dimensions of the area that shows the paper.
-    /// Map a paper-space viewport into screen-pixel coordinates.
-    /// Used to position the active-viewport widget in MSPACE paper space.
+    /// Uses the same camera-based ortho transform as `PaperCanvas::draw()` so
+    /// that the overlay lands exactly over the drawn viewport border regardless
+    /// of zoom or pan level.
     pub fn viewport_screen_rect(
         &self,
         vp_handle: Handle,
         canvas_px: (f32, f32),
     ) -> Option<iced::Rectangle> {
-        let ((px0, py0), (px1, py1)) = self.paper_limits()?;
-        let paper_w = (px1 - px0) as f32;
-        let paper_h = (py1 - py0) as f32;
-        if paper_w < 1e-6 || paper_h < 1e-6 {
-            return None;
-        }
-
         let vp = match self.document.get_entity(vp_handle) {
             Some(EntityType::Viewport(vp)) => vp,
             _ => return None,
         };
 
         let (canvas_w, canvas_h) = canvas_px;
-        let sx = canvas_w / paper_w;
-        let sy = canvas_h / paper_h;
+        if canvas_w < 1.0 || canvas_h < 1.0 {
+            return None;
+        }
+
+        let cam = self.camera.borrow();
+        let aspect = canvas_w / canvas_h;
+        let half_h = cam.ortho_size();
+        let half_w = half_h * aspect;
+        let tx = cam.target.x;
+        let ty = cam.target.y;
+        drop(cam);
+
+        // Mirror the to_px closure in PaperCanvas::draw().
+        let to_px = |wx: f32, wy: f32| -> (f32, f32) {
+            let x = (wx - tx + half_w) / (2.0 * half_w) * canvas_w;
+            let y = (ty + half_h - wy) / (2.0 * half_h) * canvas_h;
+            (x, y)
+        };
 
         let cx = vp.center.x as f32;
         let cy = vp.center.y as f32;
         let hw = (vp.width / 2.0) as f32;
         let hh = (vp.height / 2.0) as f32;
 
-        // Map paper coords (origin = paper min, Y up) → screen (Y down).
-        let screen_x = (cx - hw - px0 as f32) * sx;
-        let screen_y = (py1 as f32 - (cy + hh)) * sy;
+        let (x0, y0) = to_px(cx - hw, cy + hh); // top-left in screen
+        let (x1, y1) = to_px(cx + hw, cy - hh); // bottom-right in screen
 
-        Some(iced::Rectangle {
-            x: screen_x,
-            y: screen_y,
-            width: vp.width as f32 * sx,
-            height: vp.height as f32 * sy,
-        })
+        let w = (x1 - x0).max(1.0);
+        let h = (y1 - y0).max(1.0);
+
+        Some(iced::Rectangle { x: x0, y: y0, width: w, height: h })
     }
 
     // ── ViewportPane helpers ──────────────────────────────────────────────

src/ui/statusbar.rs

@@ -92,7 +92,7 @@ impl StatusBar {
             osnap_btn(osnap_active, snapper.snap_enabled, popup_open),
             tip(
                 space_mode_btn(&current_layout, in_mspace),
-                "Toggle Model/Paper space\nDouble-click viewport to enter MSPACE",
+                "PAPER: double-click viewport to enter MSPACE\nMODEL: click to switch to Model Space",
             ),
             status_pill(scale_label),
         ]
@@ -460,9 +460,9 @@ fn space_mode_btn(current_layout: &str, in_mspace: bool) -> Element<'static, Mes
     //   PAPER = currently in paper-space editing
     //   MODEL = currently in model-space editing (either the Model tab or MSPACE)
     let (label, active, on_press) = if is_model_tab {
-        ("MODEL", false, None)
+        ("MODEL", false, None::<Message>)
     } else if in_mspace {
-        ("MODEL", true, Some(Message::ExitViewport))
+        ("MODEL", true, Some(Message::LayoutSwitch("Model".to_string())))
     } else {
         ("PAPER", false, Some(Message::MspaceCommand))
     };