feat(paper): render the sheet as a top-locked GPU viewport

Paper space now renders its sheet through the unified shader, the same
path as model space: a full-canvas "sheet" ViewportInstance draws the
layout's own entities + viewport borders + interim/preview wires, with a
camera locked to the top/plan orientation (paper is 2-D and never
orbits). Floating content viewports overlay it.

- PaperCanvas no longer draws entity wires (still paints the desk, sheet
  and hatch/wipeout/image fills; full removal is the next step).
- Right-drag on the paper sheet no longer orbits the camera (it would
  corrupt the frame and skew subsequent pans).
- Fix floating-viewport orbit oscillation: orbit_active_viewport wrote
  view_direction.y negated while camera_for_viewport reads it un-negated,
  so each drag step read back a Y-mirrored camera and flipped the model
  between a rotation and its opposite every frame. Store eye directly,
  matching snap_active_viewport_to_direction and the reader.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/update.rs

@@ -1951,12 +1951,18 @@ impl OpenCADStudio {
                                 drop(sel);
                                 self.tabs[i].scene.orbit_active_viewport(dx, dy);
                                 return Task::none();
-                            } else {
+                            } else if self.tabs[i].scene.current_layout == "Model" {
                                 sel.right_last_pos = Some(p);
                                 drop(sel);
                                 self.tabs[i].scene.camera.borrow_mut().orbit(dx, dy);
                                 self.tabs[i].scene.camera_generation += 1;
                                 return Task::none();
+                            } else {
+                                // Paper sheet is top-locked: right-drag never
+                                // orbits it (orbiting would corrupt the camera
+                                // frame and skew subsequent pans).
+                                sel.right_last_pos = Some(p);
+                                return Task::none();
                             }
                         } else {
                             sel.right_last_pos = Some(p);

src/scene/mod.rs

@@ -436,6 +436,10 @@ pub struct ViewportInstance {
     pub render_mode: acadrust::entities::ViewportRenderMode,
     /// `true` when this is the viewport receiving cursor input.
     pub active: bool,
+    /// `true` for the full-canvas paper "sheet" viewport — the layout's own
+    /// view (paper-space entities, top-locked), the paper equivalent of the
+    /// Model view. Floating content viewports overlay it.
+    pub paper_sheet: bool,
 }
 
 /// One pane of the Model-space tiled viewport layout: the normalized screen
@@ -2691,16 +2695,18 @@ impl Scene {
         };
         cam.orbit(delta_x, delta_y);
         // yaw_pitch_to_quat(y,p)*Z = (cos(p)*sin(y), -cos(p)*cos(y), sin(p))
-        // but view_direction convention (matching snap_active_viewport_to_angles) is
+        // `camera_for_viewport` reconstructs the rotation so that
-        //   (cos(p)*sin(y), +cos(p)*cos(y), sin(p))  ← Y has opposite sign.
+        // `rotation * Z == view_direction` exactly (its `yaw = atan2(x, -y)`
-        // Negate Y when writing back so camera_for_viewport round-trips correctly.
+        // cancels the sign). Store `eye` directly so the orbit round-trips —
+        // negating Y here made each drag step read back a Y-mirrored camera,
+        // flipping the model between a rotation and its opposite every frame.
         let eye = cam.rotation * glam::Vec3::Z;
         if let Some(acadrust::EntityType::Viewport(vp)) = self.document.get_entity_mut(vp_handle) {
             if vp.status.locked {
                 return;
             }
             vp.view_direction.x = eye.x as f64;
-            vp.view_direction.y = -eye.y as f64;
+            vp.view_direction.y = eye.y as f64;
             vp.view_direction.z = eye.z as f64;
         }
     }
@@ -5699,12 +5705,37 @@ impl Scene {
                         camera,
                         render_mode: model_mode,
                         active: i == active,
+                        paper_sheet: false,
                     }
                 })
                 .collect();
         }
         let layout_block = self.current_layout_block_handle();
         let mut out: Vec<ViewportInstance> = Vec::new();
+        // The full-canvas sheet viewport renders the paper-space entities
+        // themselves — the layout's own view, drawn first so the floating
+        // content viewports overlay it. Its camera keeps the paper pan/zoom
+        // (target + ortho size) but is LOCKED to the top/plan orientation:
+        // paper is 2-D, so the sheet never orbits.
+        let mut sheet_cam = self.camera.borrow().clone();
+        sheet_cam.yaw = 0.0;
+        sheet_cam.pitch = std::f32::consts::FRAC_PI_2;
+        sheet_cam.rotation = camera::yaw_pitch_to_quat(0.0, std::f32::consts::FRAC_PI_2, 0.0);
+        sheet_cam.projection = camera::Projection::Orthographic;
+        out.push(ViewportInstance {
+            handle: Handle::NULL,
+            tile_idx: None,
+            screen_rect: iced::Rectangle {
+                x: 0.0,
+                y: 0.0,
+                width: canvas_w,
+                height: canvas_h,
+            },
+            camera: sheet_cam,
+            render_mode: acadrust::entities::ViewportRenderMode::Wireframe2D,
+            active: false,
+            paper_sheet: true,
+        });
         for e in self.document.entities() {
             let EntityType::Viewport(vp) = e else {
                 continue;
@@ -5728,6 +5759,7 @@ impl Scene {
                 camera,
                 render_mode: vp.render_mode,
                 active: self.active_viewport == Some(h),
+                paper_sheet: false,
             });
         }
         out
@@ -5796,6 +5828,7 @@ impl Scene {
     /// All wires needed to render the paper-space canvas (2D widget path).
     /// Includes paper entities, paper boundary, inactive viewport projections
     /// (excluding the active MSPACE viewport), plus interim/preview wires.
+    #[allow(dead_code)] // superseded by the GPU sheet viewport; removed in Stage 2.
     pub fn paper_canvas_wires(&self) -> Arc<Vec<WireModel>> {
         {
             let cache = self.paper_canvas_cache.borrow();
@@ -5919,6 +5952,7 @@ impl Scene {
         Arc::new(models)
     }
 
+    #[allow(dead_code)] // superseded by the GPU sheet viewport; removed in Stage 2.
     pub(super) fn paper_sheet_wires(&self) -> Vec<WireModel> {
         (*self.paper_sheet_wires_arc()).clone()
     }

src/scene/paper_canvas.rs

@@ -100,62 +100,9 @@ impl<'a> canvas::Program<Message> for PaperCanvas<'a> {
             draw_hatch(&mut frame, hatch, &to_px);
         }
 
-        // px-per-world-unit scale for linetype dash lengths.
+        // Entity lines, viewport borders and interim/preview wires are now
-        let world_to_px_scale = if half_w > 0.0 {
+        // rendered by the GPU shader's full-canvas paper "sheet" viewport, so
-            bounds.width / (2.0 * half_w)
+        // the 2-D canvas only paints the desk + sheet + fills beneath it.
-        } else {
-            1.0
-        };
-
-        // ── Wires (entity lines + inactive viewport projections) ──────────────
-        for wire in self.scene.paper_canvas_wires().iter() {
-            let [r, g, b, a] = wire.color;
-            let color = Color { r, g, b, a };
-
-            let path = canvas::Path::new(|b| {
-                let mut started = false;
-                for &[wx, wy, _] in &wire.points {
-                    if wx.is_nan() || wy.is_nan() {
-                        started = false;
-                        continue;
-                    }
-                    let p = to_px(wx, wy);
-                    if started {
-                        b.line_to(p);
-                    } else {
-                        b.move_to(p);
-                        started = true;
-                    }
-                }
-            });
-
-            // Convert WireModel linetype pattern (world units) to pixel lengths.
-            // Keep the Vec alive for the duration of frame.stroke().
-            let dash_segments: Vec<f32> = if wire.pattern_length > 0.0 {
-                wire.pattern
-                    .iter()
-                    .take_while(|&&v| v != 0.0)
-                    .map(|&v| v.abs() * world_to_px_scale)
-                    .collect()
-            } else {
-                vec![]
-            };
-
-            frame.stroke(
-                &path,
-                canvas::Stroke {
-                    style: canvas::Style::Solid(color),
-                    width: wire.line_weight_px.max(1.0),
-                    line_cap: canvas::LineCap::Square,
-                    line_join: canvas::LineJoin::Miter,
-                    line_dash: canvas::LineDash {
-                        segments: &dash_segments,
-                        offset: 0,
-                    },
-                },
-            );
-        }
-
         vec![frame.into_geometry()]
     }
 }

src/scene/render.rs

@@ -597,6 +597,11 @@ impl Scene {
                 1.0
             };
             self.model_tile_wires_arc(tile_idx, &inst.camera, aspect, full.height)
+        } else if inst.paper_sheet {
+            // The sheet renders the paper block's own entities + viewport
+            // borders — NOT the projected viewport content (the GPU content
+            // viewports draw that themselves).
+            self.paper_sheet_wires_arc()
         } else if inst.handle == acadrust::Handle::NULL {
             self.entity_wires_arc()
         } else {
@@ -648,13 +653,22 @@ impl Scene {
             height: visible_h / canvas.1,
         };
 
+        // Stage 1: the paper sheet instance contributes only entity wires; its
+        // hatch / wipeout / image fills are still drawn by PaperCanvas beneath
+        // the shader. (Stage 2 moves those into the shader and drops PaperCanvas.)
+        let (hatches, wipeout_hatches, images) = if inst.paper_sheet {
+            (Arc::new(Vec::new()), Arc::new(Vec::new()), Arc::new(Vec::new()))
+        } else {
+            (self.hatch_models_arc(), self.wipeout_models_arc(), self.images_arc())
+        };
+
         Some(ViewportData {
             wires: all_wires,
             face3d_wires: Arc::new(face3d_wires),
             draw_depths: self.draw_depth_map(),
-            hatches: self.hatch_models_arc(),
+            hatches,
-            wipeout_hatches: self.wipeout_models_arc(),
+            wipeout_hatches,
-            images: self.images_arc(),
+            images,
             meshes: self.meshes_arc(),
             uniforms,
             cam_rotation: inst.camera.view_rotation_mat(),