feat: introduce ViewportPane widget for unified model/paper rendering

- Remove Scene: shader::Program<Msg> impl; all GPU rendering now goes
  through ViewportPane which supports three modes:
    Model      — full model-space view (unchanged behaviour)
    PaperSheet — paper-space entities only, using the paper camera
    Paper      — model content through a specific viewport's own camera

- Add Scene helper methods (render.rs):
    build_primitive / build_paper_sheet_primitive / build_viewport_primitive
    update_viewcube_state / viewcube_mouse_interaction

- Add Scene helpers (mod.rs):
    paper_sheet_wires()        — paper entities without viewport projection
    camera_for_viewport()      — derive Camera from Viewport entity data
    model_wires_for_viewport() — model wires filtered by per-viewport frozen layers
    viewport_screen_rect()     — paper-space → pixel rect mapping

- Add paper_canvas_view() in view.rs:
    Stack of PaperSheet + one Paper widget per Viewport entity,
    positioned with Space offsets from viewport_screen_rect()

- Update VIEWPORT_WIDGET_PLAN.md: mark Steps 1–4 done, document remaining work

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

src/app/view.rs

@@ -4,6 +4,7 @@ use super::history::history_dropdown_labels;
 use super::helpers::grid_plane_from_camera;
 use crate::scene::{VIEWCUBE_DRAW_PX, VIEWCUBE_PAD};
 use crate::scene::grip::grips_to_screen;
+use crate::scene::viewport_pane::ViewportPane;
 use crate::ui::overlay;
 use iced::widget::{button, column, container, mouse_area, row, shader, stack, text, Row, Space};
 use iced::window;
@@ -103,7 +104,14 @@ impl H7CAD {
         let i = self.active_tab;
         let tab = &self.tabs[i];
         let is_paper = tab.scene.current_layout != "Model";
-        let viewport_3d = shader(&tab.scene).width(Fill).height(Fill);
+        let viewport_3d: Element<'_, Message> = if is_paper {
+            paper_canvas_view(tab)
+        } else {
+            shader(ViewportPane::model(&tab.scene))
+                .width(Fill)
+                .height(Fill)
+                .into()
+        };
 
         let selection_overlay = {
             let sel = tab.scene.selection.borrow().clone();
@@ -447,6 +455,65 @@ impl H7CAD {
     }
 }
 
+// ── Paper canvas ──────────────────────────────────────────────────────────
+//
+// In paper space, the main canvas is composed of:
+//   1. A full-size PaperSheet layer — renders paper-space entities (title blocks,
+//      frames, borders) with the paper-space camera.
+//   2. One ViewportPane::Paper widget per Viewport entity, positioned at its
+//      paper-space coordinates and sized to its paper-space dimensions.
+//      Each renders model-space content through its own viewport camera.
+//   3. The existing selection_overlay / viewport_mouse / nav stack on top
+//      (unchanged — mouse event routing stays the same).
+
+fn paper_canvas_view<'a>(tab: &'a super::document::DocumentTab) -> Element<'a, Message> {
+    let scene = &tab.scene;
+
+    // Layer 1: paper-space entities (title blocks, frames, paper boundary).
+    let paper_sheet = shader(ViewportPane::paper_sheet(scene))
+        .width(Fill)
+        .height(Fill);
+
+    // Use the last-rendered canvas size to compute pixel positions.
+    // On the very first frame this is (0,0); correct positions appear from
+    // the second frame onward (imperceptible in practice).
+    let (canvas_w, canvas_h) = scene.selection.borrow().vp_size;
+    let vp_list = scene.viewport_list();
+
+    let mut layers = stack![paper_sheet];
+
+    for (handle, _, _) in &vp_list {
+        let Some(rect) = scene.viewport_screen_rect(*handle, (canvas_w, canvas_h)) else {
+            continue;
+        };
+
+        // Clamp to avoid zero-size or negative widgets.
+        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);
+
+        // Layer 2+: one shader widget per viewport, positioned with Space offsets.
+        let vp_widget = shader(ViewportPane::paper(scene, *handle))
+            .width(iced::Length::Fixed(w))
+            .height(iced::Length::Fixed(h));
+
+        let positioned = column![
+            Space::new().height(iced::Length::Fixed(y)),
+            row![
+                Space::new().width(iced::Length::Fixed(x)),
+                vp_widget,
+            ],
+        ]
+        .width(Fill)
+        .height(Fill);
+
+        layers = layers.push(positioned);
+    }
+
+    layers.width(Fill).height(Fill).into()
+}
+
 // ── Document tab bar ───────────────────────────────────────────────────────
 
 pub(super) fn doc_tab_bar<'a>(tabs: &'a [DocumentTab], active_tab: usize) -> Element<'a, Message> {

src/scene/mod.rs

@@ -18,6 +18,7 @@ pub mod solid3d_tess;
 pub mod tessellate;
 pub mod transform;
 pub mod truck_tess;
+pub mod viewport_pane;
 pub mod wire_model;
 
 use camera::Camera;
@@ -2095,6 +2096,156 @@ impl Scene {
     }
 
     pub fn update(&mut self, _dt: Duration) {}
+
+    // ── Paper-space coordinate helpers ───────────────────────────────────
+
+    /// 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.
+    /// Paper limits from the current layout define the paper-space extent;
+    /// the viewport is mapped linearly into that area (Y-axis flipped).
+    /// Returns `None` if the viewport handle is invalid or paper limits are missing.
+    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;
+
+        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;
+
+        Some(iced::Rectangle {
+            x: screen_x,
+            y: screen_y,
+            width: vp.width as f32 * sx,
+            height: vp.height as f32 * sy,
+        })
+    }
+
+    // ── ViewportPane helpers ──────────────────────────────────────────────
+
+    /// Paper-space entity wires only (title blocks, frames, borders).
+    /// Does NOT include viewport content projection — that is handled by
+    /// individual ViewportPane::Paper widgets layered on top.
+    pub(super) fn paper_sheet_wires(&self) -> Vec<WireModel> {
+        let layout_block = self.current_layout_block_handle();
+        let mut wires = self.wires_for_block(layout_block);
+        if let Some(((x0, y0), (x1, y1))) = self.paper_limits() {
+            wires.insert(0, paper_boundary_wire(x0 as f32, y0 as f32, x1 as f32, y1 as f32));
+        }
+        wires
+    }
+
+    /// Build a Camera oriented and scaled to match a paper-space Viewport entity.
+    /// Used by `ViewportPane::Paper` to render model-space content through the
+    /// viewport's own view direction and scale.
+    fn camera_for_viewport(&self, vp_handle: Handle) -> Option<camera::Camera> {
+        let vp = match self.document.get_entity(vp_handle) {
+            Some(EntityType::Viewport(vp)) => vp,
+            _ => return None,
+        };
+
+        let vd = glam::Vec3::new(
+            vp.view_direction.x as f32,
+            vp.view_direction.y as f32,
+            vp.view_direction.z as f32,
+        )
+        .normalize_or(glam::Vec3::Z);
+
+        let pitch = vd.z.clamp(-0.999, 0.999).asin();
+        let yaw = vd.x.atan2(vd.y);
+
+        let target = glam::Vec3::new(
+            vp.view_target.x as f32,
+            vp.view_target.y as f32,
+            vp.view_target.z as f32,
+        );
+
+        let fov_y = 45.0_f32.to_radians();
+        let view_height = if vp.view_height.abs() > 1e-9 {
+            vp.view_height as f32
+        } else {
+            vp.height as f32
+        };
+        // ortho_size = distance * tan(fov_y/2)  =>  distance = view_height/2 / tan(fov_y/2)
+        let distance = ((view_height / 2.0) / (fov_y * 0.5).tan()).max(0.001);
+
+        Some(camera::Camera {
+            target,
+            rotation: camera::yaw_pitch_to_quat(yaw, pitch),
+            distance,
+            fov_y,
+            projection: camera::Projection::Orthographic,
+            yaw,
+            pitch,
+        })
+    }
+
+    /// Collect model-space WireModels visible through `vp_handle`, respecting
+    /// global layer visibility and the viewport's per-viewport layer freeze list.
+    fn model_wires_for_viewport(&self, vp_handle: Handle) -> Vec<WireModel> {
+        use std::collections::HashSet as HSet;
+
+        let frozen: HSet<Handle> = match self.document.get_entity(vp_handle) {
+            Some(EntityType::Viewport(vp)) => vp.frozen_layers.iter().cloned().collect(),
+            _ => HSet::new(),
+        };
+
+        let model_block = self.model_space_block_handle();
+
+        self.document
+            .entities()
+            .filter(|e| {
+                let c = e.common();
+                if c.invisible || matches!(e, EntityType::Viewport(_)) {
+                    return false;
+                }
+                if !self.belongs_to_visible_block(c.handle, c.owner_handle, model_block) {
+                    return false;
+                }
+                if self
+                    .document
+                    .layers
+                    .get(&c.layer)
+                    .map(|l| l.flags.off || l.flags.frozen)
+                    .unwrap_or(false)
+                {
+                    return false;
+                }
+                if !frozen.is_empty() {
+                    if let Some(lh) = self.document.layers.get(&c.layer).map(|l| l.handle) {
+                        if frozen.contains(&lh) {
+                            return false;
+                        }
+                    }
+                }
+                true
+            })
+            .flat_map(|e| self.tessellate_one(e))
+            .collect()
+    }
 }
 
 impl Default for Scene {

src/scene/render.rs

@@ -3,11 +3,11 @@
 
 use acadrust::tables::LineType;
 use acadrust::types::{Color as AcadColor, LineWeight};
-use acadrust::EntityType;
+use acadrust::{EntityType, Handle};
 use glam::Mat4;
 use iced::mouse;
 use iced::widget::shader::{self, Viewport};
-use iced::{Event, Rectangle, Size};
+use iced::{Rectangle, Size};
 
 use super::pipeline::viewcube::{hover_id, VIEWCUBE_PX};
 use super::pipeline::Pipeline;
@@ -40,84 +40,6 @@ pub struct Primitive {
     pub(super) bg_color: [f32; 4],
 }
 
-// ── shader::Program impl ──────────────────────────────────────────────────
-
-impl<Msg: std::fmt::Debug + Clone> shader::Program<Msg> for Scene {
-    type State = CameraState;
-    type Primitive = Primitive;
-
-    fn draw(
-        &self,
-        state: &Self::State,
-        _cursor: mouse::Cursor,
-        bounds: Rectangle,
-    ) -> Self::Primitive {
-        let cam = self.camera.borrow();
-        self.selection.borrow_mut().vp_size = (bounds.width, bounds.height);
-
-        let mut all_wires = self.entity_wires();
-        if let Some(iw) = &self.interim_wire {
-            all_wires.push(iw.clone());
-        }
-        all_wires.extend(self.preview_wires.iter().cloned());
-
-        let bg_color = if self.current_layout == "Model" {
-            self.bg_color
-        } else {
-            self.paper_bg_color
-        };
-
-        Primitive {
-            wires: all_wires,
-            hatches: self.synced_hatch_models(),
-            wipeout_hatches: self.wipeout_models(),
-            images: self.images.values().cloned().collect(),
-            meshes: self.meshes.values().cloned().collect(),
-            uniforms: Uniforms::new(&cam, bounds),
-            cam_rotation: cam.view_rotation_mat(),
-            hover_region: state.hover_region,
-            bg_color,
-        }
-    }
-
-    fn update(
-        &self,
-        state: &mut Self::State,
-        event: &Event,
-        bounds: Rectangle,
-        cursor: mouse::Cursor,
-    ) -> Option<iced::widget::Action<Msg>> {
-        let pos = cursor.position_in(bounds);
-        let cam_rotation = { self.camera.borrow().view_rotation_mat() };
-        if let Some(p) = pos {
-            state.hover_region = hover_id(
-                p.x,
-                p.y,
-                bounds.width,
-                bounds.height,
-                cam_rotation,
-                VIEWCUBE_PX,
-            );
-        } else {
-            state.hover_region = None;
-        }
-        let _ = event;
-        None
-    }
-
-    fn mouse_interaction(
-        &self,
-        state: &Self::State,
-        _b: Rectangle,
-        _c: mouse::Cursor,
-    ) -> mouse::Interaction {
-        if state.hover_region.is_some() {
-            return mouse::Interaction::Pointer;
-        }
-        mouse::Interaction::default()
-    }
-}
-
 // ── shader::Primitive impl ────────────────────────────────────────────────
 
 impl shader::Primitive for Primitive {
@@ -236,6 +158,142 @@ impl Scene {
     }
 }
 
+// ── Primitive builder helpers (called by ViewportPane's shader::Program impl) ──
+
+impl Scene {
+    /// Build a full-scene Primitive for the model or paper view (the camera
+    /// stored in `self.camera` is used as-is).
+    pub(super) fn build_primitive(
+        &self,
+        hover_region: Option<usize>,
+        bounds: Rectangle,
+    ) -> Primitive {
+        let cam = self.camera.borrow();
+        self.selection.borrow_mut().vp_size = (bounds.width, bounds.height);
+
+        let mut all_wires = self.entity_wires();
+        if let Some(iw) = &self.interim_wire {
+            all_wires.push(iw.clone());
+        }
+        all_wires.extend(self.preview_wires.iter().cloned());
+
+        let bg_color = if self.current_layout == "Model" {
+            self.bg_color
+        } else {
+            self.paper_bg_color
+        };
+
+        Primitive {
+            wires: all_wires,
+            hatches: self.synced_hatch_models(),
+            wipeout_hatches: self.wipeout_models(),
+            images: self.images.values().cloned().collect(),
+            meshes: self.meshes.values().cloned().collect(),
+            uniforms: Uniforms::new(&cam, bounds),
+            cam_rotation: cam.view_rotation_mat(),
+            hover_region,
+            bg_color,
+        }
+    }
+
+    /// Build a Primitive for the paper-sheet background: paper-space entities
+    /// (title blocks, frames, borders) using the paper-space camera.
+    /// Viewport content is NOT included — it is rendered by separate
+    /// `ViewportPane::Paper` widgets stacked on top.
+    pub(super) fn build_paper_sheet_primitive(
+        &self,
+        hover_region: Option<usize>,
+        bounds: Rectangle,
+    ) -> Primitive {
+        let cam = self.camera.borrow();
+        self.selection.borrow_mut().vp_size = (bounds.width, bounds.height);
+
+        let mut wires = self.paper_sheet_wires();
+        if let Some(iw) = &self.interim_wire {
+            wires.push(iw.clone());
+        }
+        wires.extend(self.preview_wires.iter().cloned());
+
+        Primitive {
+            wires,
+            hatches: self.synced_hatch_models(),
+            wipeout_hatches: self.wipeout_models(),
+            images: self.images.values().cloned().collect(),
+            meshes: self.meshes.values().cloned().collect(),
+            uniforms: Uniforms::new(&cam, bounds),
+            cam_rotation: cam.view_rotation_mat(),
+            hover_region,
+            bg_color: self.paper_bg_color,
+        }
+    }
+
+    /// Build a Primitive that renders model-space content through a specific
+    /// paper-space viewport's camera, applying its layer-freeze list.
+    pub(super) fn build_viewport_primitive(
+        &self,
+        vp_handle: Handle,
+        hover_region: Option<usize>,
+        bounds: Rectangle,
+    ) -> Primitive {
+        let cam = match self.camera_for_viewport(vp_handle) {
+            Some(c) => c,
+            None => return self.build_primitive(hover_region, bounds),
+        };
+
+        let mut all_wires = self.model_wires_for_viewport(vp_handle);
+        if let Some(iw) = &self.interim_wire {
+            all_wires.push(iw.clone());
+        }
+        all_wires.extend(self.preview_wires.iter().cloned());
+
+        Primitive {
+            wires: all_wires,
+            hatches: self.synced_hatch_models(),
+            wipeout_hatches: self.wipeout_models(),
+            images: self.images.values().cloned().collect(),
+            meshes: self.meshes.values().cloned().collect(),
+            uniforms: Uniforms::new(&cam, bounds),
+            cam_rotation: cam.view_rotation_mat(),
+            hover_region,
+            bg_color: self.bg_color,
+        }
+    }
+
+    /// Update viewcube hover state from cursor position within `bounds`.
+    pub(super) fn update_viewcube_state(
+        &self,
+        state: &mut CameraState,
+        bounds: Rectangle,
+        cursor: mouse::Cursor,
+    ) {
+        let pos = cursor.position_in(bounds);
+        let cam_rotation = self.camera.borrow().view_rotation_mat();
+        if let Some(p) = pos {
+            state.hover_region = hover_id(
+                p.x,
+                p.y,
+                bounds.width,
+                bounds.height,
+                cam_rotation,
+                VIEWCUBE_PX,
+            );
+        } else {
+            state.hover_region = None;
+        }
+    }
+
+    pub(super) fn viewcube_mouse_interaction(
+        &self,
+        state: &CameraState,
+    ) -> mouse::Interaction {
+        if state.hover_region.is_some() {
+            mouse::Interaction::Pointer
+        } else {
+            mouse::Interaction::default()
+        }
+    }
+}
+
 // ── Linetype pattern helper ───────────────────────────────────────────────
 
 pub(super) fn resolve_pattern(

src/scene/viewport_pane.rs

@@ -0,0 +1,94 @@
+use super::render::{CameraState, Primitive};
+use super::Scene;
+use acadrust::Handle;
+use iced::widget::shader;
+use iced::{mouse, Event, Rectangle};
+
+// ── Mode ──────────────────────────────────────────────────────────────────
+
+pub enum ViewportPaneMode {
+    /// Full model space — fills whatever bounds Iced assigns.
+    Model,
+    /// Paper-space entities only (title blocks, frames, borders) using the
+    /// paper-space camera. No viewport content projection.
+    PaperSheet,
+    /// Model-space content seen through a specific paper-space Viewport entity.
+    Paper { handle: Handle },
+}
+
+// ── Widget struct ─────────────────────────────────────────────────────────
+
+pub struct ViewportPane<'a> {
+    pub scene: &'a Scene,
+    pub mode: ViewportPaneMode,
+}
+
+impl<'a> ViewportPane<'a> {
+    pub fn model(scene: &'a Scene) -> Self {
+        Self { scene, mode: ViewportPaneMode::Model }
+    }
+
+    /// Paper-sheet layer: paper-space entities rendered with the paper camera.
+    pub fn paper_sheet(scene: &'a Scene) -> Self {
+        Self { scene, mode: ViewportPaneMode::PaperSheet }
+    }
+
+    /// One paper-space viewport: model content rendered through its own camera.
+    pub fn paper(scene: &'a Scene, handle: Handle) -> Self {
+        Self { scene, mode: ViewportPaneMode::Paper { handle } }
+    }
+}
+
+// ── shader::Program impl ──────────────────────────────────────────────────
+
+impl<'a, Msg: std::fmt::Debug + Clone> shader::Program<Msg> for ViewportPane<'a> {
+    type State = CameraState;
+    type Primitive = Primitive;
+
+    fn draw(
+        &self,
+        state: &Self::State,
+        _cursor: mouse::Cursor,
+        bounds: Rectangle,
+    ) -> Self::Primitive {
+        match &self.mode {
+            ViewportPaneMode::Model => {
+                self.scene.build_primitive(state.hover_region, bounds)
+            }
+            ViewportPaneMode::PaperSheet => {
+                self.scene.build_paper_sheet_primitive(state.hover_region, bounds)
+            }
+            ViewportPaneMode::Paper { handle } => {
+                self.scene.build_viewport_primitive(*handle, state.hover_region, bounds)
+            }
+        }
+    }
+
+    fn update(
+        &self,
+        state: &mut Self::State,
+        event: &Event,
+        bounds: Rectangle,
+        cursor: mouse::Cursor,
+    ) -> Option<iced::widget::Action<Msg>> {
+        // ViewCube hover only makes sense in the full model-space view.
+        if matches!(self.mode, ViewportPaneMode::Model | ViewportPaneMode::PaperSheet) {
+            self.scene.update_viewcube_state(state, bounds, cursor);
+        }
+        let _ = event;
+        None
+    }
+
+    fn mouse_interaction(
+        &self,
+        state: &Self::State,
+        _b: Rectangle,
+        _c: mouse::Cursor,
+    ) -> mouse::Interaction {
+        if matches!(self.mode, ViewportPaneMode::Model | ViewportPaneMode::PaperSheet) {
+            self.scene.viewcube_mouse_interaction(state)
+        } else {
+            mouse::Interaction::default()
+        }
+    }
+}