fix(viewport): blit UV crop for off-canvas viewports

Refactor the per-viewport blit so a content viewport whose screen rect
extends off the canvas composites the correct slice of its MSAA target
to the visible portion of the surface, instead of being slammed against
the canvas corner by an `(f32 as u32)` saturation on the negative
offset.

- blit.wgsl + Pipeline gain a small uniform buffer (`uv_offset` /
  `uv_scale`). `Primitive::prepare` computes the per-viewport crop from
  `screen_rect`'s on-canvas fraction and writes it via the new
  `upload_blit_uv` helper while it still has the `Queue`.
- `Pipeline::render` now takes `vp_size` (full vp — MSAA / camera size)
  and `surface_dest` (visible intersection — blit destination) and
  reads the crop from the uniform; passes that don't have a non-empty
  surface intersection are skipped.
- `Primitive::render` clips the full vp rect against the surface clip
  in `i32` and skips empty intersections. The earlier "skip GPU for
  inactive off-canvas viewports" workaround in `active_viewports` is
  no longer needed.
- ViewCube draws only when its hosting vp is fully on-canvas (its own
  pipeline writes directly to the surface and would distort under a
  clamped `set_viewport`).

With the GPU path correct for every vp position, drop
`viewport_content_wires` from `paper_canvas_wires` — the inactive
viewport CPU projection was only there to mask the blit bug and was
duplicating the unified shader's output. The hit-test / snap path
(`entity_wires_arc`, `wires_for_block`) still calls
`viewport_content_wires` directly, so picking through inactive
viewports keeps working.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/scene/pipeline/mod.rs

@@ -8,6 +8,7 @@ pub mod viewcube;
 pub mod wire_gpu;
 
 use iced::wgpu;
+use iced::wgpu::util::DeviceExt;
 use iced::{Rectangle, Size};
 
 pub use face3d_gpu::Face3DGpu;
@@ -69,6 +70,10 @@ pub struct Pipeline {
     blit_bind_group_layout: wgpu::BindGroupLayout,
     blit_sampler: wgpu::Sampler,
     blit_bind_group: wgpu::BindGroup,
+    /// UV transform (offset + scale) consumed by the blit shader so a
+    /// partially off-canvas viewport still composites the right portion of
+    /// its resolve texture to the visible portion of the surface.
+    blit_uniform_buffer: wgpu::Buffer,
     /// Cached texture format (needed to recreate MSAA / depth textures on resize).
     surface_format: wgpu::TextureFormat,
     gpu_wires: Vec<WireGpu>,
@@ -743,9 +748,28 @@ impl Pipeline {
                     ty: wgpu::BindingType::Sampler(wgpu::SamplerBindingType::Filtering),
                     count: None,
                 },
+                wgpu::BindGroupLayoutEntry {
+                    binding: 2,
+                    visibility: wgpu::ShaderStages::FRAGMENT,
+                    ty: wgpu::BindingType::Buffer {
+                        ty: wgpu::BufferBindingType::Uniform,
+                        has_dynamic_offset: false,
+                        min_binding_size: None,
+                    },
+                    count: None,
+                },
             ],
         });
 
+        // UV crop uniform: [uv_offset_x, uv_offset_y, uv_scale_x, uv_scale_y]
+        // padded to 16 bytes (std140 vec2 alignment). Defaulted to the
+        // identity crop (offset 0, scale 1) for the common on-canvas case.
+        let blit_uniform_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
+            label: Some("blit.uniform_buffer"),
+            contents: bytemuck::cast_slice(&[0.0f32, 0.0, 1.0, 1.0]),
+            usage: wgpu::BufferUsages::UNIFORM | wgpu::BufferUsages::COPY_DST,
+        });
+
         let blit_layout = device.create_pipeline_layout(&wgpu::PipelineLayoutDescriptor {
             label: Some("blit.pipeline_layout"),
             bind_group_layouts: &[&blit_bgl],
@@ -810,6 +834,10 @@ impl Pipeline {
                     binding: 1,
                     resource: wgpu::BindingResource::Sampler(&blit_sampler),
                 },
+                wgpu::BindGroupEntry {
+                    binding: 2,
+                    resource: blit_uniform_buffer.as_entire_binding(),
+                },
             ],
         });
 
@@ -837,6 +865,7 @@ impl Pipeline {
             blit_bind_group_layout: blit_bgl,
             blit_sampler,
             blit_bind_group,
+            blit_uniform_buffer,
             surface_format: format,
             gpu_wires: vec![],
             wire_pixel_scissors: vec![],
@@ -1030,17 +1059,39 @@ impl Pipeline {
         queue.write_buffer(&self.uniform_buffer, 0, bytemuck::bytes_of(uniforms));
     }
 
+    /// Write the blit shader's UV crop uniform. Call in `prepare` (the only
+    /// place with a `&Queue`) — `render` then just submits the draw call.
+    pub fn upload_blit_uv(&self, queue: &wgpu::Queue, uv_offset: [f32; 2], uv_scale: [f32; 2]) {
+        queue.write_buffer(
+            &self.blit_uniform_buffer,
+            0,
+            bytemuck::cast_slice(&[uv_offset[0], uv_offset[1], uv_scale[0], uv_scale[1]]),
+        );
+    }
+
+    /// Render the geometry passes at `vp_size` (the full viewport size — the
+    /// MSAA / resolve textures are this size) and blit the resulting resolve
+    /// to `surface_dest` on the swap-chain. The UV crop is read from the
+    /// blit uniform buffer (written by `upload_blit_uv` during `prepare`)
+    /// so a viewport that hangs off the canvas still composites the correct
+    /// sub-rectangle to the visible portion of the surface.
     pub fn render(
         &self,
         encoder: &mut wgpu::CommandEncoder,
         target: &wgpu::TextureView,
-        clip_bounds: Rectangle<u32>,
+        vp_size: Size<u32>,
+        surface_dest: Rectangle<u32>,
         bg_color: [f32; 4],
         mesh_wireframe: bool,
         hidden_line: bool,
         show_3d_edges: bool,
     ) {
-        let vp = clip_bounds;
+        let vp = Rectangle::<u32> {
+            x: 0,
+            y: 0,
+            width: vp_size.width,
+            height: vp_size.height,
+        };
         let msaa = &self.msaa_view;
         let [r, g, b, a] = bg_color;
         let clear_color = wgpu::Color {
@@ -1543,10 +1594,12 @@ impl Pipeline {
             // No draw calls — the pass itself triggers the MSAA resolve.
         }
 
-        // ── Blit resolve texture → surface target at clip_bounds position ──
-        // The viewport maps the full-screen NDC quad to exactly clip_bounds
-        // in the surface, leaving all other widgets untouched.
-        {
+        // ── Blit resolve texture → surface target at surface_dest position ──
+        // The viewport maps the NDC quad to exactly `surface_dest` in the
+        // swap-chain; `uv_offset` + `uv_scale` (passed through the blit
+        // uniform) crop the resolve so we sample only the visible portion
+        // of the full viewport's MSAA texture.
+        if surface_dest.width > 0 && surface_dest.height > 0 {
             let mut pass = encoder.begin_render_pass(&wgpu::RenderPassDescriptor {
                 label: Some("blit.render_pass"),
                 color_attachments: &[Some(wgpu::RenderPassColorAttachment {
@@ -1563,10 +1616,10 @@ impl Pipeline {
                 occlusion_query_set: None,
             });
             pass.set_viewport(
-                vp.x as f32,
-                vp.y as f32,
-                vp.width as f32,
-                vp.height as f32,
+                surface_dest.x as f32,
+                surface_dest.y as f32,
+                surface_dest.width as f32,
+                surface_dest.height as f32,
                 0.0,
                 1.0,
             );
@@ -1596,6 +1649,10 @@ impl Pipeline {
                         binding: 1,
                         resource: wgpu::BindingResource::Sampler(&self.blit_sampler),
                     },
+                    wgpu::BindGroupEntry {
+                        binding: 2,
+                        resource: self.blit_uniform_buffer.as_entire_binding(),
+                    },
                 ],
             });
             self.resolve_view = resolve_view;