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/render.rs

@@ -178,13 +178,28 @@ impl shader::Primitive for Primitive {
 
         for (i, vp) in self.viewports.iter().enumerate() {
             let inner = &mut pipeline.inners[i];
-            // Physical MSAA/depth size for this viewport's sub-rect.
+            // The MSAA / depth / resolve textures are always sized to the
+            // FULL viewport rectangle (not the on-canvas-visible portion)
+            // so the camera matrices render at consistent aspect / scale.
+            // The blit step picks the visible sub-rectangle out via the
+            // shader's UV crop uniform, which lets partially off-canvas
+            // viewports composite to their visible surface area without
+            // drift.
             let clip_size = Size::new(
                 (vp.screen_rect.width * bounds.width * scale).ceil().max(1.0) as u32,
                 (vp.screen_rect.height * bounds.height * scale).ceil().max(1.0) as u32,
             );
             inner.ensure_depth_texture(device, clip_size);
             inner.viewcube.ensure_depth_texture(device, full_size);
+            // Compute the UV crop for this viewport. `screen_rect` is in
+            // normalized canvas units (0..1) but may extend negative or
+            // beyond 1 when the viewport hangs off the canvas. The on-
+            // canvas portion in viewport-local UV is straightforward to
+            // derive from how much sticks out on each side.
+            let sr = vp.screen_rect;
+            let (uo_x, us_x) = uv_crop_axis(sr.x, sr.width);
+            let (uo_y, us_y) = uv_crop_axis(sr.y, sr.height);
+            inner.upload_blit_uv(queue, [uo_x, uo_y], [us_x, us_y]);
             inner.upload_uniforms(queue, &vp.uniforms);
             let cur_key = (vp.geometry_epoch, vp.camera_generation);
             let fill_mode = vp.fill_mode;
@@ -244,20 +259,35 @@ impl shader::Primitive for Primitive {
         target: &iced::wgpu::TextureView,
         clip: &Rectangle<u32>,
     ) {
+        let cw = clip.width as f32;
+        let ch = clip.height as f32;
+        let clip_right = clip.x + clip.width;
+        let clip_bottom = clip.y + clip.height;
         for (i, vp) in self.viewports.iter().enumerate() {
             let Some(inner) = pipeline.inners.get(i) else {
                 break;
             };
-            // Derive this viewport's physical sub-clip from the normalized
-            // screen_rect and the surface clip.
-            let cw = clip.width as f32;
-            let ch = clip.height as f32;
-            let vp_clip = Rectangle {
-                x: clip.x + (vp.screen_rect.x * cw) as u32,
-                y: clip.y + (vp.screen_rect.y * ch) as u32,
-                width: (vp.screen_rect.width * cw).max(1.0) as u32,
-                height: (vp.screen_rect.height * ch).max(1.0) as u32,
+            // Where the viewport would land on the surface in absolute
+            // pixels (i32 because either edge may stick off the canvas).
+            let vp_full_x = clip.x as i32 + (vp.screen_rect.x * cw) as i32;
+            let vp_full_y = clip.y as i32 + (vp.screen_rect.y * ch) as i32;
+            let vp_full_w = (vp.screen_rect.width * cw).max(1.0) as i32;
+            let vp_full_h = (vp.screen_rect.height * ch).max(1.0) as i32;
+            // Intersect with the surface clip — that's the slice we blit.
+            let dest_x = vp_full_x.max(clip.x as i32);
+            let dest_y = vp_full_y.max(clip.y as i32);
+            let dest_right = (vp_full_x + vp_full_w).min(clip_right as i32);
+            let dest_bottom = (vp_full_y + vp_full_h).min(clip_bottom as i32);
+            if dest_right <= dest_x || dest_bottom <= dest_y {
+                continue;
+            }
+            let surface_dest = Rectangle {
+                x: dest_x as u32,
+                y: dest_y as u32,
+                width: (dest_right - dest_x) as u32,
+                height: (dest_bottom - dest_y) as u32,
             };
+            let vp_size = Size::new(vp_full_w.max(1) as u32, vp_full_h.max(1) as u32);
             // `mesh_fill` is false for Wireframe 2D / Wireframe 3D — flip
             // the draw path so meshes use the wireframe pipeline + the
             // pre-built triangle-edge index buffer.
@@ -265,19 +295,50 @@ impl shader::Primitive for Primitive {
             inner.render(
                 encoder,
                 target,
-                vp_clip,
+                vp_size,
+                surface_dest,
                 self.bg_color,
                 mesh_wireframe,
                 vp.hidden_line,
                 vp.show_3d_edges,
             );
-            if vp.show_viewcube {
+            // The ViewCube renders directly to the surface at the full
+            // viewport rect. Skip it when the viewport's top-right corner
+            // (where the cube sits) is off-canvas — wgpu's `set_viewport`
+            // rejects negative origins, and a clamped cube would scale
+            // distortedly. The active viewport is normally fully visible.
+            if vp.show_viewcube
+                && vp_full_x >= clip.x as i32
+                && vp_full_y >= clip.y as i32
+                && vp_full_x + vp_full_w <= clip_right as i32
+                && vp_full_y + vp_full_h <= clip_bottom as i32
+            {
+                let vp_clip = Rectangle {
+                    x: vp_full_x as u32,
+                    y: vp_full_y as u32,
+                    width: vp_full_w as u32,
+                    height: vp_full_h as u32,
+                };
                 inner.viewcube.render(encoder, target, vp_clip);
             }
         }
     }
 }
 
+/// On-canvas-visible UV crop on one axis. `pos` and `size` are in the
+/// shader widget's normalized 0..1 coords. Returns `(uv_offset, uv_scale)`
+/// applied as `actual_uv = quad_uv * uv_scale + uv_offset` in the blit
+/// shader — identity `(0.0, 1.0)` for fully on-canvas viewports.
+fn uv_crop_axis(pos: f32, size: f32) -> (f32, f32) {
+    if size <= 0.0 {
+        return (0.0, 1.0);
+    }
+    let left_off = (-pos).max(0.0);
+    let right_off = (pos + size - 1.0).max(0.0);
+    let visible = (size - left_off - right_off).max(0.0);
+    (left_off / size, visible / size)
+}
+
 // ── Render-style helpers (impl Scene) ────────────────────────────────────
 
 impl Scene {