Write a layout's paper-space viewport (id 1) ahead of its other viewports (#1782)

DWG stores no viewport ids; readers number a layout's viewports in order. A drawing whose draw
order (SORTENTS) put the paper-space viewport behind a model viewport came back from DWG with
that model viewport as the sheet.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: Brandon Thomas <echelon@gmail.com>

crates/io/src/dxf_write.rs

@@ -1629,6 +1629,15 @@ fn paper_views(d: &Drawing) -> HashMap<Handle, usize> {
     out
 }
 
+/// A layout's entities with its paper-space viewport (id 1) first. DWG stores no viewport ids and
+/// readers number a layout's viewports in order, so the sheet must lead even when the draw order
+/// (SORTENTS) put it behind a model viewport; otherwise that viewport becomes the sheet.
+fn sheet_first(l: &Layout) -> Vec<&Entity> {
+    let sheet = |e: &&Entity| matches!(&e.kind, EntityKind::Viewport(v) if v.id == 1);
+    let all = || l.entities.iter().map(|e| e.as_ref());
+    all().filter(sheet).chain(all().filter(|e| !sheet(e))).collect()
+}
+
 /// The paper-space viewport (id 1) of the layout whose first viewport is `e`, if it needs one
 /// ([`paper_views`]): the whole sheet, viewed 1:1. Marked with CADCraft xdata so that our reader
 /// leaves it implicit again.
@@ -2340,7 +2349,7 @@ fn tags_for(d: &Drawing, version: Version, target: Version) -> Vec<Tag> {
         if i == 0 {
             block(&mut w, n, "", h, Vec3::ZERO, 0, &mut std::iter::empty(), true);
         } else {
-            block(&mut w, n, "", h, Vec3::ZERO, 0, &mut l.entities.iter().map(|e| e.as_ref()), true);
+            block(&mut w, n, "", h, Vec3::ZERO, 0, &mut sheet_first(l).into_iter(), true);
         }
     }
     for (n, h, b) in &user_blocks {
@@ -2371,7 +2380,7 @@ fn tags_for(d: &Drawing, version: Version, target: Version) -> Vec<Tag> {
         entity(&mut w, d, e, &ms_br, false, &cx);
     }
     if let Some((_, h, l)) = ps_brs.first() {
-        for e in l.entities.iter() {
+        for e in sheet_first(l) {
             entity(&mut w, d, e, h, true, &cx);
         }
     }

crates/io/tests/it/dwg_roundtrip_losses.rs

@@ -138,3 +138,24 @@ fn viewport_ids_survive_dwg_and_the_paper_viewport_stays_implicit() {
     assert!(!text.contains("PAPERVIEW"));
     assert_eq!(viewport_ids(&cadcraft_io::read_dxf(text.as_bytes()).unwrap().layouts[0]), [1, 2]);
 }
+
+/// The paper-space viewport (id 1) of a drawing from another program can come after a model
+/// viewport in the layout, e.g. when the draw order (SORTENTS) put it behind. DWG numbers in
+/// order, so the writer puts it first: otherwise the model viewport became the sheet on reopen.
+#[test]
+fn a_paper_viewport_behind_a_model_viewport_stays_id_1_through_dwg() {
+    let mut d = Drawing::new_metric();
+    let names: Vec<String> = d.layouts.iter().map(|l| l.name.clone()).collect();
+    for name in &names {
+        let space = Space::Paper(name.clone());
+        let model = d.add(&space, Common::default(), viewport(2)).unwrap();
+        let sheet = d.add(&space, Common::default(), viewport(1)).unwrap();
+        let back = through_dwg(&d);
+        let l = back.layouts.iter().find(|l| &l.name == name).unwrap();
+        let id = |h: Handle| match &l.entities.get(h).map(|e| &e.kind) {
+            Some(EntityKind::Viewport(v)) => Some(v.id),
+            _ => None,
+        };
+        assert_eq!((id(model), id(sheet)), (Some(2), Some(1)), "layout {name}");
+    }
+}