fix: make viewports from external DXF/DWG files visible and enterable

Two root causes prevented paper-space viewports from loading correctly:

1. acadrust's ViewportStatusFlags::from_bits() maps bit 0 → is_on, but
   the real DXF/DWG spec uses bit 15 (0x8000) for "viewport on". Post-load
   fixup in io/mod.rs now corrects this for both DXF and DWG files: when
   bit 0 is clear but bit 15 is set, is_on is forced true and locked is
   derived from bit 14 (0x4000).

2. Some DXF exporters write id=-1 for all viewport entities instead of the
   standard id=1 (sheet) / id≥2 (user) convention. The is_user_viewport_id()
   helper now treats any id ≠ 0 and ≠ 1 as a user viewport, covering both
   the standard id≥2 and the non-standard id=-1 cases.

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

src/app/update.rs

@@ -1696,7 +1696,7 @@ impl H7CAD {
                                     if let Some(AcadEntityType::Viewport(vp)) =
                                         self.tabs[i].scene.document.get_entity(h)
                                     {
-                                        if vp.id > 1 { Some(h) } else { None }
+                                        if Scene::is_user_viewport_id(vp.id) { Some(h) } else { None }
                                     } else {
                                         None
                                     }

src/io/mod.rs

@@ -58,8 +58,12 @@ pub fn load_file(path: &Path) -> Result<CadDocument, String> {
 
     match ext.as_str() {
         "dwg" => {
-            let mut reader = DwgReader::from_file(path).map_err(|e| e.to_string())?;
-            reader.read().map_err(|e| e.to_string())
+            let mut doc = DwgReader::from_file(path)
+                .map_err(|e| e.to_string())?
+                .read()
+                .map_err(|e| e.to_string())?;
+            fix_viewport_status_flags(&mut doc);
+            Ok(doc)
         }
         "dxf" => {
             let mut doc = DxfReader::from_file(path)
@@ -67,6 +71,7 @@ pub fn load_file(path: &Path) -> Result<CadDocument, String> {
                 .read()
                 .map_err(|e| e.to_string())?;
             fix_dxf_dimension_rotations(&mut doc);
+            fix_viewport_status_flags(&mut doc);
             Ok(doc)
         }
         _ => Err(format!("Unsupported file format: .{ext}")),
@@ -168,7 +173,25 @@ pub fn save_dxf(doc: &CadDocument, path: &Path) -> Result<(), String> {
         .map_err(|e| e.to_string())
 }
 
-// ── DXF post-load fixups ──────────────────────────────────────────────────
+// ── Post-load fixups ──────────────────────────────────────────────────────
+
+/// acadrust's ViewportStatusFlags::from_bits() maps bit 0 → is_on and bit 15 → locked,
+/// but the real DXF/DWG spec uses bit 15 (0x8000) → viewport on and bit 14 (0x4000) → locked.
+/// Files from AutoCAD and other tools always set bit 15 for active viewports, leaving bit 0
+/// clear, so acadrust reads every such viewport as off.  Correct that here after loading.
+fn fix_viewport_status_flags(doc: &mut CadDocument) {
+    for entity in doc.entities_mut() {
+        if let EntityType::Viewport(vp) = entity {
+            let bits = vp.status.to_bits();
+            // If bit 0 is not set but bit 15 is, this is an external-format viewport:
+            // treat bit 15 as "on" and bit 14 as "locked".
+            if (bits & 0x0001) == 0 && (bits & 0x8000) != 0 {
+                vp.status.is_on = true;
+                vp.status.locked = (bits & 0x4000) != 0;
+            }
+        }
+    }
+}
 
 /// The acadrust DXF reader stores several rotation fields directly from DXF
 /// group code 50 in degrees, while DWG and our own creation code store radians.

src/scene/mod.rs

@@ -196,6 +196,14 @@ impl Scene {
         }
     }
 
+    /// Returns true if this viewport ID identifies a user viewport (not the
+    /// sheet/overall viewport).  The standard sheet viewport has id=1; id=0
+    /// means "unset".  External DXF exporters sometimes use id=-1 for all
+    /// viewports, so any negative id is also treated as a user viewport.
+    pub fn is_user_viewport_id(id: i16) -> bool {
+        id != 0 && id != 1
+    }
+
     /// Public accessor for the block-record handle of the current layout.
     /// Used by external callers (e.g. `commit_entity`) that need the handle
     /// without going through private API.
@@ -325,7 +333,7 @@ impl Scene {
         }
         self.document.entities().find_map(|e| {
             if let EntityType::Viewport(vp) = e {
-                if vp.id > 1 && vp.common.owner_handle == layout_block {
+                if Self::is_user_viewport_id(vp.id) && vp.common.owner_handle == layout_block {
                     let scale = if vp.custom_scale.abs() > 1e-9 {
                         vp.custom_scale
                     } else if vp.view_height.abs() > 1e-9 {
@@ -354,7 +362,7 @@ impl Scene {
             .entities()
             .filter_map(|e| {
                 if let EntityType::Viewport(vp) = e {
-                    if vp.id > 1 && vp.common.owner_handle == layout_block {
+                    if Self::is_user_viewport_id(vp.id) && vp.common.owner_handle == layout_block {
                         Some((vp.common.handle, vp.id, vp.frozen_layers.clone()))
                     } else {
                         None
@@ -365,7 +373,15 @@ impl Scene {
             })
             .collect::<Vec<_>>()
             .into_iter()
-            .map(|(h, id, frozen)| (h, format!("VP {}", id - 1), frozen))
+            .enumerate()
+            .map(|(i, (h, id, frozen))| {
+                let label = if id > 1 {
+                    format!("VP {}", id - 1)
+                } else {
+                    format!("VP {}", i + 1)
+                };
+                (h, label, frozen)
+            })
             .collect();
         result.sort_by_key(|(_, label, _)| label.clone());
         result
@@ -382,7 +398,7 @@ impl Scene {
         }
         self.document.entities().filter(|e| {
             if let EntityType::Viewport(vp) = e {
-                vp.id > 1 && vp.common.owner_handle == layout_block
+                Self::is_user_viewport_id(vp.id) && vp.common.owner_handle == layout_block
             } else {
                 false
             }
@@ -786,7 +802,7 @@ impl Scene {
                 if let EntityType::Viewport(vp) = e { Some(vp) } else { None }
             })
             .filter(|vp| {
-                vp.id > 1
+                Self::is_user_viewport_id(vp.id)
                     && vp.common.owner_handle == paper_block
                     && vp.status.is_on
                     && only_vp.map_or(true, |h| vp.common.handle == h)
@@ -1126,7 +1142,7 @@ impl Scene {
             .entities()
             .find_map(|e| {
                 let EntityType::Viewport(vp) = e else { return None; };
-                if vp.id <= 1 || vp.common.owner_handle != layout_block || !vp.status.is_on {
+                if !Self::is_user_viewport_id(vp.id) || vp.common.owner_handle != layout_block || !vp.status.is_on {
                     return None;
                 }
                 let hw = (vp.width / 2.0) as f32;
@@ -1147,7 +1163,7 @@ impl Scene {
         let layout_block = self.current_layout_block_handle();
         self.document.entities().find_map(|e| {
             let EntityType::Viewport(vp) = e else { return None; };
-            if vp.id > 1 && vp.common.owner_handle == layout_block && vp.status.is_on {
+            if Self::is_user_viewport_id(vp.id) && vp.common.owner_handle == layout_block && vp.status.is_on {
                 Some(vp.common.handle)
             } else {
                 None