feat(layout): reopen a drawing in the space it was saved in

Files saved while a paper layout was active reopened in Model space
because the loader hardcoded the current layout to "Model" and never
consulted the space recorded in the file.

The active space is a standard part of the format and acadrust already
round-trips it: $TILEMODE (header.show_model_space) for model-vs-paper,
and the CTAB current-tab variable for the exact layout name.

- Read: on open, pick the current layout from CTAB (when it names a
  layout that exists), else $TILEMODE (Model, or the first paper layout).
- Write: Scene::sync_active_space_to_document() mirrors the active layout
  back into the document (show_model_space + CTAB). It runs from
  set_current_layout, so every active-layout change — the status-bar tab,
  the Layout Manager's Set Current, create / rename / delete, and
  undo/redo — keeps the document in sync for the next save.

$TILEMODE is the guaranteed mechanism (read and written for both DXF and
DWG); CTAB refines the exact tab when the variable is present.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/update/file.rs

@@ -489,7 +489,28 @@ pub(super) fn on_open_file(&mut self) -> Task<Message> {
                 }
                 self.tabs[i].scene.selected = rustc_hash::FxHashSet::default();
                 self.tabs[i].scene.preview_wires = vec![];
-                self.tabs[i].scene.current_layout = "Model".to_string();
+                // Reopen in whichever space the file was saved in — the CTAB
+                // tab name when recorded, else the $TILEMODE model/paper flag —
+                // instead of always landing in Model.
+                {
+                    let names = self.tabs[i].scene.layout_names();
+                    let saved = crate::io::saved_active_layout(&self.tabs[i].scene.document)
+                        .and_then(|n| {
+                            names.iter().find(|x| x.eq_ignore_ascii_case(&n)).cloned()
+                        });
+                    self.tabs[i].scene.current_layout = match saved {
+                        Some(n) => n,
+                        None if self.tabs[i].scene.document.header.show_model_space => {
+                            "Model".to_string()
+                        }
+                        // Paper was active but no CTAB — fall back to the first
+                        // paper layout (names[0] is always "Model").
+                        None => names
+                            .into_iter()
+                            .nth(1)
+                            .unwrap_or_else(|| "Model".to_string()),
+                    };
+                }
                 // Rebuild the Isolate/Hide set from the entities the file itself
                 // marks invisible (DXF code 60), so hidden objects stay hidden on
                 // reopen and End Isolation can bring them back.

src/app/update/mod.rs

@@ -2569,7 +2569,7 @@ impl OpenCADStudio {
                     self.push_undo_snapshot(i, "LAYOUT RENAME");
                     self.tabs[i].scene.rename_layout(&old_name, &new_name);
                     if self.tabs[i].scene.current_layout == old_name {
-                        self.tabs[i].scene.current_layout = new_name.clone();
+                        self.tabs[i].scene.set_current_layout(new_name.clone());
                     }
                     self.layout_manager_selected = new_name.clone();
                     self.tabs[i].dirty = true;
@@ -2610,8 +2610,7 @@ impl OpenCADStudio {
                     self.tabs[i].dirty = true;
                     // Switch to Model if active layout was deleted.
                     if self.tabs[i].scene.current_layout == name {
-                        self.tabs[i].scene.current_layout = "Model".to_string();
-                        self.tabs[i].scene.bump_geometry();
+                        self.tabs[i].scene.set_current_layout("Model".to_string());
                     }
                     self.layout_manager_selected = "Model".to_string();
                     self.layout_manager_rename_buf = String::new();
@@ -2658,8 +2657,7 @@ impl OpenCADStudio {
             Message::LayoutManagerSetCurrent => {
                 let i = self.active_tab;
                 let name = self.layout_manager_selected.clone();
-                self.tabs[i].scene.current_layout = name.clone();
-                self.tabs[i].scene.bump_geometry();
+                self.tabs[i].scene.set_current_layout(name.clone());
                 self.command_line
                     .push_output(&format!("Switched to layout '{name}'."));
                 Task::none()

src/app/update/viewport.rs

@@ -3050,7 +3050,7 @@ pub(super) fn on_tick(&mut self, t: Instant) -> Task<Message> {
                                 }
                             }
                         }
-                        self.tabs[i].scene.current_layout = new_name.clone();
+                        self.tabs[i].scene.set_current_layout(new_name.clone());
                         // Safety net — `add_layout` already creates the overall
                         // sheet viewport; this covers any path that doesn't.
                         self.tabs[i].scene.ensure_sheet_viewport(&new_name);
@@ -3085,7 +3085,7 @@ pub(super) fn on_tick(&mut self, t: Instant) -> Task<Message> {
                             self.push_undo_snapshot(i, "LAYOUT RENAME");
                             self.tabs[i].scene.rename_layout(&orig, &new_name);
                             if self.tabs[i].scene.current_layout == orig {
-                                self.tabs[i].scene.current_layout = new_name.clone();
+                                self.tabs[i].scene.set_current_layout(new_name.clone());
                             }
                             self.tabs[i].dirty = true;
                             self.command_line

src/io/mod.rs

@@ -530,6 +530,20 @@ fn set_vardict_value(doc: &mut CadDocument, name: &str, value: &str) {
     }
 }
 
+/// The layout tab that was active when the drawing was saved — the `CTAB`
+/// system variable (stored in the variable dictionary as a `DICTIONARYVAR`).
+/// `None` when the file recorded no current tab.
+pub fn saved_active_layout(doc: &CadDocument) -> Option<String> {
+    vardict_value(doc, "CTAB").filter(|s| !s.is_empty())
+}
+
+/// Record `name` as the active layout tab (`CTAB`) so the next save round-trips
+/// which space was open. No-op when the document has no `CTAB` entry yet; the
+/// `$TILEMODE` header (model vs paper) remains the guaranteed fallback.
+pub fn set_saved_active_layout(doc: &mut CadDocument, name: &str) {
+    set_vardict_value(doc, "CTAB", name);
+}
+
 /// Materialise the current-style choices into their format-specific storage
 /// before saving, treating the current-style *names* as the single source of
 /// truth:

src/scene/layout.rs

@@ -89,6 +89,7 @@ impl Scene {
         // If the deleted layout was active, fall back to Model space.
         if self.current_layout == name {
             self.current_layout = "Model".to_string();
+            self.sync_active_space_to_document();
         }
 
         self.bump_geometry();

src/scene/mod.rs

@@ -1333,11 +1333,22 @@ impl Scene {
     pub fn set_current_layout(&mut self, name: String) {
         if self.current_layout != name {
             self.current_layout = name;
+            self.sync_active_space_to_document();
             self.recolor_meshes();
             self.bump_geometry();
         }
     }
 
+    /// Mirror the active space (Model tile-mode vs a paper layout) into the
+    /// document's persisted settings so it round-trips on save: the `$TILEMODE`
+    /// header (`show_model_space`) and the `CTAB` current-tab variable. The
+    /// reader restores it via [`current_layout`] on open. Called whenever the
+    /// active layout changes; the file otherwise always reopened in Model.
+    pub fn sync_active_space_to_document(&mut self) {
+        self.document.header.show_model_space = self.current_layout == "Model";
+        crate::io::set_saved_active_layout(&mut self.document, &self.current_layout);
+    }
+
     /// Returns true if this viewport should display model-space content
     /// (i.e. it is a user viewport, not the sheet/overall viewport).
     ///