diff --git a/src/draw.rs b/src/draw.rs
index 326ce25..660a3e3 100644
--- a/src/draw.rs
+++ b/src/draw.rs
@@ -188,8 +188,9 @@ fn draw(f: &mut Frame, app: &App) {
 }
 
 fn draw_title(f: &mut Frame, area: Rect, app: &App) {
-    let dirty = if app.dirty { " [+]" } else { "" };
+    let dirty = if app.model_state.dirty { " [+]" } else { "" };
     let file = app
+        .model_state
         .file_path
         .as_ref()
         .and_then(|p| p.file_name())
@@ -198,7 +199,7 @@ fn draw_title(f: &mut Frame, area: Rect, app: &App) {
         .unwrap_or_default();
     let title = format!(
         "  improvise  ·  {}{}{} ",
-        app.workbook.model.name, file, dirty
+        app.model_state.workbook.model.name, file, dirty
     );
     let right = " ?:help  :q quit ";
     let line = fill_line(title, right, area.width);
@@ -240,15 +241,15 @@ fn draw_content(f: &mut Frame, area: Rect, app: &App) {
 
         if app.formula_panel_open {
             let a = Rect::new(side.x, y, side.width, ph);
-            let content = FormulaContent::new(&app.workbook.model, &app.mode);
+            let content = FormulaContent::new(&app.model_state.workbook.model, &app.mode);
             f.render_widget(Panel::new(content, &app.mode, app.formula_cursor), a);
             y += ph;
         }
         if app.category_panel_open {
             let a = Rect::new(side.x, y, side.width, ph);
             let content = CategoryContent::new(
-                &app.workbook.model,
-                app.workbook.active_view(),
+                &app.model_state.workbook.model,
+                app.model_state.workbook.active_view(),
                 &app.expanded_cats,
             );
             f.render_widget(Panel::new(content, &app.mode, app.cat_panel_cursor), a);
@@ -256,7 +257,7 @@ fn draw_content(f: &mut Frame, area: Rect, app: &App) {
         }
         if app.view_panel_open {
             let a = Rect::new(side.x, y, side.width, ph);
-            let content = ViewContent::new(&app.workbook);
+            let content = ViewContent::new(&app.model_state.workbook);
             f.render_widget(Panel::new(content, &app.mode, app.view_panel_cursor), a);
         }
     } else {
@@ -265,9 +266,9 @@ fn draw_content(f: &mut Frame, area: Rect, app: &App) {
 
     f.render_widget(
         GridWidget::new(
-            &app.workbook.model,
-            app.workbook.active_view(),
-            &app.workbook.active_view,
+            &app.model_state.workbook.model,
+            app.model_state.workbook.active_view(),
+            &app.model_state.workbook.active_view,
             &app.layout,
             &app.mode,
             &app.search_query,
@@ -281,8 +282,8 @@ fn draw_content(f: &mut Frame, area: Rect, app: &App) {
 fn draw_tile_bar(f: &mut Frame, area: Rect, app: &App) {
     f.render_widget(
         TileBar::new(
-            &app.workbook.model,
-            app.workbook.active_view(),
+            &app.model_state.workbook.model,
+            app.model_state.workbook.active_view(),
             &app.mode,
             app.tile_cat_idx,
         ),
@@ -326,7 +327,7 @@ fn draw_status(f: &mut Frame, area: Rect, app: &App) {
     };
 
     let yank_indicator = if app.yanked.is_some() { " [yank]" } else { "" };
-    let view_badge = format!("  {}{}  ", app.workbook.active_view, yank_indicator);
+    let view_badge = format!("  {}{}  ", app.model_state.workbook.active_view, yank_indicator);
 
     let left = format!("  {}{search_part}  {msg}", mode_name(&app.mode));
     let line = fill_line(left, &view_badge, area.width);
diff --git a/src/main.rs b/src/main.rs
index 7f6339f..fadaf18 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -353,7 +353,7 @@ fn run_headless_commands(cmds: &[String], file: &Option<PathBuf>) -> Result<()>
     }
 
     if let Some(path) = file {
-        persistence::save(&app.workbook, path)?;
+        persistence::save(&app.model_state.workbook, path)?;
     }
 
     std::process::exit(exit_code);
diff --git a/src/ui/app.rs b/src/ui/app.rs
index dd063a5..c455c2e 100644
--- a/src/ui/app.rs
+++ b/src/ui/app.rs
@@ -187,10 +187,23 @@ impl AppMode {
 
 /// Document state slice: the workbook and its IO bookkeeping. Distinct from
 /// `Workbook` itself (which is pure document semantics in `improvise-core`)
-/// because `file_path` and `dirty` are persistence-layer concerns. Filled in
-/// by improvise-x2c (vb4 step 2).
-#[derive(Debug, Default)]
-pub struct ModelState {}
+/// because `file_path` and `dirty` are persistence-layer concerns.
+#[derive(Debug)]
+pub struct ModelState {
+    pub workbook: Workbook,
+    pub file_path: Option<PathBuf>,
+    pub dirty: bool,
+}
+
+impl Default for ModelState {
+    fn default() -> Self {
+        Self {
+            workbook: Workbook::new("Untitled"),
+            file_path: None,
+            dirty: false,
+        }
+    }
+}
 
 /// UI session-state slice: mode, cursors, panels, buffers, navigation stacks,
 /// and other per-session state that does not persist to disk. Filled in by
@@ -199,8 +212,7 @@ pub struct ModelState {}
 pub struct ViewState {}
 
 pub struct App {
-    pub workbook: Workbook,
-    pub file_path: Option<PathBuf>,
+    pub model_state: ModelState,
     pub mode: AppMode,
     pub status_msg: String,
     pub wizard: Option<ImportWizard>,
@@ -213,7 +225,6 @@ pub struct App {
     pub cat_panel_cursor: usize,
     pub view_panel_cursor: usize,
     pub formula_cursor: usize,
-    pub dirty: bool,
     /// Yanked cell value for `p` paste
     pub yanked: Option<CellValue>,
     /// Tile select cursor (which category index is highlighted)
@@ -264,8 +275,11 @@ impl App {
             GridLayout::with_frozen_records(&workbook.model, view, None)
         };
         Self {
-            workbook,
-            file_path,
+            model_state: ModelState {
+                workbook,
+                file_path,
+                dirty: false,
+            },
             mode: AppMode::Normal,
             status_msg: String::new(),
             wizard: None,
@@ -278,7 +292,6 @@ impl App {
             cat_panel_cursor: 0,
             view_panel_cursor: 0,
             formula_cursor: 0,
-            dirty: false,
             yanked: None,
             tile_cat_idx: 0,
             view_back_stack: Vec::new(),
@@ -299,20 +312,20 @@ impl App {
     /// Rebuild the grid layout from current workbook, active view, and drill
     /// state. Note: `with_frozen_records` already handles pruning internally.
     pub fn rebuild_layout(&mut self) {
-        let none_cats = self.workbook.active_view().none_cats();
-        self.workbook.model.recompute_formulas(&none_cats);
-        let view = self.workbook.active_view();
+        let none_cats = self.model_state.workbook.active_view().none_cats();
+        self.model_state.workbook.model.recompute_formulas(&none_cats);
+        let view = self.model_state.workbook.active_view();
         let frozen = self.drill_state.as_ref().map(|s| Rc::clone(&s.records));
-        self.layout = GridLayout::with_frozen_records(&self.workbook.model, view, frozen);
+        self.layout = GridLayout::with_frozen_records(&self.model_state.workbook.model, view, frozen);
     }
 
     pub fn cmd_context(&self, key: KeyCode, _mods: KeyModifiers) -> CmdContext<'_> {
-        let view = self.workbook.active_view();
+        let view = self.model_state.workbook.active_view();
         let layout = &self.layout;
         let (sel_row, sel_col) = view.selected;
         CmdContext {
-            model: &self.workbook.model,
-            workbook: &self.workbook,
+            model: &self.model_state.workbook.model,
+            workbook: &self.model_state.workbook,
             view,
             layout,
             registry: self.keymap_set.registry(),
@@ -322,7 +335,7 @@ impl App {
             col_offset: view.col_offset,
             search_query: &self.search_query,
             yanked: &self.yanked,
-            dirty: self.dirty,
+            dirty: self.model_state.dirty,
             search_mode: self.search_mode,
             formula_panel_open: self.formula_panel_open,
             category_panel_open: self.category_panel_open,
@@ -345,7 +358,7 @@ impl App {
                             .or_else(|| layout.resolve_display(k))
                             .unwrap_or_default()
                     } else {
-                        self.workbook
+                        self.model_state.workbook
                             .model
                             .get_cell(k)
                             .map(|v| v.to_string())
@@ -359,7 +372,7 @@ impl App {
             visible_cols: {
                 let (fmt_comma, fmt_decimals) = parse_number_format(&view.number_format);
                 let col_widths =
-                    compute_col_widths(&self.workbook.model, layout, fmt_comma, fmt_decimals);
+                    compute_col_widths(&self.model_state.workbook.model, layout, fmt_comma, fmt_decimals);
                 let row_header_width = compute_row_header_width(layout);
                 compute_visible_cols(
                     &col_widths,
@@ -392,7 +405,7 @@ impl App {
     /// Virtual categories (_Index, _Dim, _Measure) are always present and don't count.
     pub fn is_empty_model(&self) -> bool {
         use crate::model::category::CategoryKind;
-        self.workbook.model.categories.values().all(|c| {
+        self.model_state.workbook.model.categories.values().all(|c| {
             matches!(
                 c.kind,
                 CategoryKind::VirtualIndex
@@ -431,12 +444,12 @@ impl App {
     }
 
     pub fn autosave_if_needed(&mut self) {
-        if self.dirty
+        if self.model_state.dirty
             && self.last_autosave.elapsed() > Duration::from_secs(30)
-            && let Some(path) = &self.file_path.clone()
+            && let Some(path) = &self.model_state.file_path.clone()
         {
             let ap = persistence::autosave_path(path);
-            let _ = persistence::save(&self.workbook, &ap);
+            let _ = persistence::save(&self.model_state.workbook, &ap);
             self.last_autosave = Instant::now();
         }
     }
@@ -494,6 +507,16 @@ mod tests {
         let _: ViewState = ViewState::default();
     }
 
+    /// improvise-x2c: ModelState owns the document slice — workbook,
+    /// file_path, and dirty. App accesses them through model_state.
+    #[test]
+    fn app_model_state_owns_workbook_file_path_and_dirty() {
+        let app = App::new(Workbook::new("T"), Some(PathBuf::from("/tmp/x")));
+        let _: &Workbook = &app.model_state.workbook;
+        let _: &Option<PathBuf> = &app.model_state.file_path;
+        let _: bool = app.model_state.dirty;
+    }
+
     fn two_col_model() -> App {
         let mut wb = Workbook::new("T");
         wb.add_category("Row").unwrap(); // → Row axis
@@ -515,7 +538,7 @@ mod tests {
 
     fn enter_advance_cmd(app: &App) -> crate::command::cmd::navigation::EnterAdvance {
         use crate::command::cmd::navigation::CursorState;
-        let view = app.workbook.active_view();
+        let view = app.model_state.workbook.active_view();
         let cursor = CursorState {
             row: view.selected.0,
             col: view.selected.1,
@@ -532,29 +555,29 @@ mod tests {
     #[test]
     fn enter_advance_moves_down_within_column() {
         let mut app = two_col_model();
-        app.workbook.active_view_mut().selected = (0, 0);
+        app.model_state.workbook.active_view_mut().selected = (0, 0);
         let cmd = enter_advance_cmd(&app);
         run_cmd(&mut app, &cmd);
-        assert_eq!(app.workbook.active_view().selected, (1, 0));
+        assert_eq!(app.model_state.workbook.active_view().selected, (1, 0));
     }
 
     #[test]
     fn enter_advance_wraps_to_top_of_next_column() {
         let mut app = two_col_model();
         // row_max = 2 (A,B,C), col 0 → should wrap to (0, 1)
-        app.workbook.active_view_mut().selected = (2, 0);
+        app.model_state.workbook.active_view_mut().selected = (2, 0);
         let cmd = enter_advance_cmd(&app);
         run_cmd(&mut app, &cmd);
-        assert_eq!(app.workbook.active_view().selected, (0, 1));
+        assert_eq!(app.model_state.workbook.active_view().selected, (0, 1));
     }
 
     #[test]
     fn enter_advance_stays_at_bottom_right() {
         let mut app = two_col_model();
-        app.workbook.active_view_mut().selected = (2, 1);
+        app.model_state.workbook.active_view_mut().selected = (2, 1);
         let cmd = enter_advance_cmd(&app);
         run_cmd(&mut app, &cmd);
-        assert_eq!(app.workbook.active_view().selected, (2, 1));
+        assert_eq!(app.model_state.workbook.active_view().selected, (2, 1));
     }
 
     #[test]
@@ -633,34 +656,34 @@ mod tests {
         }
 
         assert_eq!(
-            app.workbook.active_view().selected.1,
+            app.model_state.workbook.active_view().selected.1,
             3,
             "cursor should be at column 3"
         );
         assert!(
-            app.workbook.active_view().col_offset > 0,
+            app.model_state.workbook.active_view().col_offset > 0,
             "col_offset should scroll when cursor moves past visible area (only ~2 cols fit \
              in 80-char terminal with 26-char-wide columns), but col_offset is {}",
-            app.workbook.active_view().col_offset
+            app.model_state.workbook.active_view().col_offset
         );
     }
 
     #[test]
     fn home_jumps_to_first_col() {
         let mut app = two_col_model();
-        app.workbook.active_view_mut().selected = (1, 1);
+        app.model_state.workbook.active_view_mut().selected = (1, 1);
         app.handle_key(KeyEvent::new(KeyCode::Home, KeyModifiers::NONE))
             .unwrap();
-        assert_eq!(app.workbook.active_view().selected, (1, 0));
+        assert_eq!(app.model_state.workbook.active_view().selected, (1, 0));
     }
 
     #[test]
     fn end_jumps_to_last_col() {
         let mut app = two_col_model();
-        app.workbook.active_view_mut().selected = (1, 0);
+        app.model_state.workbook.active_view_mut().selected = (1, 0);
         app.handle_key(KeyEvent::new(KeyCode::End, KeyModifiers::NONE))
             .unwrap();
-        assert_eq!(app.workbook.active_view().selected, (1, 1));
+        assert_eq!(app.model_state.workbook.active_view().selected, (1, 1));
     }
 
     #[test]
@@ -668,40 +691,40 @@ mod tests {
         let mut app = two_col_model();
         // Add enough rows
         for i in 0..30 {
-            app.workbook
+            app.model_state.workbook
                 .model
                 .category_mut("Row")
                 .unwrap()
                 .add_item(format!("R{i}"));
         }
         app.term_height = 28; // ~20 visible rows → delta = 15
-        app.workbook.active_view_mut().selected = (0, 0);
+        app.model_state.workbook.active_view_mut().selected = (0, 0);
         app.handle_key(KeyEvent::new(KeyCode::PageDown, KeyModifiers::NONE))
             .unwrap();
-        assert_eq!(app.workbook.active_view().selected.1, 0, "column preserved");
+        assert_eq!(app.model_state.workbook.active_view().selected.1, 0, "column preserved");
         assert!(
-            app.workbook.active_view().selected.0 > 0,
+            app.model_state.workbook.active_view().selected.0 > 0,
             "row should advance on PageDown"
         );
         // 3/4 of ~20 = 15
-        assert_eq!(app.workbook.active_view().selected.0, 15);
+        assert_eq!(app.model_state.workbook.active_view().selected.0, 15);
     }
 
     #[test]
     fn page_up_scrolls_backward() {
         let mut app = two_col_model();
         for i in 0..30 {
-            app.workbook
+            app.model_state.workbook
                 .model
                 .category_mut("Row")
                 .unwrap()
                 .add_item(format!("R{i}"));
         }
         app.term_height = 28;
-        app.workbook.active_view_mut().selected = (20, 0);
+        app.model_state.workbook.active_view_mut().selected = (20, 0);
         app.handle_key(KeyEvent::new(KeyCode::PageUp, KeyModifiers::NONE))
             .unwrap();
-        assert_eq!(app.workbook.active_view().selected.0, 5);
+        assert_eq!(app.model_state.workbook.active_view().selected.0, 5);
     }
 
     #[test]
@@ -709,22 +732,22 @@ mod tests {
         let mut app = two_col_model();
         // Total rows: A, B, C + R0..R9 = 13 rows. Last row = 12.
         for i in 0..10 {
-            app.workbook
+            app.model_state.workbook
                 .model
                 .category_mut("Row")
                 .unwrap()
                 .add_item(format!("R{i}"));
         }
         app.term_height = 13; // ~5 visible rows
-        app.workbook.active_view_mut().selected = (0, 0);
+        app.model_state.workbook.active_view_mut().selected = (0, 0);
         // G jumps to last row (row 12)
         app.handle_key(KeyEvent::new(KeyCode::Char('G'), KeyModifiers::NONE))
             .unwrap();
-        let last = app.workbook.active_view().selected.0;
+        let last = app.model_state.workbook.active_view().selected.0;
         assert_eq!(last, 12, "should be at last row");
         // With only ~5 visible rows and 13 rows, offset should scroll.
         // Bug: hardcoded 20 means `12 >= 0 + 20` is false → no scroll.
-        let offset = app.workbook.active_view().row_offset;
+        let offset = app.model_state.workbook.active_view().row_offset;
         assert!(
             offset > 0,
             "row_offset should scroll when last row is beyond visible area, but is {offset}"
@@ -735,34 +758,34 @@ mod tests {
     fn ctrl_d_scrolls_viewport_with_small_terminal() {
         let mut app = two_col_model();
         for i in 0..30 {
-            app.workbook
+            app.model_state.workbook
                 .model
                 .category_mut("Row")
                 .unwrap()
                 .add_item(format!("R{i}"));
         }
         app.term_height = 13; // ~5 visible rows
-        app.workbook.active_view_mut().selected = (0, 0);
+        app.model_state.workbook.active_view_mut().selected = (0, 0);
         // Ctrl+d scrolls by 5 rows
         app.handle_key(KeyEvent::new(KeyCode::Char('d'), KeyModifiers::CONTROL))
             .unwrap();
-        assert_eq!(app.workbook.active_view().selected.0, 5);
+        assert_eq!(app.model_state.workbook.active_view().selected.0, 5);
         // Press Ctrl+d again — now at row 10 with only 5 visible rows,
         // row_offset should have scrolled (not stay at 0 due to hardcoded 20)
         app.handle_key(KeyEvent::new(KeyCode::Char('d'), KeyModifiers::CONTROL))
             .unwrap();
-        assert_eq!(app.workbook.active_view().selected.0, 10);
+        assert_eq!(app.model_state.workbook.active_view().selected.0, 10);
         assert!(
-            app.workbook.active_view().row_offset > 0,
+            app.model_state.workbook.active_view().row_offset > 0,
             "row_offset should scroll with small terminal, but is {}",
-            app.workbook.active_view().row_offset
+            app.model_state.workbook.active_view().row_offset
         );
     }
 
     #[test]
     fn tab_in_edit_mode_commits_and_moves_right() {
         let mut app = two_col_model();
-        app.workbook.active_view_mut().selected = (0, 0);
+        app.model_state.workbook.active_view_mut().selected = (0, 0);
         // Enter edit mode
         app.handle_key(KeyEvent::new(KeyCode::Char('i'), KeyModifiers::NONE))
             .unwrap();
@@ -780,7 +803,7 @@ mod tests {
             app.mode
         );
         assert_eq!(
-            app.workbook.active_view().selected.1,
+            app.model_state.workbook.active_view().selected.1,
             1,
             "should have moved to column 1"
         );
@@ -885,7 +908,7 @@ mod tests {
             .unwrap();
 
         assert_eq!(
-            app.workbook.model.get_cell(&CellKey::new(vec![(
+            app.model_state.workbook.model.get_cell(&CellKey::new(vec![(
                 "_Measure".to_string(),
                 "Rev".to_string(),
             )])),
@@ -957,11 +980,11 @@ mod tests {
         use crate::model::cell::{CellKey, CellValue};
         let mut app = records_model_with_two_rows();
         // Simulate Tab-at-bottom-right having produced an empty-key cell.
-        app.workbook
+        app.model_state.workbook
             .model
             .set_cell(CellKey::new(vec![]), CellValue::Number(0.0));
         assert!(
-            app.workbook
+            app.model_state.workbook
                 .model
                 .data
                 .iter_cells()
@@ -976,7 +999,7 @@ mod tests {
             "setup: should have left records mode"
         );
         assert!(
-            !app.workbook
+            !app.model_state.workbook
                 .model
                 .data
                 .iter_cells()
@@ -994,7 +1017,7 @@ mod tests {
         let mut app = records_model_with_two_rows();
         let last_row = app.layout.row_count() - 1;
         let last_col = app.layout.col_count() - 1;
-        app.workbook.active_view_mut().selected = (last_row, last_col);
+        app.model_state.workbook.active_view_mut().selected = (last_row, last_col);
 
         app.handle_key(KeyEvent::new(KeyCode::Char('i'), KeyModifiers::NONE))
             .unwrap();
@@ -1025,7 +1048,7 @@ mod tests {
 
         let last_row = initial_rows - 1;
         let last_col = app.layout.col_count() - 1;
-        app.workbook.active_view_mut().selected = (last_row, last_col);
+        app.model_state.workbook.active_view_mut().selected = (last_row, last_col);
 
         // Enter edit mode on the bottom-right cell
         app.handle_key(KeyEvent::new(KeyCode::Char('i'), KeyModifiers::NONE))
@@ -1042,7 +1065,7 @@ mod tests {
             "TAB on bottom-right should insert a record below"
         );
         assert_eq!(
-            app.workbook.active_view().selected,
+            app.model_state.workbook.active_view().selected,
             (initial_rows, 0),
             "TAB should move to first cell of the new row"
         );
@@ -1078,7 +1101,7 @@ mod tests {
         let value_col = (0..app.layout.col_count())
             .find(|&col| app.layout.col_label(col) == "Value")
             .expect("drill view should include a Value column");
-        app.workbook.active_view_mut().selected = (0, value_col);
+        app.model_state.workbook.active_view_mut().selected = (0, value_col);
         app.handle_key(KeyEvent::new(KeyCode::Char('i'), KeyModifiers::NONE))
             .unwrap();
         app.handle_key(KeyEvent::new(KeyCode::Backspace, KeyModifiers::NONE))
@@ -1089,7 +1112,7 @@ mod tests {
             .unwrap();
 
         assert_eq!(
-            app.workbook.model.get_cell(&record_key),
+            app.model_state.workbook.model.get_cell(&record_key),
             Some(&CellValue::Number(1.0)),
             "drill edit should remain staged until leaving the drill view"
         );
@@ -1107,7 +1130,7 @@ mod tests {
             .unwrap();
 
         assert_eq!(
-            app.workbook.model.get_cell(&record_key),
+            app.model_state.workbook.model.get_cell(&record_key),
             Some(&CellValue::Number(9.0)),
             "leaving drill view should apply the staged edit"
         );
@@ -1140,7 +1163,7 @@ mod tests {
             .unwrap();
 
         assert!(
-            !app.workbook
+            !app.model_state.workbook
                 .model
                 .category("Region")
                 .unwrap()
diff --git a/src/ui/effect.rs b/src/ui/effect.rs
index bdf7285..208209a 100644
--- a/src/ui/effect.rs
+++ b/src/ui/effect.rs
@@ -24,7 +24,7 @@ pub trait Effect: Debug {
 pub struct AddCategory(pub String);
 impl Effect for AddCategory {
     fn apply(&self, app: &mut App) {
-        let _ = app.workbook.add_category(&self.0);
+        let _ = app.model_state.workbook.add_category(&self.0);
     }
 }
 
@@ -35,7 +35,7 @@ pub struct AddItem {
 }
 impl Effect for AddItem {
     fn apply(&self, app: &mut App) {
-        if let Some(cat) = app.workbook.model.category_mut(&self.category) {
+        if let Some(cat) = app.model_state.workbook.model.category_mut(&self.category) {
             cat.add_item(&self.item);
         } else {
             app.status_msg = format!("Unknown category '{}'", self.category);
@@ -51,7 +51,7 @@ pub struct AddItemInGroup {
 }
 impl Effect for AddItemInGroup {
     fn apply(&self, app: &mut App) {
-        if let Some(cat) = app.workbook.model.category_mut(&self.category) {
+        if let Some(cat) = app.model_state.workbook.model.category_mut(&self.category) {
             cat.add_item_in_group(&self.item, &self.group);
         } else {
             app.status_msg = format!("Unknown category '{}'", self.category);
@@ -63,7 +63,7 @@ impl Effect for AddItemInGroup {
 pub struct SortData;
 impl Effect for SortData {
     fn apply(&self, app: &mut App) {
-        app.workbook.model.data.sort_by_key();
+        app.model_state.workbook.model.data.sort_by_key();
     }
 }
 
@@ -71,7 +71,7 @@ impl Effect for SortData {
 pub struct SetCell(pub CellKey, pub CellValue);
 impl Effect for SetCell {
     fn apply(&self, app: &mut App) {
-        app.workbook.model.set_cell(self.0.clone(), self.1.clone());
+        app.model_state.workbook.model.set_cell(self.0.clone(), self.1.clone());
     }
 }
 
@@ -79,7 +79,7 @@ impl Effect for SetCell {
 pub struct ClearCell(pub CellKey);
 impl Effect for ClearCell {
     fn apply(&self, app: &mut App) {
-        app.workbook.model.clear_cell(&self.0);
+        app.model_state.workbook.model.clear_cell(&self.0);
     }
 }
 
@@ -96,11 +96,11 @@ impl Effect for AddFormula {
                 // appears in the grid. _Measure targets are dynamically included
                 // via Model::measure_item_names().
                 if formula.target_category != "_Measure"
-                    && let Some(cat) = app.workbook.model.category_mut(&formula.target_category)
+                    && let Some(cat) = app.model_state.workbook.model.category_mut(&formula.target_category)
                 {
                     cat.add_item(&formula.target);
                 }
-                app.workbook.model.add_formula(formula);
+                app.model_state.workbook.model.add_formula(formula);
             }
             Err(e) => {
                 app.status_msg = format!("Formula error: {e}");
@@ -116,7 +116,7 @@ pub struct RemoveFormula {
 }
 impl Effect for RemoveFormula {
     fn apply(&self, app: &mut App) {
-        app.workbook
+        app.model_state.workbook
             .model
             .remove_formula(&self.target, &self.target_category);
     }
@@ -153,7 +153,7 @@ impl Effect for EnterEditAtCursor {
 pub struct TogglePruneEmpty;
 impl Effect for TogglePruneEmpty {
     fn apply(&self, app: &mut App) {
-        let v = app.workbook.active_view_mut();
+        let v = app.model_state.workbook.active_view_mut();
         v.prune_empty = !v.prune_empty;
     }
 }
@@ -175,7 +175,7 @@ pub struct RemoveItem {
 }
 impl Effect for RemoveItem {
     fn apply(&self, app: &mut App) {
-        app.workbook.model.remove_item(&self.category, &self.item);
+        app.model_state.workbook.model.remove_item(&self.category, &self.item);
     }
 }
 
@@ -183,7 +183,7 @@ impl Effect for RemoveItem {
 pub struct RemoveCategory(pub String);
 impl Effect for RemoveCategory {
     fn apply(&self, app: &mut App) {
-        app.workbook.remove_category(&self.0);
+        app.model_state.workbook.remove_category(&self.0);
     }
 }
 
@@ -193,7 +193,7 @@ impl Effect for RemoveCategory {
 pub struct CreateView(pub String);
 impl Effect for CreateView {
     fn apply(&self, app: &mut App) {
-        app.workbook.create_view(&self.0);
+        app.model_state.workbook.create_view(&self.0);
     }
 }
 
@@ -201,7 +201,7 @@ impl Effect for CreateView {
 pub struct DeleteView(pub String);
 impl Effect for DeleteView {
     fn apply(&self, app: &mut App) {
-        let _ = app.workbook.delete_view(&self.0);
+        let _ = app.model_state.workbook.delete_view(&self.0);
     }
 }
 
@@ -209,7 +209,7 @@ impl Effect for DeleteView {
 pub struct SwitchView(pub String);
 impl Effect for SwitchView {
     fn apply(&self, app: &mut App) {
-        let current = app.workbook.active_view.clone();
+        let current = app.model_state.workbook.active_view.clone();
         if current != self.0 {
             app.view_back_stack.push(ViewFrame {
                 view_name: current,
@@ -217,7 +217,7 @@ impl Effect for SwitchView {
             });
             app.view_forward_stack.clear();
         }
-        let _ = app.workbook.switch_view(&self.0);
+        let _ = app.model_state.workbook.switch_view(&self.0);
     }
 }
 
@@ -227,12 +227,12 @@ pub struct ViewBack;
 impl Effect for ViewBack {
     fn apply(&self, app: &mut App) {
         if let Some(frame) = app.view_back_stack.pop() {
-            let current = app.workbook.active_view.clone();
+            let current = app.model_state.workbook.active_view.clone();
             app.view_forward_stack.push(ViewFrame {
                 view_name: current,
                 mode: app.mode.clone(),
             });
-            let _ = app.workbook.switch_view(&frame.view_name);
+            let _ = app.model_state.workbook.switch_view(&frame.view_name);
             app.mode = frame.mode;
         }
     }
@@ -244,12 +244,12 @@ pub struct ViewForward;
 impl Effect for ViewForward {
     fn apply(&self, app: &mut App) {
         if let Some(frame) = app.view_forward_stack.pop() {
-            let current = app.workbook.active_view.clone();
+            let current = app.model_state.workbook.active_view.clone();
             app.view_back_stack.push(ViewFrame {
                 view_name: current,
                 mode: app.mode.clone(),
             });
-            let _ = app.workbook.switch_view(&frame.view_name);
+            let _ = app.model_state.workbook.switch_view(&frame.view_name);
             app.mode = frame.mode;
         }
     }
@@ -262,7 +262,7 @@ pub struct SetAxis {
 }
 impl Effect for SetAxis {
     fn apply(&self, app: &mut App) {
-        app.workbook
+        app.model_state.workbook
             .active_view_mut()
             .set_axis(&self.category, self.axis);
     }
@@ -275,7 +275,7 @@ pub struct SetPageSelection {
 }
 impl Effect for SetPageSelection {
     fn apply(&self, app: &mut App) {
-        app.workbook
+        app.model_state.workbook
             .active_view_mut()
             .set_page_selection(&self.category, &self.item);
     }
@@ -288,7 +288,7 @@ pub struct ToggleGroup {
 }
 impl Effect for ToggleGroup {
     fn apply(&self, app: &mut App) {
-        app.workbook
+        app.model_state.workbook
             .active_view_mut()
             .toggle_group_collapse(&self.category, &self.group);
     }
@@ -301,7 +301,7 @@ pub struct HideItem {
 }
 impl Effect for HideItem {
     fn apply(&self, app: &mut App) {
-        app.workbook
+        app.model_state.workbook
             .active_view_mut()
             .hide_item(&self.category, &self.item);
     }
@@ -314,7 +314,7 @@ pub struct ShowItem {
 }
 impl Effect for ShowItem {
     fn apply(&self, app: &mut App) {
-        app.workbook
+        app.model_state.workbook
             .active_view_mut()
             .show_item(&self.category, &self.item);
     }
@@ -324,7 +324,7 @@ impl Effect for ShowItem {
 pub struct TransposeAxes;
 impl Effect for TransposeAxes {
     fn apply(&self, app: &mut App) {
-        app.workbook.active_view_mut().transpose_axes();
+        app.model_state.workbook.active_view_mut().transpose_axes();
     }
 }
 
@@ -332,7 +332,7 @@ impl Effect for TransposeAxes {
 pub struct CycleAxis(pub String);
 impl Effect for CycleAxis {
     fn apply(&self, app: &mut App) {
-        app.workbook.active_view_mut().cycle_axis(&self.0);
+        app.model_state.workbook.active_view_mut().cycle_axis(&self.0);
     }
 }
 
@@ -340,7 +340,7 @@ impl Effect for CycleAxis {
 pub struct SetNumberFormat(pub String);
 impl Effect for SetNumberFormat {
     fn apply(&self, app: &mut App) {
-        app.workbook.active_view_mut().number_format = self.0.clone();
+        app.model_state.workbook.active_view_mut().number_format = self.0.clone();
     }
 }
 
@@ -350,7 +350,7 @@ impl Effect for SetNumberFormat {
 pub struct SetSelected(pub usize, pub usize);
 impl Effect for SetSelected {
     fn apply(&self, app: &mut App) {
-        app.workbook.active_view_mut().selected = (self.0, self.1);
+        app.model_state.workbook.active_view_mut().selected = (self.0, self.1);
     }
 }
 
@@ -358,7 +358,7 @@ impl Effect for SetSelected {
 pub struct SetRowOffset(pub usize);
 impl Effect for SetRowOffset {
     fn apply(&self, app: &mut App) {
-        app.workbook.active_view_mut().row_offset = self.0;
+        app.model_state.workbook.active_view_mut().row_offset = self.0;
     }
 }
 
@@ -366,7 +366,7 @@ impl Effect for SetRowOffset {
 pub struct SetColOffset(pub usize);
 impl Effect for SetColOffset {
     fn apply(&self, app: &mut App) {
-        app.workbook.active_view_mut().col_offset = self.0;
+        app.model_state.workbook.active_view_mut().col_offset = self.0;
     }
 }
 
@@ -395,7 +395,7 @@ impl Effect for SetStatus {
 pub struct MarkDirty;
 impl Effect for MarkDirty {
     fn apply(&self, app: &mut App) {
-        app.dirty = true;
+        app.model_state.dirty = true;
     }
 }
 
@@ -480,25 +480,25 @@ impl Effect for ApplyAndClearDrill {
             if col_name == "Value" {
                 // Update the cell's value
                 let value = if new_value.is_empty() {
-                    app.workbook.model.clear_cell(orig_key);
+                    app.model_state.workbook.model.clear_cell(orig_key);
                     continue;
                 } else if let Ok(n) = new_value.parse::<f64>() {
                     CellValue::Number(n)
                 } else {
                     CellValue::Text(new_value.clone())
                 };
-                app.workbook.model.set_cell(orig_key.clone(), value);
+                app.model_state.workbook.model.set_cell(orig_key.clone(), value);
             } else {
                 if new_value.is_empty() {
                     app.status_msg = RECORD_COORDS_CANNOT_BE_EMPTY.to_string();
                     continue;
                 }
                 // Rename a coordinate: remove old cell, insert new with updated coord
-                let value = match app.workbook.model.get_cell(orig_key) {
+                let value = match app.model_state.workbook.model.get_cell(orig_key) {
                     Some(v) => v.clone(),
                     None => continue,
                 };
-                app.workbook.model.clear_cell(orig_key);
+                app.model_state.workbook.model.clear_cell(orig_key);
                 // Build new key by replacing the coord
                 let new_coords: Vec<(String, String)> = orig_key
                     .0
@@ -513,13 +513,13 @@ impl Effect for ApplyAndClearDrill {
                     .collect();
                 let new_key = CellKey::new(new_coords);
                 // Ensure the new item exists in that category
-                if let Some(cat) = app.workbook.model.category_mut(col_name) {
+                if let Some(cat) = app.model_state.workbook.model.category_mut(col_name) {
                     cat.add_item(new_value.clone());
                 }
-                app.workbook.model.set_cell(new_key, value);
+                app.model_state.workbook.model.set_cell(new_key, value);
             }
         }
-        app.dirty = true;
+        app.model_state.dirty = true;
     }
 }
 
@@ -547,10 +547,10 @@ impl Effect for SetDrillPendingEdit {
 pub struct Save;
 impl Effect for Save {
     fn apply(&self, app: &mut App) {
-        if let Some(ref path) = app.file_path {
-            match crate::persistence::save(&app.workbook, path) {
+        if let Some(ref path) = app.model_state.file_path {
+            match crate::persistence::save(&app.model_state.workbook, path) {
                 Ok(()) => {
-                    app.dirty = false;
+                    app.model_state.dirty = false;
                     app.status_msg = format!("Saved to {}", path.display());
                 }
                 Err(e) => {
@@ -567,10 +567,10 @@ impl Effect for Save {
 pub struct SaveAs(pub PathBuf);
 impl Effect for SaveAs {
     fn apply(&self, app: &mut App) {
-        match crate::persistence::save(&app.workbook, &self.0) {
+        match crate::persistence::save(&app.model_state.workbook, &self.0) {
             Ok(()) => {
-                app.file_path = Some(self.0.clone());
-                app.dirty = false;
+                app.model_state.file_path = Some(self.0.clone());
+                app.model_state.dirty = false;
                 app.status_msg = format!("Saved to {}", self.0.display());
             }
             Err(e) => {
@@ -685,9 +685,9 @@ impl Effect for WizardKey {
                 crossterm::event::KeyCode::Enter => match wizard.build_model() {
                     Ok(mut workbook) => {
                         workbook.normalize_view_state();
-                        app.workbook = workbook;
+                        app.model_state.workbook = workbook;
                         app.formula_cursor = 0;
-                        app.dirty = true;
+                        app.model_state.dirty = true;
                         app.status_msg = "Import successful! Press :w <path> to save.".to_string();
                         app.mode = AppMode::Normal;
                         app.wizard = None;
@@ -738,8 +738,8 @@ impl Effect for StartImportWizard {
 pub struct ExportCsv(pub PathBuf);
 impl Effect for ExportCsv {
     fn apply(&self, app: &mut App) {
-        let view_name = app.workbook.active_view.clone();
-        match crate::persistence::export_csv(&app.workbook, &view_name, &self.0) {
+        let view_name = app.model_state.workbook.active_view.clone();
+        match crate::persistence::export_csv(&app.model_state.workbook, &view_name, &self.0) {
             Ok(()) => {
                 app.status_msg = format!("Exported to {}", self.0.display());
             }
@@ -758,7 +758,7 @@ impl Effect for LoadModel {
         match crate::persistence::load(&self.0) {
             Ok(mut loaded) => {
                 loaded.normalize_view_state();
-                app.workbook = loaded;
+                app.model_state.workbook = loaded;
                 app.status_msg = format!("Loaded from {}", self.0.display());
             }
             Err(e) => {
@@ -869,7 +869,7 @@ impl Effect for ImportJsonHeadless {
 
         match pipeline.build_model() {
             Ok(new_workbook) => {
-                app.workbook = new_workbook;
+                app.model_state.workbook = new_workbook;
                 app.status_msg = "Imported successfully".to_string();
             }
             Err(e) => {
@@ -953,6 +953,8 @@ pub struct CleanEmptyRecords;
 impl Effect for CleanEmptyRecords {
     fn apply(&self, app: &mut App) {
         let empties: Vec<CellKey> = app
+            .model_state
+
             .workbook
             .model
             .data
@@ -960,7 +962,7 @@ impl Effect for CleanEmptyRecords {
             .filter_map(|(k, _)| if k.0.is_empty() { Some(k) } else { None })
             .collect();
         for key in empties {
-            app.workbook.model.clear_cell(&key);
+            app.model_state.workbook.model.clear_cell(&key);
         }
     }
 }
@@ -1045,7 +1047,7 @@ mod tests {
     fn add_category_effect() {
         let mut app = test_app();
         AddCategory("Region".to_string()).apply(&mut app);
-        assert!(app.workbook.model.category("Region").is_some());
+        assert!(app.model_state.workbook.model.category("Region").is_some());
     }
 
     #[test]
@@ -1057,6 +1059,8 @@ mod tests {
         }
         .apply(&mut app);
         let items: Vec<&str> = app
+            .model_state
+
             .workbook
             .model
             .category("Type")
@@ -1087,12 +1091,12 @@ mod tests {
         ]);
         SetCell(key.clone(), CellValue::Number(42.0)).apply(&mut app);
         assert_eq!(
-            app.workbook.model.get_cell(&key),
+            app.model_state.workbook.model.get_cell(&key),
             Some(&CellValue::Number(42.0))
         );
 
         ClearCell(key.clone()).apply(&mut app);
-        assert_eq!(app.workbook.model.get_cell(&key), None);
+        assert_eq!(app.model_state.workbook.model.get_cell(&key), None);
     }
 
     #[test]
@@ -1103,7 +1107,7 @@ mod tests {
             target_category: "Type".to_string(),
         }
         .apply(&mut app);
-        assert!(!app.workbook.model.formulas().is_empty());
+        assert!(!app.model_state.workbook.model.formulas().is_empty());
     }
 
     /// Regression: AddFormula must add the target item to the target category
@@ -1114,7 +1118,7 @@ mod tests {
         let mut app = test_app();
         // "Margin" does not exist as an item in "Type" before adding the formula
         assert!(
-            !app.workbook
+            !app.model_state.workbook
                 .model
                 .category("Type")
                 .unwrap()
@@ -1127,6 +1131,8 @@ mod tests {
         }
         .apply(&mut app);
         let items: Vec<&str> = app
+            .model_state
+
             .workbook
             .model
             .category("Type")
@@ -1152,7 +1158,7 @@ mod tests {
         }
         .apply(&mut app);
         // Should appear in effective_item_names (used by layout)
-        let effective = app.workbook.model.effective_item_names("_Measure");
+        let effective = app.model_state.workbook.model.effective_item_names("_Measure");
         assert!(
             effective.contains(&"Margin".to_string()),
             "formula target 'Margin' should appear in effective _Measure items, got: {:?}",
@@ -1160,7 +1166,7 @@ mod tests {
         );
         // Should NOT be in the category's own items
         assert!(
-            !app.workbook
+            !app.model_state.workbook
                 .model
                 .category("_Measure")
                 .unwrap()
@@ -1189,13 +1195,13 @@ mod tests {
             target_category: "Type".to_string(),
         }
         .apply(&mut app);
-        assert!(!app.workbook.model.formulas().is_empty());
+        assert!(!app.model_state.workbook.model.formulas().is_empty());
         RemoveFormula {
             target: "Clothing".to_string(),
             target_category: "Type".to_string(),
         }
         .apply(&mut app);
-        assert!(app.workbook.model.formulas().is_empty());
+        assert!(app.model_state.workbook.model.formulas().is_empty());
     }
 
     // ── View effects ────────────────────────────────────────────────────
@@ -1203,11 +1209,11 @@ mod tests {
     #[test]
     fn switch_view_pushes_to_back_stack() {
         let mut app = test_app();
-        app.workbook.create_view("View 2");
+        app.model_state.workbook.create_view("View 2");
         assert!(app.view_back_stack.is_empty());
 
         SwitchView("View 2".to_string()).apply(&mut app);
-        assert_eq!(app.workbook.active_view.as_str(), "View 2");
+        assert_eq!(app.model_state.workbook.active_view.as_str(), "View 2");
         assert_eq!(app.view_back_stack.len(), 1);
         assert_eq!(app.view_back_stack[0].view_name, "Default");
         // Forward stack should be cleared
@@ -1224,20 +1230,20 @@ mod tests {
     #[test]
     fn view_back_and_forward() {
         let mut app = test_app();
-        app.workbook.create_view("View 2");
+        app.model_state.workbook.create_view("View 2");
         SwitchView("View 2".to_string()).apply(&mut app);
-        assert_eq!(app.workbook.active_view.as_str(), "View 2");
+        assert_eq!(app.model_state.workbook.active_view.as_str(), "View 2");
 
         // Go back
         ViewBack.apply(&mut app);
-        assert_eq!(app.workbook.active_view.as_str(), "Default");
+        assert_eq!(app.model_state.workbook.active_view.as_str(), "Default");
         assert_eq!(app.view_forward_stack.len(), 1);
         assert_eq!(app.view_forward_stack[0].view_name, "View 2");
         assert!(app.view_back_stack.is_empty());
 
         // Go forward
         ViewForward.apply(&mut app);
-        assert_eq!(app.workbook.active_view.as_str(), "View 2");
+        assert_eq!(app.model_state.workbook.active_view.as_str(), "View 2");
         assert_eq!(app.view_back_stack.len(), 1);
         assert_eq!(app.view_back_stack[0].view_name, "Default");
         assert!(app.view_forward_stack.is_empty());
@@ -1246,19 +1252,19 @@ mod tests {
     #[test]
     fn view_back_with_empty_stack_is_noop() {
         let mut app = test_app();
-        let before = app.workbook.active_view.clone();
+        let before = app.model_state.workbook.active_view.clone();
         ViewBack.apply(&mut app);
-        assert_eq!(app.workbook.active_view, before);
+        assert_eq!(app.model_state.workbook.active_view, before);
     }
 
     #[test]
     fn create_and_delete_view() {
         let mut app = test_app();
         CreateView("View 2".to_string()).apply(&mut app);
-        assert!(app.workbook.views.contains_key("View 2"));
+        assert!(app.model_state.workbook.views.contains_key("View 2"));
 
         DeleteView("View 2".to_string()).apply(&mut app);
-        assert!(!app.workbook.views.contains_key("View 2"));
+        assert!(!app.model_state.workbook.views.contains_key("View 2"));
     }
 
     #[test]
@@ -1269,13 +1275,15 @@ mod tests {
             axis: Axis::Page,
         }
         .apply(&mut app);
-        assert_eq!(app.workbook.active_view().axis_of("Type"), Axis::Page);
+        assert_eq!(app.model_state.workbook.active_view().axis_of("Type"), Axis::Page);
     }
 
     #[test]
     fn transpose_axes_effect() {
         let mut app = test_app();
         let row_before: Vec<String> = app
+            .model_state
+
             .workbook
             .active_view()
             .categories_on(Axis::Row)
@@ -1283,6 +1291,8 @@ mod tests {
             .map(String::from)
             .collect();
         let col_before: Vec<String> = app
+            .model_state
+
             .workbook
             .active_view()
             .categories_on(Axis::Column)
@@ -1291,6 +1301,8 @@ mod tests {
             .collect();
         TransposeAxes.apply(&mut app);
         let row_after: Vec<String> = app
+            .model_state
+
             .workbook
             .active_view()
             .categories_on(Axis::Row)
@@ -1298,6 +1310,8 @@ mod tests {
             .map(String::from)
             .collect();
         let col_after: Vec<String> = app
+            .model_state
+
             .workbook
             .active_view()
             .categories_on(Axis::Column)
@@ -1314,7 +1328,7 @@ mod tests {
     fn set_selected_effect() {
         let mut app = test_app();
         SetSelected(3, 5).apply(&mut app);
-        assert_eq!(app.workbook.active_view().selected, (3, 5));
+        assert_eq!(app.model_state.workbook.active_view().selected, (3, 5));
     }
 
     #[test]
@@ -1322,8 +1336,8 @@ mod tests {
         let mut app = test_app();
         SetRowOffset(10).apply(&mut app);
         SetColOffset(5).apply(&mut app);
-        assert_eq!(app.workbook.active_view().row_offset, 10);
-        assert_eq!(app.workbook.active_view().col_offset, 5);
+        assert_eq!(app.model_state.workbook.active_view().row_offset, 10);
+        assert_eq!(app.model_state.workbook.active_view().col_offset, 5);
     }
 
     // ── App state effects ───────────────────────────────────────────────
@@ -1369,7 +1383,7 @@ mod tests {
         let mut app = test_app();
         // An empty-key cell (the bug: produced by AddRecordRow when no page
         // filters are set).
-        app.workbook
+        app.model_state.workbook
             .model
             .set_cell(CellKey::new(vec![]), CellValue::Number(0.0));
         // Plus a well-formed cell that must survive.
@@ -1377,15 +1391,15 @@ mod tests {
             ("Type".to_string(), "Food".to_string()),
             ("Month".to_string(), "Jan".to_string()),
         ]);
-        app.workbook
+        app.model_state.workbook
             .model
             .set_cell(valid.clone(), CellValue::Number(42.0));
-        assert_eq!(app.workbook.model.data.iter_cells().count(), 2);
+        assert_eq!(app.model_state.workbook.model.data.iter_cells().count(), 2);
 
         CleanEmptyRecords.apply(&mut app);
 
         assert!(
-            !app.workbook
+            !app.model_state.workbook
                 .model
                 .data
                 .iter_cells()
@@ -1393,7 +1407,7 @@ mod tests {
             "empty-key cell should be gone"
         );
         assert_eq!(
-            app.workbook.model.get_cell(&valid),
+            app.model_state.workbook.model.get_cell(&valid),
             Some(&CellValue::Number(42.0)),
             "valid cell must survive"
         );
@@ -1459,9 +1473,9 @@ mod tests {
     #[test]
     fn mark_dirty_effect() {
         let mut app = test_app();
-        assert!(!app.dirty);
+        assert!(!app.model_state.dirty);
         MarkDirty.apply(&mut app);
-        assert!(app.dirty);
+        assert!(app.model_state.dirty);
     }
 
     #[test]
@@ -1587,7 +1601,7 @@ mod tests {
         // Apply with no pending edits — should just clear state
         ApplyAndClearDrill.apply(&mut app);
         assert!(app.drill_state.is_none());
-        assert!(!app.dirty); // no edits → not dirty
+        assert!(!app.model_state.dirty); // no edits → not dirty
     }
 
     #[test]
@@ -1598,7 +1612,7 @@ mod tests {
             ("Month".into(), "Jan".into()),
         ]);
         // Set original cell
-        app.workbook
+        app.model_state.workbook
             .model
             .set_cell(key.clone(), CellValue::Number(42.0));
 
@@ -1615,9 +1629,9 @@ mod tests {
 
         ApplyAndClearDrill.apply(&mut app);
         assert!(app.drill_state.is_none());
-        assert!(app.dirty);
+        assert!(app.model_state.dirty);
         assert_eq!(
-            app.workbook.model.get_cell(&key),
+            app.model_state.workbook.model.get_cell(&key),
             Some(&CellValue::Number(99.0))
         );
     }
@@ -1629,7 +1643,7 @@ mod tests {
             ("Type".into(), "Food".into()),
             ("Month".into(), "Jan".into()),
         ]);
-        app.workbook
+        app.model_state.workbook
             .model
             .set_cell(key.clone(), CellValue::Number(42.0));
 
@@ -1645,20 +1659,22 @@ mod tests {
         .apply(&mut app);
 
         ApplyAndClearDrill.apply(&mut app);
-        assert!(app.dirty);
+        assert!(app.model_state.dirty);
         // Old cell should be gone
-        assert_eq!(app.workbook.model.get_cell(&key), None);
+        assert_eq!(app.model_state.workbook.model.get_cell(&key), None);
         // New cell should exist
         let new_key = CellKey::new(vec![
             ("Type".into(), "Drink".into()),
             ("Month".into(), "Jan".into()),
         ]);
         assert_eq!(
-            app.workbook.model.get_cell(&new_key),
+            app.model_state.workbook.model.get_cell(&new_key),
             Some(&CellValue::Number(42.0))
         );
         // "Drink" should have been added as an item
         let items: Vec<&str> = app
+            .model_state
+
             .workbook
             .model
             .category("Type")
@@ -1676,7 +1692,7 @@ mod tests {
             ("Type".into(), "Food".into()),
             ("Month".into(), "Jan".into()),
         ]);
-        app.workbook
+        app.model_state.workbook
             .model
             .set_cell(key.clone(), CellValue::Number(42.0));
 
@@ -1692,7 +1708,7 @@ mod tests {
         .apply(&mut app);
 
         ApplyAndClearDrill.apply(&mut app);
-        assert_eq!(app.workbook.model.get_cell(&key), None);
+        assert_eq!(app.model_state.workbook.model.get_cell(&key), None);
     }
 
     // ── Toggle effects ──────────────────────────────────────────────────
@@ -1700,11 +1716,11 @@ mod tests {
     #[test]
     fn toggle_prune_empty_effect() {
         let mut app = test_app();
-        let before = app.workbook.active_view().prune_empty;
+        let before = app.model_state.workbook.active_view().prune_empty;
         TogglePruneEmpty.apply(&mut app);
-        assert_ne!(app.workbook.active_view().prune_empty, before);
+        assert_ne!(app.model_state.workbook.active_view().prune_empty, before);
         TogglePruneEmpty.apply(&mut app);
-        assert_eq!(app.workbook.active_view().prune_empty, before);
+        assert_eq!(app.model_state.workbook.active_view().prune_empty, before);
     }
 
     #[test]
@@ -1726,6 +1742,8 @@ mod tests {
         }
         .apply(&mut app);
         let items: Vec<&str> = app
+            .model_state
+
             .workbook
             .model
             .category("Type")
@@ -1736,7 +1754,7 @@ mod tests {
         assert!(!items.contains(&"Food"));
 
         RemoveCategory("Month".to_string()).apply(&mut app);
-        assert!(app.workbook.model.category("Month").is_none());
+        assert!(app.model_state.workbook.model.category("Month").is_none());
     }
 
     // ── Number format ───────────────────────────────────────────────────
@@ -1745,7 +1763,7 @@ mod tests {
     fn set_number_format_effect() {
         let mut app = test_app();
         SetNumberFormat(",.2f".to_string()).apply(&mut app);
-        assert_eq!(app.workbook.active_view().number_format, ",.2f");
+        assert_eq!(app.model_state.workbook.active_view().number_format, ",.2f");
     }
 
     // ── Page selection ──────────────────────────────────────────────────
@@ -1759,7 +1777,7 @@ mod tests {
         }
         .apply(&mut app);
         assert_eq!(
-            app.workbook.active_view().page_selection("Type"),
+            app.model_state.workbook.active_view().page_selection("Type"),
             Some("Food")
         );
     }
@@ -1774,14 +1792,14 @@ mod tests {
             item: "Food".to_string(),
         }
         .apply(&mut app);
-        assert!(app.workbook.active_view().is_hidden("Type", "Food"));
+        assert!(app.model_state.workbook.active_view().is_hidden("Type", "Food"));
 
         ShowItem {
             category: "Type".to_string(),
             item: "Food".to_string(),
         }
         .apply(&mut app);
-        assert!(!app.workbook.active_view().is_hidden("Type", "Food"));
+        assert!(!app.model_state.workbook.active_view().is_hidden("Type", "Food"));
     }
 
     // ── Toggle group ────────────────────────────────────────────────────
@@ -1795,7 +1813,7 @@ mod tests {
         }
         .apply(&mut app);
         assert!(
-            app.workbook
+            app.model_state.workbook
                 .active_view()
                 .is_group_collapsed("Type", "MyGroup")
         );
@@ -1805,7 +1823,7 @@ mod tests {
         }
         .apply(&mut app);
         assert!(
-            !app.workbook
+            !app.model_state.workbook
                 .active_view()
                 .is_group_collapsed("Type", "MyGroup")
         );
@@ -1816,9 +1834,9 @@ mod tests {
     #[test]
     fn cycle_axis_effect() {
         let mut app = test_app();
-        let before = app.workbook.active_view().axis_of("Type");
+        let before = app.model_state.workbook.active_view().axis_of("Type");
         CycleAxis("Type".to_string()).apply(&mut app);
-        let after = app.workbook.active_view().axis_of("Type");
+        let after = app.model_state.workbook.active_view().axis_of("Type");
         assert_ne!(before, after);
     }