fix(viewport): preserve tiled pane controls

src/app/view/mod.rs

@@ -218,6 +218,7 @@ impl OpenCADStudio {
             )
             .width(Fill)
             .height(Fill)
+            .min_size(scene.model_pane_min_px())
             .spacing(crate::scene::TILE_DIVIDER_PX);
             stack![size_probe, shaders].width(Fill).height(Fill).into()
         };
@@ -236,6 +237,7 @@ impl OpenCADStudio {
                 })
                 .width(Fill)
                 .height(Fill)
+                .min_size(scene.model_pane_min_px())
                 .spacing(crate::scene::TILE_DIVIDER_PX)
                 .on_resize(6.0, Message::PaneResized)
                 .into(),
@@ -782,6 +784,7 @@ impl OpenCADStudio {
                     .into(),
             ])
             .report_width0(self.render_bar_w.clone())
+            .report_width0(tab.scene.model_pane_min_reporter())
             .into();
             // Pin the bar to the active model tile's top-left corner so it
             // follows the active panel in a tiled layout.

src/scene/layout.rs

@@ -4,6 +4,24 @@ use super::*;
 impl Scene {
     // ── Layout management ─────────────────────────────────────────────────
 
+    pub(crate) fn model_pane_min_px(&self) -> f32 {
+        let measured = f32::from_bits(
+            self.model_pane_min_px
+                .load(std::sync::atomic::Ordering::Relaxed),
+        );
+        if measured.is_finite() && measured > 1.0 {
+            measured
+        } else {
+            MODEL_PANE_MIN_FALLBACK_PX
+        }
+    }
+
+    pub(crate) fn model_pane_min_reporter(
+        &self,
+    ) -> std::sync::Arc<std::sync::atomic::AtomicU32> {
+        self.model_pane_min_px.clone()
+    }
+
     /// Rename a paper-space layout.  Updates the Layout object name in the document.
     pub fn rename_layout(&mut self, old_name: &str, new_name: &str) {
         for obj in self.document.objects.values_mut() {
@@ -361,7 +379,11 @@ impl Scene {
         let half = TILE_DIVIDER_PX * 0.5;
         self.model_panes
             .layout()
-            .split_regions(TILE_DIVIDER_PX, 0.0, iced::Size::new(vw, vh))
+            .split_regions(
+                TILE_DIVIDER_PX,
+                self.model_pane_min_px(),
+                iced::Size::new(vw, vh),
+            )
             .values()
             .map(|(axis, region, ratio)| match axis {
                 Axis::Vertical => {
@@ -394,7 +416,7 @@ impl Scene {
         }
         let regions = self.model_panes.layout().pane_regions(
             TILE_DIVIDER_PX,
-            0.0,
+            self.model_pane_min_px(),
             iced::Size::new(vw, vh),
         );
         self.model_panes
@@ -421,7 +443,7 @@ impl Scene {
         }
         let regions = self.model_panes.layout().pane_regions(
             TILE_DIVIDER_PX,
-            0.0,
+            self.model_pane_min_px(),
             iced::Size::new(canvas_w, canvas_h),
         );
         let mut tiles = self.model_tiles.borrow_mut();

src/scene/mod.rs

@@ -1110,6 +1110,9 @@ pub(crate) struct ModelTile {
 /// the drawn viewports line up exactly with the pane_grid layout.
 pub const TILE_DIVIDER_PX: f32 = 2.0;
 
+/// Minimum used until the viewport render-mode bar reports its natural width.
+pub const MODEL_PANE_MIN_FALLBACK_PX: f32 = 50.0;
+
 /// Shift every vertex of a freshly tessellated `MeshLodSet` into the
 /// scene's local f32 space by subtracting `world_offset`. ACIS / SAT
 /// tessellation hands us WCS coordinates; the wire / hatch / face3d
@@ -1502,6 +1505,10 @@ pub struct Scene {
     /// Plain field (not a `RefCell`) so the view can borrow it for the
     /// `PaneGrid` widget's lifetime; mutated through `&mut Scene` in update.
     pub(crate) model_panes: iced::widget::pane_grid::State<usize>,
+    /// Natural width of the Model viewport control bar. `DensitySwap` updates
+    /// it during layout; every pane-grid geometry calculation reads it on the
+    /// next frame so a pane cannot become narrower than its controls.
+    model_pane_min_px: std::sync::Arc<std::sync::atomic::AtomicU32>,
     pub selection: Rc<RefCell<SelectionState>>,
     /// The CAD document — single source of truth for all entities.
     pub document: CadDocument,
@@ -1903,6 +1910,7 @@ impl Scene {
             last_sdf_text: RefCell::new(HashMap::default()),
             // One pane mapped to tile 0 — matches the single default tile above.
             model_panes: iced::widget::pane_grid::State::new(0).0,
+            model_pane_min_px: std::sync::Arc::new(std::sync::atomic::AtomicU32::new(0)),
             selection: Rc::new(RefCell::new(SelectionState::default())),
             document: CadDocument::new(),
             object_data_cache: crate::entities::object_data::ObjectDataCache::default(),

src/ui/wrap_bar.rs

@@ -761,7 +761,7 @@ pub struct DensitySwap<'a> {
     /// measured every layout regardless of which variant is shown — so a caller
     /// can place a neighbouring widget relative to the full-size content even
     /// while a narrower variant is on screen.
-    width0_out: Option<Arc<AtomicU32>>,
+    width0_out: Vec<Arc<AtomicU32>>,
 }
 
 impl<'a> DensitySwap<'a> {
@@ -770,7 +770,7 @@ impl<'a> DensitySwap<'a> {
             variants,
             chosen: Cell::new(0),
             height_out: None,
-            width0_out: None,
+            width0_out: Vec::new(),
         }
     }
 
@@ -783,7 +783,7 @@ impl<'a> DensitySwap<'a> {
 
     /// Report the first variant's natural (unconstrained) width — see `width0_out`.
     pub fn report_width0(mut self, out: Arc<AtomicU32>) -> Self {
-        self.width0_out = Some(out);
+        self.width0_out.push(out);
         self
     }
 }
@@ -818,7 +818,7 @@ impl<'a> Widget<Message, Theme, Renderer> for DensitySwap<'a> {
         for (i, v) in self.variants.iter_mut().enumerate() {
             let n = v.as_widget_mut().layout(&mut tree.children[i], renderer, &natural);
             if i == 0 {
-                if let Some(out) = &self.width0_out {
+                for out in &self.width0_out {
                     out.store(n.size().width.to_bits(), Ordering::Relaxed);
                 }
             }