feat(viewport): reorder model controls, add close-vport button

Reorder the model-tile control chip to grid, grid snap, render mode,
vertical split, horizontal split. Add a close button (shown only with
more than one model tile) that merges the active pane into its
longest-contact neighbour.

Divider drags now clamp each side to the minimum tile size, so dragging
a boundary to the screen edge no longer collapses a pane — closing is
done with the button instead.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/app/mod.rs

@@ -1078,6 +1078,9 @@ pub enum Message {
     /// Split the active Model viewport in two. `true` → horizontal divider
     /// (top / bottom); `false` → vertical divider (left / right).
     SplitModelViewport(bool),
+    /// Close the active Model viewport, merging it into a neighbour.
+    /// Only meaningful when more than one model tile exists.
+    CloseModelViewport,
     /// Recall previous command in history (↑ arrow key).
     CommandHistoryPrev,
     /// Recall next command in history (↓ arrow key).

src/app/update.rs

@@ -53,10 +53,9 @@ const VIEWCUBE_HIT_SIZE: f32 = VIEWCUBE_DRAW_PX;
 /// Pixel distance from a Model-tile inner divider that still registers as
 /// a resize grip on the press.
 const TILE_EDGE_HIT_PX: f32 = 4.0;
-/// Normalized minimum tile size before `collapse_small_model_tiles` merges
-/// it into a neighbour. Sized to comfortably contain the ViewCube + its
-/// padding so a tile that's still GPU-rendered always has room to show
-/// the gizmo.
+/// Normalized minimum tile size a divider drag clamps to. Sized to
+/// comfortably contain the ViewCube + its padding so every tile always
+/// has room to show the gizmo.
 fn tile_min_norm(canvas_w: f32, canvas_h: f32) -> (f32, f32) {
     let px = VIEWCUBE_DRAW_PX + 2.0 * VIEWCUBE_PAD + 16.0;
     (
@@ -1562,6 +1561,13 @@ impl OpenCADStudio {
                 Task::none()
             }
 
+            Message::CloseModelViewport => {
+                let i = self.active_tab;
+                self.tabs[i].scene.close_active_model_tile();
+                self.tabs[i].scene.camera_generation += 1;
+                Task::none()
+            }
+
             Message::CommandHistoryPrev => {
                 // Grip popup wins first — arrow keys walk its items.
                 if let Some(popup) = self.grip_popup.as_mut() {
@@ -3472,15 +3478,10 @@ impl OpenCADStudio {
                     return Task::none();
                 }
 
-                // End an in-flight tile-divider drag. Any tile that fell
-                // below the minimum (viewcube fits comfortably) gets
-                // absorbed into its longest-contact neighbour, so the
-                // user can drag a divider all the way to one side to
-                // remove a pane.
+                // End an in-flight tile-divider drag. The move clamps
+                // each side to its minimum, so panes never collapse from
+                // dragging — use the close button to remove a pane.
                 if self.tile_drag.take().is_some() {
-                    let (vw, vh) = self.tabs[i].scene.selection.borrow().vp_size;
-                    let (min_w, min_h) = tile_min_norm(vw, vh);
-                    self.tabs[i].scene.collapse_small_model_tiles(min_w, min_h);
                     self.tabs[i].scene.camera_generation += 1;
                     return Task::none();
                 }

src/app/view.rs

@@ -436,6 +436,7 @@ impl OpenCADStudio {
                 self.show_grid,
                 self.snapper.grid_snap(),
                 true,
+                tab.scene.model_tiles.borrow().len(),
             );
             // Position the bar at the active model tile's top-left corner so
             // it follows the active panel in a tiled layout (full canvas when
@@ -520,6 +521,7 @@ impl OpenCADStudio {
                         self.show_grid,
                         self.snapper.grid_snap(),
                         false,
+                        0,
                     )),
                 ],
             ]
@@ -4504,6 +4506,7 @@ fn viewport_controls<'a>(
     show_grid: bool,
     snap_on: bool,
     include_split: bool,
+    tile_count: usize,
 ) -> Element<'a, Message> {
     use acadrust::entities::ViewportRenderMode as M;
     let render_modes: Vec<RenderModeChoice> = vec![
@@ -4586,19 +4589,25 @@ fn viewport_controls<'a>(
     let mut bar = row![]
         .spacing(3)
         .align_y(iced::alignment::Vertical::Center);
-    if include_split {
-        bar = bar
-            .push(icon_btn(crate::ui::icons::SPLIT_H, false, Message::SplitModelViewport(true)))
-            .push(sep())
-            .push(icon_btn(crate::ui::icons::SPLIT_V, false, Message::SplitModelViewport(false)))
-            .push(sep());
-    }
     bar = bar
-        .push(picker)
-        .push(sep())
         .push(icon_btn(crate::ui::icons::GRID, show_grid, Message::ToggleGrid))
         .push(sep())
-        .push(icon_btn(crate::ui::icons::SNAP, snap_on, Message::ToggleGridSnap));
+        .push(icon_btn(crate::ui::icons::SNAP, snap_on, Message::ToggleGridSnap))
+        .push(sep())
+        .push(picker);
+    if include_split {
+        bar = bar
+            .push(sep())
+            .push(icon_btn(crate::ui::icons::SPLIT_V, false, Message::SplitModelViewport(false)))
+            .push(sep())
+            .push(icon_btn(crate::ui::icons::SPLIT_H, false, Message::SplitModelViewport(true)));
+        // Close button: only meaningful with more than one model tile.
+        if tile_count > 1 {
+            bar = bar
+                .push(sep())
+                .push(icon_btn(crate::ui::icons::CLOSE, false, Message::CloseModelViewport));
+        }
+    }
 
     container(bar)
         .padding(2)