Tabs: Add support for adding buttons before the left arrow in the tab bar.

examples/advanced.rs

@@ -135,11 +135,22 @@ impl egui_tiles::Behavior<Pane> for TreeBehavior {
         _tabs: &egui_tiles::Tabs,
         _scroll_offset: &mut f32,
     ) {
+        let _response = ui.small_button("⚙");
         if ui.button("➕").clicked() {
             self.add_child_to = Some(tile_id);
         }
     }
 
+    fn tab_bar_left_ui(
+        &mut self,
+        _tiles: &egui_tiles::Tiles<Pane>,
+        ui: &mut egui::Ui,
+        _tile_id: egui_tiles::TileId,
+        _tabs: &egui_tiles::Tabs,
+    ) {
+        ui.label("☰");
+    }
+
     // ---
     // Settings:
 

src/behavior.rs

@@ -254,6 +254,24 @@ pub trait Behavior<Pane> {
         // }
     }
 
+    /// Adds custom UI to the leftmost edge of each tab bar, pinned outside the
+    /// scrollable tab strip and to the LEFT of the left scroll-arrow.
+    ///
+    /// Width consumed by widgets added here is automatically subtracted from
+    /// the area available to the scrollable tab strip — tabs shrink rather
+    /// than overlap. The slot does not move when tabs are scrolled.
+    ///
+    /// To match the visual height of the scroll arrows, use
+    /// `ui.add_sized(egui::Vec2::splat(self.tab_bar_height(...)), widget)`.
+    fn tab_bar_left_ui(
+        &mut self,
+        _tiles: &Tiles<Pane>,
+        _ui: &mut Ui,
+        _tile_id: TileId,
+        _tabs: &crate::Tabs,
+    ) {
+    }
+
     /// The height of the bar holding tab titles.
     fn tab_bar_height(&self, _style: &egui::Style) -> f32 {
         24.0

src/container/tabs.rs

@@ -255,8 +255,18 @@ impl Tabs {
                 ui.available_size(),
                 egui::Layout::left_to_right(egui::Align::Center),
                 |ui| {
+                    // Left custom slot — first call in this LTR child layout
+                    // means leftmost on screen, so it sits to the left of the
+                    // left scroll-arrow.
+                    behavior.tab_bar_left_ui(&tree.tiles, ui, tile_id, self);
+
                     scroll_state.left_arrow(ui, arrow_size);
 
+                    // Clamp the precomputed width so it can't exceed what's
+                    // left after the leading slot + left arrow consumed space
+                    // inside this LTR child ui.
+                    let scroll_area_width = scroll_area_width.min(ui.available_width()).max(0.0);
+
                     // Prepare to show the scroll area with the tabs:
 
                     scroll_state.offset = scroll_state