Add a hook for views to add additional UI in the tab title bar

crates/viewer/re_viewer_context/src/space_view/space_view_class.rs

@@ -1,4 +1,5 @@
 use nohash_hasher::IntSet;
+
 use re_entity_db::EntityDb;
 use re_log_types::EntityPath;
 use re_types::{ComponentName, SpaceViewClassIdentifier};
@@ -180,6 +181,20 @@ pub trait SpaceViewClass: Send + Sync {
         Ok(())
     }
 
+    /// Additional UI displayed in the tab title bar, between the "maximize" and "help" buttons.
+    ///
+    /// Note: this is a right-to-left layout.
+    fn extra_title_bar_ui(
+        &self,
+        _ctx: &ViewerContext<'_>,
+        _ui: &mut egui::Ui,
+        _state: &mut dyn SpaceViewState,
+        _space_origin: &EntityPath,
+        _space_view_id: SpaceViewId,
+    ) -> Result<(), SpaceViewSystemExecutionError> {
+        Ok(())
+    }
+
     /// Draws the ui for this space view class and handles ui events.
     ///
     /// The passed state is kept frame-to-frame.

crates/viewer/re_viewport/src/viewport.rs

@@ -671,7 +671,8 @@ impl<'a, 'b> egui_tiles::Behavior<SpaceViewId> for TabViewer<'a, 'b> {
         };
         let space_view_id = *space_view_id;
 
-        let Some(space_view) = self.viewport_blueprint.space_views.get(&space_view_id) else {
+        let Some(space_view_blueprint) = self.viewport_blueprint.space_views.get(&space_view_id)
+        else {
             return;
         };
         let num_space_views = tiles.tiles().filter(|tile| tile.is_pane()).count();
@@ -699,9 +700,29 @@ impl<'a, 'b> egui_tiles::Behavior<SpaceViewId> for TabViewer<'a, 'b> {
             }
         }
 
-        let help_markdown = space_view
-            .class(self.ctx.space_view_class_registry)
-            .help_markdown(self.ctx.egui_ctx);
+        let space_view_class = space_view_blueprint.class(self.ctx.space_view_class_registry);
+
+        // give the view a chance to display some extra UI in the top bar.
+        let view_state = self
+            .view_states
+            .get_mut_or_create(space_view_id, space_view_class);
+        space_view_class
+            .extra_title_bar_ui(
+                self.ctx,
+                ui,
+                view_state,
+                &space_view_blueprint.space_origin,
+                space_view_id,
+            )
+            .unwrap_or_else(|err| {
+                re_log::error!(
+                    "Error in view title bar UI (class: {}, display name: {}): {err}",
+                    space_view_blueprint.class_identifier(),
+                    space_view_class.display_name(),
+                );
+            });
+
+        let help_markdown = space_view_class.help_markdown(self.ctx.egui_ctx);
         ui.help_hover_button().on_hover_ui(|ui| {
             ui.markdown_ui(&help_markdown);
         });