Enforce root layout container to be always present (rerun-io#5856)

### What

Previously, it was possible to end up without a root blueprint container
which can cause weird ui behavior like not being able to add space
views.
This is now solved by making up a root container upon reading the
blueprint layout from the data store.

(Review with whitespace diff disabled!)

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested the web demo (if applicable):
* Using newly built examples:
[rerun.io/viewer](https://rerun.io/viewer/pr/5856)
* Using examples from latest `main` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/5856?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/5856?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG
* [x] If applicable, add a new check to the [release
checklist](https://github.com/rerun-io/rerun/blob/main/tests/python/release_checklist)!

- [PR Build Summary](https://build.rerun.io/pr/5856)
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

crates/viewer/re_blueprint_tree/src/blueprint_tree.rs

@@ -184,10 +184,7 @@ impl BlueprintTree {
         blueprint: &ViewportBlueprint,
         ui: &mut egui::Ui,
     ) {
-        let Some(container_id) = blueprint.root_container else {
-            // nothing to draw if there is no root container
-            return;
-        };
+        let container_id = blueprint.root_container;
 
         let Some(container_blueprint) = blueprint.containers.get(&container_id) else {
             re_log::warn_once!("Cannot find root container {container_id}");
@@ -655,14 +652,12 @@ impl BlueprintTree {
             // root container.
             let target_container_id =
                 if let Some(Item::Container(container_id)) = ctx.selection().single_item() {
-                    Some(*container_id)
+                    *container_id
                 } else {
                     blueprint.root_container
                 };
 
-            if let Some(target_container_id) = target_container_id {
-                show_add_space_view_or_container_modal(target_container_id);
-            }
+            show_add_space_view_or_container_modal(target_container_id);
         }
     }
 
@@ -821,15 +816,11 @@ impl BlueprintTree {
         // TODO(ab): this is a rather primitive behavior. Ideally we should allow dropping in the last container based
         //           on the horizontal position of the cursor.
 
-        let Some(root_container_id) = blueprint.root_container else {
-            return;
-        };
-
         if ui.rect_contains_pointer(empty_space) {
             let drop_target = re_ui::drag_and_drop::DropTarget::new(
                 empty_space.x_range(),
                 empty_space.top(),
-                Contents::Container(root_container_id),
+                Contents::Container(blueprint.root_container),
                 usize::MAX,
             );
 

crates/viewer/re_context_menu/src/actions/move_contents_to_new_container.rs

@@ -22,9 +22,7 @@ impl ContextMenuAction for MoveContentsToNewContainerAction {
 
         ctx.selection.iter().all(|(item, _)| match item {
             Item::SpaceView(_) => true,
-            Item::Container(container_id) => {
-                ctx.viewport_blueprint.root_container != Some(*container_id)
-            }
+            Item::Container(container_id) => ctx.viewport_blueprint.root_container != *container_id,
             _ => false,
         })
     }
@@ -36,9 +34,7 @@ impl ContextMenuAction for MoveContentsToNewContainerAction {
     fn supports_item(&self, ctx: &ContextMenuContext<'_>, item: &Item) -> bool {
         match item {
             Item::SpaceView(_) => true,
-            Item::Container(container_id) => {
-                ctx.viewport_blueprint.root_container != Some(*container_id)
-            }
+            Item::Container(container_id) => ctx.viewport_blueprint.root_container != *container_id,
             _ => false,
         }
     }
@@ -57,26 +53,25 @@ impl ContextMenuAction for MoveContentsToNewContainerAction {
     }
 
     fn process_selection(&self, ctx: &ContextMenuContext<'_>) {
-        if let Some(root_container_id) = ctx.viewport_blueprint.root_container {
-            let (target_container_id, target_position) = ctx
-                .clicked_item_enclosing_container_id_and_position()
-                .unwrap_or((root_container_id, 0));
+        let root_container_id = ctx.viewport_blueprint.root_container;
+        let (target_container_id, target_position) = ctx
+            .clicked_item_enclosing_container_id_and_position()
+            .unwrap_or((root_container_id, 0));
 
-            let contents = ctx
-                .selection
-                .iter()
-                .filter_map(|(item, _)| item.try_into().ok())
-                .collect();
+        let contents = ctx
+            .selection
+            .iter()
+            .filter_map(|(item, _)| item.try_into().ok())
+            .collect();
 
-            ctx.viewport_blueprint.move_contents_to_new_container(
-                contents,
-                self.0,
-                target_container_id,
-                target_position,
-            );
+        ctx.viewport_blueprint.move_contents_to_new_container(
+            contents,
+            self.0,
+            target_container_id,
+            target_position,
+        );
 
-            ctx.viewport_blueprint
-                .mark_user_interaction(ctx.viewer_context);
-        }
+        ctx.viewport_blueprint
+            .mark_user_interaction(ctx.viewer_context);
     }
 }

crates/viewer/re_context_menu/src/actions/remove.rs

@@ -14,9 +14,7 @@ impl ContextMenuAction for RemoveAction {
     fn supports_item(&self, ctx: &ContextMenuContext<'_>, item: &Item) -> bool {
         match item {
             Item::SpaceView(_) => true,
-            Item::Container(container_id) => {
-                ctx.viewport_blueprint.root_container != Some(*container_id)
-            }
+            Item::Container(container_id) => ctx.viewport_blueprint.root_container != *container_id,
             Item::DataResult(_, instance_path) => instance_path.is_all(),
             _ => false,
         }

crates/viewer/re_context_menu/src/actions/show_hide.rs

@@ -15,7 +15,7 @@ impl ContextMenuAction for ShowAction {
             Item::Container(container_id) => {
                 !ctx.viewport_blueprint
                     .is_contents_visible(&Contents::Container(*container_id))
-                    && ctx.viewport_blueprint.root_container != Some(*container_id)
+                    && ctx.viewport_blueprint.root_container != *container_id
             }
             Item::DataResult(space_view_id, instance_path) => {
                 data_result_visible(ctx, space_view_id, instance_path).is_some_and(|vis| !vis)
@@ -69,7 +69,7 @@ impl ContextMenuAction for HideAction {
             Item::Container(container_id) => {
                 ctx.viewport_blueprint
                     .is_contents_visible(&Contents::Container(*container_id))
-                    && ctx.viewport_blueprint.root_container != Some(*container_id)
+                    && ctx.viewport_blueprint.root_container != *container_id
             }
             Item::DataResult(space_view_id, instance_path) => {
                 data_result_visible(ctx, space_view_id, instance_path).unwrap_or(false)

crates/viewer/re_viewport/src/viewport.rs

@@ -244,9 +244,8 @@ impl<'a> Viewport<'a> {
                         );
 
                         reset = true;
-                    } else if let Some(parent_id) =
-                        parent_container.or(self.blueprint.root_container)
-                    {
+                    } else {
+                        let parent_id = parent_container.unwrap_or(self.blueprint.root_container);
                         let tile_id = self.tree.tiles.insert_pane(space_view_id);
                         let container_tile_id = blueprint_id_to_tile_id(&parent_id);
                         if let Some(egui_tiles::Tile::Container(container)) =
@@ -266,31 +265,26 @@ impl<'a> Viewport<'a> {
                             re_log::trace!("Root was not a container - will re-run auto-layout");
                             reset = true;
                         }
-                    } else {
-                        re_log::trace!("No root found - will re-run auto-layout");
                     }
 
                     self.tree_edited = true;
                 }
                 TreeAction::AddContainer(container_kind, parent_container) => {
-                    if let Some(parent_id) = parent_container.or(self.blueprint.root_container) {
-                        let tile_id = self
-                            .tree
-                            .tiles
-                            .insert_container(egui_tiles::Container::new(container_kind, vec![]));
-                        if let Some(egui_tiles::Tile::Container(container)) =
-                            self.tree.tiles.get_mut(blueprint_id_to_tile_id(&parent_id))
-                        {
-                            re_log::trace!("Inserting new space view into container {parent_id:?}");
-                            container.add_child(tile_id);
-                        } else {
-                            re_log::trace!(
-                                "Parent or root was not a container - will re-run auto-layout"
-                            );
-                            reset = true;
-                        }
+                    let parent_id = parent_container.unwrap_or(self.blueprint.root_container);
+                    let tile_id = self
+                        .tree
+                        .tiles
+                        .insert_container(egui_tiles::Container::new(container_kind, vec![]));
+                    if let Some(egui_tiles::Tile::Container(container)) =
+                        self.tree.tiles.get_mut(blueprint_id_to_tile_id(&parent_id))
+                    {
+                        re_log::trace!("Inserting new space view into container {parent_id:?}");
+                        container.add_child(tile_id);
                     } else {
-                        re_log::trace!("No root found - will re-run auto-layout");
+                        re_log::trace!(
+                            "Parent or root was not a container - will re-run auto-layout"
+                        );
+                        reset = true;
                     }
 
                     self.tree_edited = true;
@@ -461,7 +455,7 @@ struct TabViewer<'a, 'b> {
     ctx: &'a ViewerContext<'b>,
     viewport_blueprint: &'a ViewportBlueprint,
     maximized: &'a mut Option<SpaceViewId>,
-    root_container_id: Option<ContainerId>,
+    root_container_id: ContainerId,
     tree_action_sender: std::sync::mpsc::Sender<TreeAction>,
 
     /// List of query & system execution results for each space view.
@@ -773,26 +767,22 @@ impl<'a, 'b> egui_tiles::Behavior<SpaceViewId> for TabViewer<'a, 'b> {
                 // drag and drop operation often lead to many spurious empty containers. To work
                 // around this, we run a simplification pass when a drop occurs.
 
-                if let Some(root_container_id) = self.root_container_id {
-                    if self
-                        .tree_action_sender
-                        .send(TreeAction::SimplifyContainer(
-                            root_container_id,
-                            egui_tiles::SimplificationOptions {
-                                prune_empty_tabs: true,
-                                prune_empty_containers: false,
-                                prune_single_child_tabs: true,
-                                prune_single_child_containers: false,
-                                all_panes_must_have_tabs: true,
-                                join_nested_linear_containers: false,
-                            },
-                        ))
-                        .is_err()
-                    {
-                        re_log::warn_once!(
-                            "Channel between ViewportBlueprint and Viewport is broken"
-                        );
-                    }
+                if self
+                    .tree_action_sender
+                    .send(TreeAction::SimplifyContainer(
+                        self.root_container_id,
+                        egui_tiles::SimplificationOptions {
+                            prune_empty_tabs: true,
+                            prune_empty_containers: false,
+                            prune_single_child_tabs: true,
+                            prune_single_child_containers: false,
+                            all_panes_must_have_tabs: true,
+                            join_nested_linear_containers: false,
+                        },
+                    ))
+                    .is_err()
+                {
+                    re_log::warn_once!("Channel between ViewportBlueprint and Viewport is broken");
                 }
 
                 self.edited = true;

crates/viewer/re_viewport_blueprint/src/container.rs

@@ -35,6 +35,22 @@ pub struct ContainerBlueprint {
     pub grid_columns: Option<u32>,
 }
 
+impl Default for ContainerBlueprint {
+    fn default() -> Self {
+        Self {
+            id: ContainerId::random(),
+            container_kind: egui_tiles::ContainerKind::Grid,
+            display_name: None,
+            contents: vec![],
+            col_shares: vec![],
+            row_shares: vec![],
+            active_tab: None,
+            visible: true,
+            grid_columns: None,
+        }
+    }
+}
+
 impl ContainerBlueprint {
     /// Attempt to load a [`ContainerBlueprint`] from the blueprint store.
     pub fn try_from_db(

crates/viewer/re_viewport_blueprint/src/viewport_blueprint.rs

@@ -33,7 +33,7 @@ pub struct ViewportBlueprint {
     pub containers: BTreeMap<ContainerId, ContainerBlueprint>,
 
     /// The root container.
-    pub root_container: Option<ContainerId>,
+    pub root_container: ContainerId,
 
     /// The layouts of all the space views.
     pub tree: egui_tiles::Tree<SpaceViewId>,
@@ -129,8 +129,6 @@ impl ViewportBlueprint {
             .map(|c| (c.id, c))
             .collect();
 
-        let root_container = root_container.map(|id| id.0.into());
-
         // Auto layouting and auto space view are only enabled if no blueprint has been provided by the user.
         // Only enable auto-space-views if this is the app-default blueprint
         let is_app_default_blueprint = blueprint_db
@@ -144,7 +142,7 @@ impl ViewportBlueprint {
         let tree = build_tree_from_space_views_and_containers(
             space_views.values(),
             containers.values(),
-            root_container,
+            root_container.clone(),
         );
 
         let past_viewer_recommendations = past_viewer_recommendations
@@ -153,10 +151,15 @@ impl ViewportBlueprint {
             .cloned()
             .collect();
 
+        let root_container_or_placeholder = root_container.clone().map_or_else(
+            || ContainerId::hashed_from_str("placeholder_root_container"),
+            |id| id.0.into(),
+        );
+
         Self {
             space_views,
             containers,
-            root_container,
+            root_container: root_container_or_placeholder,
             tree,
             maximized: maximized.map(|id| id.0.into()),
             auto_layout,
@@ -455,9 +458,7 @@ impl ViewportBlueprint {
     ///
     /// See [`Self::visit_contents_in_container`] for details.
     pub fn visit_contents(&self, visitor: &mut impl FnMut(&Contents, &SmallVec<[ContainerId; 4]>)) {
-        if let Some(root_container) = self.root_container {
-            self.visit_contents_in_container(&root_container, visitor);
-        }
+        self.visit_contents_in_container(&self.root_container, visitor);
     }
 
     /// Walk the subtree defined by the provided container id and call `visitor` for each
@@ -501,11 +502,7 @@ impl ViewportBlueprint {
     /// Given a predicate, finds the (first) matching contents by recursively walking from the root
     /// container.
     pub fn find_contents_by(&self, predicate: &impl Fn(&Contents) -> bool) -> Option<Contents> {
-        if let Some(root_container) = self.root_container {
-            self.find_contents_in_container_by(predicate, &root_container)
-        } else {
-            None
-        }
+        self.find_contents_in_container_by(predicate, &self.root_container)
     }
 
     /// Given a predicate, finds the (first) matching contents by recursively walking from the given
@@ -555,15 +552,11 @@ impl ViewportBlueprint {
         &self,
         contents: &Contents,
     ) -> Option<(ContainerId, usize)> {
-        if let Some(container_id) = self.root_container {
-            if *contents == Contents::Container(container_id) {
-                // root doesn't have a parent
-                return None;
-            }
-            self.find_parent_and_position_index_impl(contents, &container_id)
-        } else {
-            None
+        if *contents == Contents::Container(self.root_container) {
+            // root doesn't have a parent
+            return None;
         }
+        self.find_parent_and_position_index_impl(contents, &self.root_container)
     }
 
     fn find_parent_and_position_index_impl(
@@ -915,7 +908,7 @@ impl ViewportBlueprint {
 fn build_tree_from_space_views_and_containers<'a>(
     space_views: impl Iterator<Item = &'a SpaceViewBlueprint>,
     containers: impl Iterator<Item = &'a ContainerBlueprint>,
-    root_container: Option<ContainerId>,
+    root_container: Option<RootContainer>,
 ) -> egui_tiles::Tree<SpaceViewId> {
     re_tracing::profile_function!();
     let mut tree = egui_tiles::Tree::empty("viewport_tree");
@@ -937,8 +930,10 @@ fn build_tree_from_space_views_and_containers<'a>(
     }
 
     // And finally, set the root
-    if let Some(root_container) = root_container.map(|id| blueprint_id_to_tile_id(&id)) {
-        tree.root = Some(root_container);
+
+    if let Some(root_container) = root_container {
+        let root_container = ContainerId::from(root_container.0);
+        tree.root = Some(blueprint_id_to_tile_id(&root_container));
     }
 
     tree