Addressed review comments from #6161

crates/re_ui/Cargo.toml

@@ -34,16 +34,15 @@ re_format.workspace = true
 re_log.workspace = true
 re_log_types.workspace = true # syntax-highlighting for EntityPath
 
+egui.workspace = true
 egui_commonmark = { workspace = true, features = ["pulldown_cmark"] }
 egui_extras.workspace = true
-egui.workspace = true
 parking_lot.workspace = true
 serde = { workspace = true, features = ["derive"] }
 serde_json.workspace = true
-strum_macros.workspace = true
 strum.workspace = true
+strum_macros.workspace = true
 sublime_fuzzy.workspace = true
-once_cell.workspace = true
 
 
 eframe = { workspace = true, default-features = false, features = ["wgpu"] }

crates/re_ui/examples/re_ui_example/right_panel.rs

@@ -285,17 +285,6 @@ impl RightPanel {
             true,
             list_item2::LabelContent::new("Other contents:"),
             |re_ui, ui| {
-                re_ui.list_item2().show_hierarchical(
-                    ui,
-                    list_item2::LabelContent::new("next line is a EmptyContent:")
-                        .subdued(true)
-                        .italics(true),
-                );
-
-                re_ui
-                    .list_item2()
-                    .show_hierarchical(ui, list_item2::EmptyContent);
-
                 re_ui.list_item2().show_hierarchical(
                     ui,
                     list_item2::DebugContent::default()

crates/re_ui/src/drag_and_drop.rs

@@ -1,6 +1,6 @@
 //! Helpers for drag and drop support for reordering hierarchical lists.
 //!
-//! Works well in combination with [`crate::ListItem`].
+//! Works well in combination with [`crate::list_item2::ListItem`].
 
 pub enum ItemKind<ItemId: Copy> {
     /// Root container item.

crates/re_ui/src/list_item2/list_item.rs

@@ -22,7 +22,19 @@ pub struct ShowCollapsingResponse<R> {
     pub body_response: Option<egui::InnerResponse<R>>,
 }
 
-/// Generic list item that delegates its content to a [`ListItemContent`] implementation.
+/// Content-generic list item.
+///
+/// The following features are supported:
+/// - Flat or collapsible hierarchical lists.
+/// - Full-span background highlighting via [`super::list_item_scope`]. TODO(#6156): fix reference
+/// - Interactive or not.
+/// - Support for drag and drop with [`crate::drag_and_drop`].
+///
+/// Besides these core features, [`ListItem`] delegates all content to the [`ListItemContent`]
+/// implementations, such as [`super::LabelContent`] and [`super::PropertyContent`].
+///
+/// Usage example can be found in `re_ui_example`.
+
 #[derive(Debug, Clone)]
 pub struct ListItem<'a> {
     re_ui: &'a ReUi,

crates/re_ui/src/list_item2/mod.rs

@@ -21,6 +21,9 @@ pub struct ContentContext<'a> {
     pub rect: egui::Rect,
 
     /// Background area
+    ///
+    /// This is the area covered by the full-span highlighting. Useful for testing if the cursor is
+    /// over the item.
     pub bg_rect: egui::Rect,
 
     /// List item response.

crates/re_ui/src/list_item2/other_contents.rs

@@ -2,35 +2,36 @@ use crate::list_item2::{ContentContext, DesiredWidth, ListItemContent};
 use crate::ReUi;
 use egui::Ui;
 
-/// Empty list item content.
-pub struct EmptyContent;
-
-impl ListItemContent for EmptyContent {
-    fn ui(
-        self: Box<Self>,
-        _re_ui: &crate::ReUi,
-        _ui: &mut egui::Ui,
-        _context: &ContentContext<'_>,
-    ) {
-    }
-}
-
 /// [`ListItemContent`] that delegates to a closure.
 #[allow(clippy::type_complexity)]
 pub struct CustomContent<'a> {
     ui: Box<dyn FnOnce(&crate::ReUi, &mut egui::Ui, &ContentContext<'_>) + 'a>,
+    desired_width: DesiredWidth,
 }
 
 impl<'a> CustomContent<'a> {
     pub fn new(ui: impl FnOnce(&crate::ReUi, &mut egui::Ui, &ContentContext<'_>) + 'a) -> Self {
-        Self { ui: Box::new(ui) }
+        Self {
+            ui: Box::new(ui),
+            desired_width: Default::default(),
+        }
+    }
+
+    #[inline]
+    pub fn with_desired_width(mut self, desired_width: DesiredWidth) -> Self {
+        self.desired_width = desired_width;
+        self
     }
 }
 
 impl ListItemContent for CustomContent<'_> {
     fn ui(self: Box<Self>, re_ui: &crate::ReUi, ui: &mut egui::Ui, context: &ContentContext<'_>) {
         (self.ui)(re_ui, ui, context);
     }
+
+    fn desired_width(&self, _re_ui: &ReUi, _ui: &Ui) -> DesiredWidth {
+        self.desired_width
+    }
 }
 
 /// [`ListItemContent`] that displays the content rect.

crates/re_ui/src/list_item2/scope.rs

@@ -1,13 +1,11 @@
 use egui::NumExt;
-use once_cell::sync::Lazy;
 
 #[derive(Debug, Clone, PartialEq)]
 pub struct State {
     /// X coordinate span to use for hover/selection highlight.
     ///
-    /// Note: this field is not, strictly speaking, part of the state, as it's overwritten with each
-    /// call of `list_item_scope`. Still, it's convenient to have it here to pass it from the scope
-    /// to the inner `ListItem`.
+    /// Note: `list_item_scope` always overwrite this field before pushing the [`State`] to the
+    /// stack.
     // TODO(#6156): this being here is a (temporary) hack (see docstring). In the future, this will
     // be generalized to some `full_span_scope` mechanism to be used by all full-span widgets beyond
     // `ListItem`.
@@ -78,19 +76,17 @@ impl State {
 #[derive(Debug, Clone, Default)]
 pub(crate) struct StateStack(Vec<State>);
 
-static STATE_STACK_ID: Lazy<egui::Id> = Lazy::new(|| egui::Id::new("re_ui_list_item_state_stack"));
-
 impl StateStack {
     fn push(ctx: &egui::Context, state: State) {
         ctx.data_mut(|writer| {
-            let stack: &mut StateStack = writer.get_temp_mut_or_default(*STATE_STACK_ID);
+            let stack: &mut StateStack = writer.get_temp_mut_or_default(egui::Id::NULL);
             stack.0.push(state);
         });
     }
 
     fn pop(ctx: &egui::Context) -> Option<State> {
         ctx.data_mut(|writer| {
-            let stack: &mut StateStack = writer.get_temp_mut_or_default(*STATE_STACK_ID);
+            let stack: &mut StateStack = writer.get_temp_mut_or_default(egui::Id::NULL);
             stack.0.pop()
         })
     }
@@ -102,7 +98,7 @@ impl StateStack {
     /// [`super::list_item_scope`].
     pub(crate) fn top(ctx: &egui::Context) -> State {
         ctx.data_mut(|writer| {
-            let stack: &mut StateStack = writer.get_temp_mut_or_default(*STATE_STACK_ID);
+            let stack: &mut StateStack = writer.get_temp_mut_or_default(egui::Id::NULL);
             let state = stack.0.last();
             if state.is_none() {
                 re_log::warn_once!(
@@ -120,7 +116,7 @@ impl StateStack {
     /// empty, the closure is not called and a warning is logged.
     pub(crate) fn top_mut(ctx: &egui::Context, state_writer: impl FnOnce(&mut State)) {
         ctx.data_mut(|writer| {
-            let stack: &mut StateStack = writer.get_temp_mut_or_default(*STATE_STACK_ID);
+            let stack: &mut StateStack = writer.get_temp_mut_or_default(egui::Id::NULL);
             let state = stack.0.last_mut();
             if let Some(state) = state {
                 state_writer(state);
@@ -135,7 +131,7 @@ impl StateStack {
 
     fn peek(ctx: &egui::Context) -> Option<State> {
         ctx.data_mut(|writer| {
-            let stack: &mut StateStack = writer.get_temp_mut_or_default(*STATE_STACK_ID);
+            let stack: &mut StateStack = writer.get_temp_mut_or_default(egui::Id::NULL);
             stack.0.last().cloned()
         })
     }