[id] smarter id selection for collapsing header

egui/src/containers/collapsing_header.rs

@@ -191,8 +191,7 @@ impl CollapsingHeader {
         // TODO: horizontal layout, with icon and text as labels. Insert background behind using Frame.
 
         let title = label.text();
-        let id_source = id_source.unwrap_or_else(|| Id::new(title));
+        let id = ui.make_unique_child_id_full(id_source, Some(title));
-        let id = ui.make_unique_child_id(id_source);
 
         let available = ui.available_finite();
         let text_pos = available.min + vec2(ui.style().indent, 0.0);

egui/src/context.rs

@@ -231,6 +231,10 @@ impl Context {
         self.register_unique_id(Id::new(source), source, pos)
     }
 
+    pub fn is_unique_id(&self, id: Id) -> bool {
+        !self.used_ids.lock().contains_key(&id)
+    }
+
     /// If the given Id is not unique, an error will be printed at the given position.
     pub fn register_unique_id(&self, id: Id, source_name: impl std::fmt::Debug, pos: Pos2) -> Id {
         if let Some(clash_pos) = self.used_ids.lock().insert(id, pos) {

egui/src/ui.rs

@@ -295,6 +295,29 @@ impl Ui {
         self.ctx.register_unique_id(id, id_source, self.cursor)
     }
 
+    /// Ideally, all widgets should use this. TODO
+    /// Widgets can set an explicit id source (user picked, e.g. some loop index),
+    /// and a defualt id source (e.g. label).
+    /// If they fail to be unique, a positional id will be used instead.
+    pub fn make_unique_child_id_full(
+        &mut self,
+        explicit_id_source: Option<Id>,
+        default_id_source: Option<&str>,
+    ) -> Id
+    {
+        let id = if let Some(explicit_id_source) = explicit_id_source {
+            self.id.with(&explicit_id_source)
+        } else {
+            let id = self.id.with(default_id_source);
+            if self.ctx.is_unique_id(id) {
+                id
+            } else {
+                self.make_position_id()
+            }
+        };
+        self.ctx.register_unique_id(id, default_id_source.unwrap_or_default(), self.cursor)
+    }
+
     /// Make an Id that is unique to this positon.
     /// Can be used for widgets that do NOT persist state in Memory
     /// but you still need to interact with (e.g. buttons, sliders).