Allow setting a layer as a sublayer of another (#4690)

When the layers are reordered at the end of the frame, the sublayers are
placed directly above their respective parents. This allows having Areas
inside Windows, e.g., for the pan-zoom container.

* Closes <#4128>

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui/src/context.rs

@@ -2374,6 +2374,17 @@ impl Context {
         self.memory_mut(|mem| mem.areas_mut().move_to_top(layer_id));
     }
 
+    /// Mark the `child` layer as a sublayer of `parent`.
+    ///
+    /// Sublayers are moved directly above the parent layer at the end of the frame. This is mainly
+    /// intended for adding a new [`Area`] inside a [`Window`].
+    ///
+    /// This currently only supports one level of nesting. If `parent` is a sublayer of another
+    /// layer, the behavior is unspecified.
+    pub fn set_sublayer(&self, parent: LayerId, child: LayerId) {
+        self.memory_mut(|mem| mem.areas_mut().set_sublayer(parent, child));
+    }
+
     /// Retrieve the [`LayerId`] of the top level windows.
     pub fn top_layer_id(&self) -> Option<LayerId> {
         self.memory(|mem| mem.areas().top_layer_id(Order::Middle))

crates/egui/src/memory.rs

@@ -1,6 +1,6 @@
 #![warn(missing_docs)] // Let's keep this file well-documented.` to memory.rs
 
-use ahash::HashMap;
+use ahash::{HashMap, HashSet};
 use epaint::emath::TSTransform;
 
 use crate::{
@@ -951,6 +951,11 @@ pub struct Areas {
     /// So if you close three windows and then reopen them all in one frame,
     /// they will all be sent to the top, but keep their previous internal order.
     wants_to_be_on_top: ahash::HashSet<LayerId>,
+
+    /// List of sublayers for each layer
+    ///
+    /// When a layer has sublayers, they are moved directly above it in the ordering.
+    sublayers: ahash::HashMap<LayerId, HashSet<LayerId>>,
 }
 
 impl Areas {
@@ -1042,27 +1047,62 @@ impl Areas {
         }
     }
 
+    /// Mark the `child` layer as a sublayer of `parent`.
+    ///
+    /// Sublayers are moved directly above the parent layer at the end of the frame. This is mainly
+    /// intended for adding a new [Area](crate::Area) inside a [Window](crate::Window).
+    ///
+    /// This currently only supports one level of nesting. If `parent` is a sublayer of another
+    /// layer, the behavior is unspecified.
+    pub fn set_sublayer(&mut self, parent: LayerId, child: LayerId) {
+        self.sublayers.entry(parent).or_default().insert(child);
+    }
+
     pub fn top_layer_id(&self, order: Order) -> Option<LayerId> {
         self.order
             .iter()
-            .filter(|layer| layer.order == order)
+            .filter(|layer| layer.order == order && !self.is_sublayer(layer))
             .last()
             .copied()
     }
 
+    pub(crate) fn is_sublayer(&self, layer: &LayerId) -> bool {
+        self.sublayers
+            .iter()
+            .any(|(_, children)| children.contains(layer))
+    }
+
     pub(crate) fn end_frame(&mut self) {
         let Self {
             visible_last_frame,
             visible_current_frame,
             order,
             wants_to_be_on_top,
+            sublayers,
             ..
         } = self;
 
         std::mem::swap(visible_last_frame, visible_current_frame);
         visible_current_frame.clear();
         order.sort_by_key(|layer| (layer.order, wants_to_be_on_top.contains(layer)));
         wants_to_be_on_top.clear();
+        // For all layers with sublayers, put the sublayers directly after the parent layer:
+        let sublayers = std::mem::take(sublayers);
+        for (parent, children) in sublayers {
+            let mut moved_layers = vec![parent];
+            order.retain(|l| {
+                if children.contains(l) {
+                    moved_layers.push(*l);
+                    false
+                } else {
+                    true
+                }
+            });
+            let Some(parent_pos) = order.iter().position(|l| l == &parent) else {
+                continue;
+            };
+            order.splice(parent_pos..=parent_pos, moved_layers);
+        }
     }
 }
 

crates/egui_demo_lib/src/demo/pan_zoom.rs

@@ -110,11 +110,10 @@ impl crate::View for PanZoom {
         .into_iter()
         .enumerate()
         {
+            let window_layer = ui.layer_id();
             let id = egui::Area::new(id.with(("subarea", i)))
                 .default_pos(pos)
-                // Need to cover up the pan_zoom demo window,
-                // but may also cover over other windows.
-                .order(egui::Order::Foreground)
+                .order(egui::Order::Middle)
                 .show(ui.ctx(), |ui| {
                     ui.set_clip_rect(transform.inverse() * rect);
                     egui::Frame::default()
@@ -130,6 +129,7 @@ impl crate::View for PanZoom {
                 .response
                 .layer_id;
             ui.ctx().set_transform_layer(id, transform);
+            ui.ctx().set_sublayer(window_layer, id);
         }
     }
 }