Prevent the last child of a space from being stolen by other space later

This caused flickering on screen where children shared by two spaces
would rotate on every render.

src/buffers/mod.rs

@@ -123,8 +123,19 @@ pub trait Buffer: Send + Sync + memuse::DynamicUsage {
 
 pub struct Buffers {
   buffers: Vec<Box<dyn Buffer>>,
-  parents: HashMap<BufferId, HashSet<BufferId>>,
+  /// Set of children of each buffer, sorted so that children explicitly listed by a
+  /// space are sorted according to
+  /// https://spec.matrix.org/v1.8/client-server-api/#ordering-of-children-within-a-space
+  /// and rooms that declare that space as their parent are sorted in a best-effort
   children: HashMap<BufferId, SortedVec<(BufferSortKey, BufferId)>>,
+
+  /// Set of parents of each buffer.
+  ///
+  /// Ideally it should be unique, but it is possible for multiple spaces to list a room,
+  /// and/or for rooms to declare multiple parents.
+  /// In order to pick a parent deterministically, we use the smallest room id.
+  parents: HashMap<BufferId, HashSet<BufferId>>,
+
   /// Set of buffers already placed after a parent space, so other spaces should not
   /// steal them, even if they are also their child (or it would cause buffers to move
   /// every time we re-sort the buffer list)
@@ -278,6 +289,7 @@ impl Buffers {
         if let Some(new_buffer) = new_buffer {
           // The parent's subtree was the last subtree
           self.buffers.push(new_buffer);
+          self.attached_to_parent.insert(new_buffer_id.clone());
         }
       },
     }