Clarify logic around how viewports are retained, and add ViewportIdSet

crates/egui/src/context.rs

@@ -1551,7 +1551,7 @@ impl Context {
                 let was_used = viewport.used;
 
                 if viewport_id == viewport.id_pair.parent {
-                    viewport.used = false;
+                    viewport.used = false; // reset so we can check again next frame
                 }
 
                 viewports.push(ViewportOutput {
@@ -1560,8 +1560,19 @@ impl Context {
                     viewport_ui_cb: viewport.viewport_ui_cb.clone(),
                 });
 
-                (was_used || viewport_id != viewport.id_pair.parent)
-                    && all_viewport_ids.contains(&viewport.id_pair.parent)
+                if !all_viewport_ids.contains(&viewport.id_pair.parent) {
+                    // Parent is gone - remove this viewport.
+                    return false;
+                }
+
+                let is_child = viewport_id == viewport.id_pair.parent;
+                if is_child {
+                    // Keep all children that have been updated this frame
+                    was_used
+                } else {
+                    // Somebody elses child - don't touch
+                    true
+                }
             });
         });
 

crates/egui/src/data/output.rs

@@ -22,8 +22,10 @@ pub struct FullOutput {
     /// You can use [`crate::Context::tessellate`] to turn this into triangles.
     pub shapes: Vec<epaint::ClippedShape>,
 
+    /// All the active viewports, including the root.
     pub viewports: Vec<ViewportOutput>,
 
+    /// Commands sent to different viewports.
     pub viewport_commands: Vec<(ViewportId, ViewportCommand)>,
 }
 

crates/egui/src/viewport.rs

@@ -641,6 +641,7 @@ pub(crate) struct Viewport {
     /// Id of us and our parent.
     pub(crate) id_pair: ViewportIdPair,
 
+    /// Has this viewport been updated this frame?
     pub(crate) used: bool,
 
     /// The user-code that shows the GUI, used for "async" viewports.