diff --git a/crates/egui/src/context.rs b/crates/egui/src/context.rs
index 4e0c427bbf3..860413c35e8 100644
--- a/crates/egui/src/context.rs
+++ b/crates/egui/src/context.rs
@@ -1459,7 +1459,7 @@ impl Context {
     pub fn end_frame(&self) -> FullOutput {
         crate::profile_function!();
 
-        let mut viewports: Vec<ViewportId> = self.write(|ctx| {
+        let mut viewports: ViewportIdSet = self.write(|ctx| {
             ctx.layer_rects_prev_frame.insert(
                 ctx.viewport_id(),
                 ctx.layer_rects_this_frame
@@ -1468,7 +1468,7 @@ impl Context {
             );
             ctx.viewports.values().map(|vp| vp.id_pair.this).collect()
         });
-        viewports.push(ViewportId::ROOT);
+        viewports.insert(ViewportId::ROOT);
 
         if self.input(|i| i.wants_repaint()) {
             self.request_repaint();
diff --git a/crates/egui/src/memory.rs b/crates/egui/src/memory.rs
index 343e0598d37..89b4a745109 100644
--- a/crates/egui/src/memory.rs
+++ b/crates/egui/src/memory.rs
@@ -6,6 +6,7 @@ use crate::{
     area,
     window::{self, WindowInteraction},
     EventFilter, Id, IdMap, InputState, LayerId, Pos2, Rect, Style, ViewportId, ViewportIdMap,
+    ViewportIdSet,
 };
 
 // ----------------------------------------------------------------------------
@@ -561,7 +562,7 @@ impl Memory {
     pub(crate) fn end_frame(
         &mut self,
         input: &InputState,
-        viewports: &[ViewportId],
+        viewports: &ViewportIdSet,
         used_ids: &IdMap<Rect>,
     ) {
         self.caches.update();