Do two passes if the option is enabled

egui/src/context.rs

@@ -102,14 +102,49 @@ impl CtxRef {
     #[must_use]
     pub fn run(
         &mut self,
-        new_input: RawInput,
+        new_raw_input: RawInput,
-        run_ui: impl FnOnce(&CtxRef),
+        mut run_ui: impl FnMut(&CtxRef),
     ) -> (Output, Vec<ClippedShape>) {
+        self.memory().begin_frame(&self.input, &new_raw_input);
+
         let mut self_: Context = (*self.0).clone();
-        self_.begin_frame_mut(new_input);
+        if let Some(new_pixels_per_point) = self_.memory.lock().new_pixels_per_point.take() {
+            self_.input.pixels_per_point = new_pixels_per_point;
+        }
+        self_.input.begin_frame(&new_raw_input);
+        self_.update_fonts(self_.input.pixels_per_point());
         *self = Self(Arc::new(self_));
 
-        run_ui(self);
+        // Ensure we register the background area so panels and background ui can catch clicks:
+        let screen_rect = self.input.screen_rect();
+        self.memory().areas.set_state(
+            LayerId::background(),
+            containers::area::State {
+                pos: screen_rect.min,
+                size: screen_rect.size(),
+                interactable: true,
+            },
+        );
+
+        if self.memory().options.multi_pass {
+            self.frame_state.lock().begin_pass(&self.input);
+            run_ui(self);
+
+            self.drain_paint_lists();
+            let mut self_: Context = (*self.0).clone();
+            self_.input.on_events(new_raw_input);
+            *self = Self(Arc::new(self_));
+
+            self.frame_state.lock().begin_pass(&self.input);
+            run_ui(self);
+        } else {
+            let mut self_: Context = (*self.0).clone();
+            self_.input.on_events(new_raw_input);
+            *self = Self(Arc::new(self_));
+
+            self.frame_state.lock().begin_pass(&self.input);
+            run_ui(self);
+        }
 
         self.end_frame()
     }
@@ -566,30 +601,6 @@ impl Context {
 
     // ---------------------------------------------------------------------
 
-    fn begin_frame_mut(&mut self, new_raw_input: RawInput) {
-        self.memory().begin_frame(&self.input, &new_raw_input);
-
-        if let Some(new_pixels_per_point) = self.memory.lock().new_pixels_per_point.take() {
-            self.input.pixels_per_point = new_pixels_per_point;
-        }
-        self.input.begin_frame(&new_raw_input);
-        self.input.on_events(new_raw_input);
-        self.frame_state.lock().begin_frame(&self.input);
-
-        self.update_fonts(self.input.pixels_per_point());
-
-        // Ensure we register the background area so panels and background ui can catch clicks:
-        let screen_rect = self.input.screen_rect();
-        self.memory().areas.set_state(
-            LayerId::background(),
-            containers::area::State {
-                pos: screen_rect.min,
-                size: screen_rect.size(),
-                interactable: true,
-            },
-        );
-    }
-
     /// Load fonts unless already loaded.
     fn update_fonts(&mut self, pixels_per_point: f32) {
         let new_font_definitions = self.memory().new_font_definitions.take();

egui/src/frame_state.rs

@@ -1,10 +1,12 @@
 use crate::*;
 
-/// State that is collected during a frame and then cleared.
+// TODO: rename `FrameState` -> `PassState` ?
-/// Short-term (single frame) memory.
+/// State that is collected during a pass and then cleared.
+///
+/// One frame consists of either one or two passes.
 #[derive(Clone)]
 pub(crate) struct FrameState {
-    /// All `Id`s that were used this frame.
+    /// All `Id`s that were used this pass.
     /// Used to debug `Id` clashes of widgets.
     pub(crate) used_ids: IdMap<Rect>,
 
@@ -20,9 +22,9 @@ pub(crate) struct FrameState {
     /// How much space is used by panels.
     pub(crate) used_by_panels: Rect,
 
-    /// If a tooltip has been shown this frame, where was it?
+    /// If a tooltip has been shown this pass, where was it?
     /// This is used to prevent multiple tooltips to cover each other.
-    /// Initialized to `None` at the start of each frame.
+    /// Initialized to `None` at the start of each pass.
     pub(crate) tooltip_rect: Option<(Id, Rect, usize)>,
 
     /// Cleared by the first `ScrollArea` that makes use of it.
@@ -46,7 +48,7 @@ impl Default for FrameState {
 }
 
 impl FrameState {
-    pub(crate) fn begin_frame(&mut self, input: &InputState) {
+    pub(crate) fn begin_pass(&mut self, input: &InputState) {
         let Self {
             used_ids,
             available_rect,