Highlight the header of the topmost Window

crates/egui/src/containers/window.rs

@@ -405,10 +405,19 @@ impl<'open> Window<'open> {
         let resize = resize.resizable(false); // We move it manually
         let mut resize = resize.id(resize_id);
 
+        let on_top = Some(area_layer_id) == ctx.top_layer_id();
         let mut area = area.begin(ctx);
 
         let title_content_spacing = 2.0 * ctx.style().spacing.item_spacing.y;
 
+        // Calculate roughly how much larger the window size is compared to the inner rect
+        let title_bar_height = if with_title_bar {
+            let style = ctx.style();
+            ctx.fonts(|f| title.font_height(f, &style)) + title_content_spacing * 2.0
+        } else {
+            0.0
+        };
+
         // First interact (move etc) to avoid frame delay:
         let last_frame_outer_rect = area.state().rect();
         let interaction = if possible.movable || possible.resizable() {
@@ -420,13 +429,6 @@ impl<'open> Window<'open> {
                 last_frame_outer_rect,
             )
             .and_then(|window_interaction| {
-                // Calculate roughly how much larger the window size is compared to the inner rect
-                let title_bar_height = if with_title_bar {
-                    let style = ctx.style();
-                    ctx.fonts(|f| title.font_height(f, &style)) + title_content_spacing
-                } else {
-                    0.0
-                };
                 let margins = frame.outer_margin.sum()
                     + frame.inner_margin.sum()
                     + vec2(0.0, title_bar_height);
@@ -453,6 +455,9 @@ impl<'open> Window<'open> {
             let mut frame = frame.begin(&mut area_content_ui);
 
             let show_close_button = open.is_some();
+
+            let where_to_put_header_background = &area_content_ui.painter().add(Shape::Noop);
+
             let title_bar = if with_title_bar {
                 let title_bar = show_title_bar(
                     &mut frame.content_ui,
@@ -489,6 +494,27 @@ impl<'open> Window<'open> {
             // END FRAME --------------------------------
 
             if let Some(title_bar) = title_bar {
+                if on_top {
+                    let rect = Rect::from_min_size(
+                        outer_rect.min,
+                        Vec2 {
+                            x: outer_rect.size().x,
+                            y: title_bar_height,
+                        },
+                    );
+                    let mut round = area_content_ui.visuals().window_rounding;
+                    if !is_collapsed {
+                        round.se = 0.0;
+                        round.sw = 0.0;
+                    }
+                    let header_color = area_content_ui.visuals().widgets.hovered.bg_fill;
+
+                    area_content_ui.painter().set(
+                        *where_to_put_header_background,
+                        RectShape::filled(rect, round, header_color),
+                    );
+                };
+
                 title_bar.ui(
                     &mut area_content_ui,
                     outer_rect,

crates/egui/src/context.rs

@@ -1982,6 +1982,11 @@ impl Context {
         self.memory_mut(|mem| mem.areas_mut().move_to_top(layer_id));
     }
 
+    /// 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))
+    }
+
     pub(crate) fn rect_contains_pointer(&self, layer_id: LayerId, rect: Rect) -> bool {
         rect.is_positive() && {
             let pointer_pos = self.input(|i| i.pointer.interact_pos());

crates/egui/src/memory.rs

@@ -1,11 +1,10 @@
 #![warn(missing_docs)] // Let's keep this file well-documented.` to memory.rs
 
-use epaint::{emath::Rangef, vec2, Vec2};
-
 use crate::{
-    area,
+    area, vec2,
     window::{self, WindowInteraction},
-    EventFilter, Id, IdMap, LayerId, Pos2, Rect, Style, ViewportId, ViewportIdMap, ViewportIdSet,
+    EventFilter, Id, IdMap, LayerId, Order, Pos2, Rangef, Rect, Style, Vec2, ViewportId,
+    ViewportIdMap, ViewportIdSet,
 };
 
 // ----------------------------------------------------------------------------
@@ -908,6 +907,14 @@ impl Areas {
         }
     }
 
+    pub fn top_layer_id(&self, order: Order) -> Option<LayerId> {
+        self.order
+            .iter()
+            .filter(|layer| layer.order == order)
+            .last()
+            .copied()
+    }
+
     pub(crate) fn end_frame(&mut self) {
         let Self {
             visible_last_frame,

crates/egui_demo_lib/src/demo/window_options.rs

@@ -138,7 +138,10 @@ impl super::View for WindowOptions {
         });
 
         ui.separator();
+        let on_top = Some(ui.layer_id()) == ui.ctx().top_layer_id();
+        ui.label(format!("This window is on top: {on_top}."));
 
+        ui.separator();
         ui.horizontal(|ui| {
             if ui.button("Disable for 2 seconds").clicked() {
                 self.disabled_time = ui.input(|i| i.time);