Add macOS-specific has_shadow and with_has_shadow to ViewportBuilder (#6850)

* [X] I have followed the instructions in the PR template

This PR fixes a ghosting issue I encountered while making a native macOS
transparent overlay app using egui and eframe by exposing the [existing
macOS window attribute
`has_shadow`](https://docs.rs/winit/latest/winit/platform/macos/trait.WindowExtMacOS.html#tymethod.has_shadow)
to the `ViewportBuilder` via a new `with_has_shadow` option.

## Example of Ghosting Issue

### Before `ViewportBuilder::with_has_shadow`
By default, the underlying `winit` window's `.has_shadow()` defaults to
`true`.


https://github.com/user-attachments/assets/c3dcc2bd-535a-4960-918e-3ae5df503b12

### After `ViewportBuilder::with_has_shadow`


https://github.com/user-attachments/assets/484462a1-ea88-43e6-85b4-0bb9724e5f14

Source code for the above example can be found here:

https://github.com/gaelanmcmillan/egui-overlay-app-with-shadow-artifacts-example/blob/main/src/main.rs


### Further background
By default on macOS, `winit` windows have a drop-shadow effect. When
creating a fully transparent overlay GUI, this drop-shadow can create a
ghosting effect, as the window content has a drop shadow which is not
cleared by the app itself. This issue has been experienced by users of
`bevy`, another Rust project that has an upstream dependency on `winit`:
bevyengine/bevy#18673

Author: gaelanmcmillan

crates/egui-winit/src/lib.rs

@@ -1620,6 +1620,7 @@ pub fn create_winit_window_attributes(
         title_shown: _title_shown,
         titlebar_buttons_shown: _titlebar_buttons_shown,
         titlebar_shown: _titlebar_shown,
+        has_shadow: _has_shadow,
 
         // Windows:
         drag_and_drop: _drag_and_drop,
@@ -1764,7 +1765,8 @@ pub fn create_winit_window_attributes(
             .with_titlebar_buttons_hidden(!_titlebar_buttons_shown.unwrap_or(true))
             .with_titlebar_transparent(!_titlebar_shown.unwrap_or(true))
             .with_fullsize_content_view(_fullsize_content_view.unwrap_or(false))
-            .with_movable_by_window_background(_movable_by_window_background.unwrap_or(false));
+            .with_movable_by_window_background(_movable_by_window_background.unwrap_or(false))
+            .with_has_shadow(_has_shadow.unwrap_or(true));
     }
 
     window_attributes

crates/egui/src/viewport.rs

@@ -294,6 +294,7 @@ pub struct ViewportBuilder {
     pub title_shown: Option<bool>,
     pub titlebar_buttons_shown: Option<bool>,
     pub titlebar_shown: Option<bool>,
+    pub has_shadow: Option<bool>,
 
     // windows:
     pub drag_and_drop: Option<bool>,
@@ -380,6 +381,10 @@ impl ViewportBuilder {
     /// The default is `false`.
     /// If this is not working, it's because the graphic context doesn't support transparency,
     /// you will need to set the transparency in the eframe!
+    ///
+    /// ## Platform-specific
+    ///
+    /// **macOS:** When using this feature to create an overlay-like UI, you likely want to combine this with [`Self::with_has_shadow`] set to `false` in order to avoid ghosting artifacts.
     #[inline]
     pub fn with_transparent(mut self, transparent: bool) -> Self {
         self.transparent = Some(transparent);
@@ -433,7 +438,6 @@ impl ViewportBuilder {
     }
 
     /// macOS: Set to `true` to allow the window to be moved by dragging the background.
-    ///
     /// Enabling this feature can result in unexpected behaviour with draggable UI widgets such as sliders.
     #[inline]
     pub fn with_movable_by_background(mut self, value: bool) -> Self {
@@ -462,6 +466,19 @@ impl ViewportBuilder {
         self
     }
 
+    /// macOS: Set to `false` to make the window render without a drop shadow.
+    ///
+    /// The default is `true`.
+    ///
+    /// Disabling this feature can solve ghosting issues experienced if using [`Self::with_transparent`].
+    ///
+    /// Look at winit for more details
+    #[inline]
+    pub fn with_has_shadow(mut self, has_shadow: bool) -> Self {
+        self.has_shadow = Some(has_shadow);
+        self
+    }
+
     /// windows: Whether show or hide the window icon in the taskbar.
     #[inline]
     pub fn with_taskbar(mut self, show: bool) -> Self {
@@ -653,6 +670,7 @@ impl ViewportBuilder {
             title_shown: new_title_shown,
             titlebar_buttons_shown: new_titlebar_buttons_shown,
             titlebar_shown: new_titlebar_shown,
+            has_shadow: new_has_shadow,
             close_button: new_close_button,
             minimize_button: new_minimize_button,
             maximize_button: new_maximize_button,
@@ -823,6 +841,11 @@ impl ViewportBuilder {
             recreate_window = true;
         }
 
+        if new_has_shadow.is_some() && self.has_shadow != new_has_shadow {
+            self.has_shadow = new_has_shadow;
+            recreate_window = true;
+        }
+
         if new_taskbar.is_some() && self.taskbar != new_taskbar {
             self.taskbar = new_taskbar;
             recreate_window = true;