add option for undecorated_shadow on windows (#2285)

* add option for undecorated_shadow on windows

* formated

core/src/window/settings/windows.rs

@@ -8,13 +8,20 @@ pub struct PlatformSpecific {
 
     /// Whether show or hide the window icon in the taskbar.
     pub skip_taskbar: bool,
+
+    /// Shows or hides the background drop shadow for undecorated windows.
+    ///
+    /// The shadow is hidden by default.
+    /// Enabling the shadow causes a thin 1px line to appear on the top of the window.
+    pub undecorated_shadow: bool,
 }
 
 impl Default for PlatformSpecific {
     fn default() -> Self {
         Self {
             drag_and_drop: true,
             skip_taskbar: false,
+            undecorated_shadow: false,
         }
     }
 }

winit/src/conversion.rs

@@ -79,6 +79,10 @@ pub fn window_attributes(
 
         attributes = attributes
             .with_skip_taskbar(settings.platform_specific.skip_taskbar);
+
+        window_builder = window_builder.with_undecorated_shadow(
+            settings.platform_specific.undecorated_shadow,
+        );
     }
 
     #[cfg(target_os = "macos")]