macOS: fix opacity handling

Not using `NSColor::clearColor()` results in Quartz thinking that the
window is not transparent at all, which results in artifacts.

However, not setting the `windowBackgroundColor` in
`Window::set_transparent` results in border not properly rendered.

Fixes: 94664ff687 (Don't set the background color)

Cargo.toml

@@ -142,6 +142,7 @@ features = [
     "NSApplication",
     "NSBitmapImageRep",
     "NSButton",
+    "NSColor",
     "NSControl",
     "NSCursor",
     "NSDragging",

src/platform_impl/macos/window_delegate.rs

@@ -10,7 +10,7 @@ use objc2::runtime::{AnyObject, ProtocolObject};
 use objc2::{declare_class, msg_send_id, mutability, sel, ClassType, DeclaredClass};
 use objc2_app_kit::{
     NSAppKitVersionNumber, NSAppKitVersionNumber10_12, NSAppearance, NSApplication,
-    NSApplicationPresentationOptions, NSBackingStoreType, NSDraggingDestination,
+    NSApplicationPresentationOptions, NSBackingStoreType, NSColor, NSDraggingDestination,
     NSFilenamesPboardType, NSPasteboard, NSRequestUserAttentionType, NSScreen, NSView,
     NSWindowButton, NSWindowDelegate, NSWindowFullScreenButton, NSWindowLevel,
     NSWindowOcclusionState, NSWindowOrderingMode, NSWindowSharingType, NSWindowStyleMask,
@@ -613,6 +613,8 @@ fn new_window(
 
         if attrs.transparent {
             window.setOpaque(false);
+            // See `set_transparent` for details on why we do this.
+            window.setBackgroundColor(unsafe { Some(&NSColor::clearColor()) });
         }
 
         // register for drag and drop operations.
@@ -821,7 +823,23 @@ impl WindowDelegate {
     }
 
     pub fn set_transparent(&self, transparent: bool) {
-        self.window().setOpaque(!transparent)
+        // This is just a hint for Quartz, it doesn't actually speculate with window alpha.
+        // Providing a wrong value here could result in visual artifacts, when the window is
+        // transparent.
+        self.window().setOpaque(!transparent);
+
+        // AppKit draws the window with a background color by default, which is usually really
+        // nice, but gets in the way when we want to allow the contents of the window to be
+        // transparent, as in that case, the transparent contents will just be drawn on top of
+        // the background color. As such, to allow the window to be transparent, we must also set
+        // the background color to one with an empty alpha channel.
+        let color = if transparent {
+            unsafe { NSColor::clearColor() }
+        } else {
+            unsafe { NSColor::windowBackgroundColor() }
+        };
+
+        self.window().setBackgroundColor(Some(&color));
     }
 
     pub fn set_blur(&self, blur: bool) {

src/window.rs

@@ -942,8 +942,7 @@ impl Window {
     ///
     /// ## Platform-specific
     ///
-    /// - **macOS:** If you're not drawing to the window yourself, you might have to set the
+    /// - **macOS:** This will reset the window's background color.
-    ///   background color of the window to enable transparency.
     /// - **Web / iOS / Android:** Unsupported.
     /// - **X11:** Can only be set while building the window, with
     ///   [`WindowAttributes::with_transparent`].