app: enable creation of top most windows

Floating windows are rendered above all other non-floating windows.

Apple Documentation: https://developer.apple.com/documentation/appkit/nswindow/level-swift.struct

Signed-off-by: Thomas Banks <thomas@tombanks.me>

app/os.go

@@ -45,6 +45,8 @@ type Config struct {
 	CustomRenderer bool
 	// Decorated reports whether window decorations are provided automatically.
 	Decorated bool
+	// TopMost windows render above all other non-top-most windows.
+	TopMost bool
 	// Focused reports whether has the keyboard focus.
 	Focused bool
 	// decoHeight is the height of the fallback decoration for platforms such

app/os_macos.go

@@ -241,6 +241,13 @@ static void setTitle(CFTypeRef windowRef, CFTypeRef titleRef) {
 	}
 }
 
+static void setWindowLevel(CFTypeRef windowRef, NSWindowLevel level) {
+	@autoreleasepool {
+		NSWindow *window = (__bridge NSWindow *)windowRef;
+		window.level = level;
+	}
+}
+
 static int isWindowZoomed(CFTypeRef windowRef) {
 	@autoreleasepool {
 		NSWindow *window = (__bridge NSWindow *)windowRef;
@@ -495,6 +502,9 @@ func (w *window) Configure(options []Option) {
 		barTrans = C.YES
 		titleVis = C.NSWindowTitleHidden
 	}
+	if cnf.TopMost {
+		C.setWindowLevel(window, C.NSFloatingWindowLevel)
+	}
 	C.setWindowTitlebarAppearsTransparent(window, barTrans)
 	C.setWindowTitleVisibility(window, titleVis)
 	C.setWindowStyleMask(window, mask)

app/window.go

@@ -969,6 +969,15 @@ func Decorated(enabled bool) Option {
 	}
 }
 
+// TopMost windows will be rendered above all other non-top-most windows.
+//
+// TopMost windows are only supported on MacOS currently.
+func TopMost(enabled bool) Option {
+	return func(_ unit.Metric, cnf *Config) {
+		cnf.TopMost = enabled
+	}
+}
+
 // flushEvent is sent to detect when the user program
 // has completed processing of all prior events. Its an
 // [io/event.Event] but only for internal use.