Add option to make window "always on top" (#528)

* macOS: always_on_top

* Windows: always_on_top

* X11: always_on_top

* Stub set_always_on_top on other platforms

src/platform/macos/ffi.rs

@@ -1,6 +1,6 @@
 // TODO: Upstream these
 
-#![allow(non_snake_case, non_upper_case_globals)]
+#![allow(dead_code, non_snake_case, non_upper_case_globals)]
 
 use cocoa::base::{class, id};
 use cocoa::foundation::{NSInteger, NSUInteger};
@@ -72,3 +72,36 @@ impl NSMutableAttributedString for id {
         msg_send![self, length]
     }
 }
+
+pub const kCGBaseWindowLevelKey: NSInteger = 0;
+pub const kCGMinimumWindowLevelKey: NSInteger = 1;
+pub const kCGDesktopWindowLevelKey: NSInteger = 2;
+pub const kCGBackstopMenuLevelKey: NSInteger = 3;
+pub const kCGNormalWindowLevelKey: NSInteger = 4;
+pub const kCGFloatingWindowLevelKey: NSInteger = 5;
+pub const kCGTornOffMenuWindowLevelKey: NSInteger = 6;
+pub const kCGDockWindowLevelKey: NSInteger = 7;
+pub const kCGMainMenuWindowLevelKey: NSInteger = 8;
+pub const kCGStatusWindowLevelKey: NSInteger = 9;
+pub const kCGModalPanelWindowLevelKey: NSInteger = 10;
+pub const kCGPopUpMenuWindowLevelKey: NSInteger = 11;
+pub const kCGDraggingWindowLevelKey: NSInteger = 12;
+pub const kCGScreenSaverWindowLevelKey: NSInteger = 13;
+pub const kCGMaximumWindowLevelKey: NSInteger = 14;
+pub const kCGOverlayWindowLevelKey: NSInteger = 15;
+pub const kCGHelpWindowLevelKey: NSInteger = 16;
+pub const kCGUtilityWindowLevelKey: NSInteger = 17;
+pub const kCGDesktopIconWindowLevelKey: NSInteger = 18;
+pub const kCGCursorWindowLevelKey: NSInteger = 19;
+pub const kCGNumberOfWindowLevelKeys: NSInteger = 20;
+
+pub enum NSWindowLevel {
+    NSNormalWindowLevel = kCGBaseWindowLevelKey as _,
+    NSFloatingWindowLevel = kCGFloatingWindowLevelKey as _,
+    NSTornOffMenuWindowLevel = kCGTornOffMenuWindowLevelKey as _,
+    NSModalPanelWindowLevel = kCGModalPanelWindowLevelKey as _,
+    NSMainMenuWindowLevel = kCGMainMenuWindowLevelKey as _,
+    NSStatusWindowLevel = kCGStatusWindowLevelKey as _,
+    NSPopUpMenuWindowLevel = kCGPopUpMenuWindowLevelKey as _,
+    NSScreenSaverWindowLevel = kCGScreenSaverWindowLevelKey as _,
+}

src/platform/macos/window.rs

@@ -25,6 +25,7 @@ use std::sync::Weak;
 use std::cell::{Cell, RefCell};
 
 use super::events_loop::{EventsLoop, Shared};
+use platform::platform::ffi;
 use platform::platform::util;
 use platform::platform::view::{new_view, set_ime_spot};
 
@@ -774,6 +775,10 @@ impl Window2 {
                 if pl_attrs.movable_by_window_background {
                     window.setMovableByWindowBackground_(YES);
                 }
+
+                if attrs.always_on_top {
+                    let _: () = msg_send![*window, setLevel:ffi::NSWindowLevel::NSFloatingWindowLevel];
+                }
                 
                 if let Some((x, y)) = pl_attrs.resize_increments {
                     if x >= 1 && y >= 1 {
@@ -1066,6 +1071,18 @@ impl Window2 {
         }
     }
 
+    #[inline]
+    pub fn set_always_on_top(&self, always_on_top: bool) {
+        unsafe {
+            let level = if always_on_top {
+                ffi::NSWindowLevel::NSFloatingWindowLevel
+            } else {
+                ffi::NSWindowLevel::NSNormalWindowLevel
+            };
+            let _: () = msg_send![*self.window, setLevel:level];
+        }
+    }
+
     #[inline]
     pub fn set_window_icon(&self, _icon: Option<::Icon>) {
         // macOS doesn't have window icons. Though, there is `setRepresentedFilename`, but that's