subclass windows in macos so they can be made resizable even with no decorations (#408)

* make windows without decorations resizable and movable in macos

fixes #368

The subclassing logic was copied from servo's fork of glutin:
63026a0f4c/src/api/cocoa/mod.rs (L418)

* remove `isMovableByWindowBackground` and `mouseDownCanMoveWindow`

* revert example changes

* remove resizable mask from decoration: false

* avoid duplicate class declarations

* update changelog

* fix changelog

* changelog whitespace

CHANGELOG.md

@@ -1,5 +1,7 @@
 # Unreleased
 
+- Added subclass to macos windows so they can be made resizable even with no decorations.
+
 # Version 0.11.3 (2018-03-28)
 
 - Added `set_min_dimensions` and `set_max_dimensions` methods to `Window`, and implemented on Windows, X11, Wayland, and OSX.

src/platform/macos/window.rs

@@ -414,10 +414,7 @@ impl Window2 {
                     NSWindowStyleMask::NSTitledWindowMask
             } else if !attrs.decorations {
                 // Window2 without a titlebar
-                NSWindowStyleMask::NSClosableWindowMask |
+                NSWindowStyleMask::NSBorderlessWindowMask
-                    NSWindowStyleMask::NSMiniaturizableWindowMask |
-                    NSWindowStyleMask::NSResizableWindowMask |
-                    NSWindowStyleMask::NSFullSizeContentViewWindowMask
             } else if pl_attrs.titlebar_hidden {
                 NSWindowStyleMask::NSBorderlessWindowMask |
                     NSWindowStyleMask::NSResizableWindowMask
@@ -442,7 +439,18 @@ impl Window2 {
                     NSWindowStyleMask::NSTitledWindowMask
             };
 
-            let window = IdRef::new(NSWindow::alloc(nil).initWithContentRect_styleMask_backing_defer_(
+            let winit_window = Class::get("WinitWindow").unwrap_or_else(|| {
+                let window_superclass = Class::get("NSWindow").unwrap();
+                let mut decl = ClassDecl::new("WinitWindow", window_superclass).unwrap();
+                decl.add_method(sel!(canBecomeMainWindow), yes as extern fn(&Object, Sel) -> BOOL);
+                decl.add_method(sel!(canBecomeKeyWindow), yes as extern fn(&Object, Sel) -> BOOL);
+                decl.register();
+                Class::get("WinitWindow").unwrap()
+            });
+
+            let window: id = msg_send![winit_window, alloc];
+
+            let window = IdRef::new(window.initWithContentRect_styleMask_backing_defer_(
                 frame,
                 masks,
                 appkit::NSBackingStoreBuffered,
@@ -796,3 +804,7 @@ impl Clone for IdRef {
         IdRef(self.0)
     }
 }
+
+extern fn yes(_: &Object, _: Sel) -> BOOL {
+    YES
+}