[Mac] Use a nonactivating panel for notifications so that mouse down doesn't activate Chrome.

ui/message_center/cocoa/popup_controller.mm

Index: ui/message_center/cocoa/popup_controller.mm
diff --git a/ui/message_center/cocoa/popup_controller.mm b/ui/message_center/cocoa/popup_controller.mm
index 7e8407d11def8ecb9bf6bb98fc25aabca7970cde..e8e77281495856a42ec053b0373fa7fec2cd64ac 100644
--- a/ui/message_center/cocoa/popup_controller.mm
+++ b/ui/message_center/cocoa/popup_controller.mm
@@ -31,7 +31,7 @@
 
 // Window Subclass /////////////////////////////////////////////////////////////
 
-@interface MCPopupWindow : NSWindow {
+@interface MCPopupWindow : NSPanel {
   // The cumulative X and Y scrollingDeltas since the -scrollWheel: event began.
   NSPoint totalScrollDelta_;
 }
@@ -98,27 +98,23 @@
 - (id)initWithNotification:(const message_center::Notification*)notification
              messageCenter:(message_center::MessageCenter*)messageCenter
            popupCollection:(MCPopupCollection*)popupCollection {
-  base::scoped_nsobject<MCPopupWindow> window(
-      [[MCPopupWindow alloc] initWithContentRect:ui::kWindowSizeDeterminedLater
-                                       styleMask:NSBorderlessWindowMask
-                                         backing:NSBackingStoreBuffered
-                                           defer:NO]);
+  base::scoped_nsobject<MCPopupWindow> window([[MCPopupWindow alloc]
+      initWithContentRect:ui::kWindowSizeDeterminedLater
+                styleMask:NSNonactivatingPanelMask
+                  backing:NSBackingStoreBuffered
+                    defer:NO]);
   if ((self = [super initWithWindow:window])) {
     messageCenter_ = messageCenter;
     popupCollection_ = popupCollection;
     notificationController_.reset(
         [[MCNotificationController alloc] initWithNotification:notification
                                                  messageCenter:messageCenter_]);
-    isClosing_ = NO;
     bounds_ = [[notificationController_ view] frame];
 
-    [window setReleasedWhenClosed:NO];
-
-    [window setLevel:NSFloatingWindowLevel];
-    [window setExcludedFromWindowsMenu:YES];
+    [window setFloatingPanel:YES];
+    [window setBecomesKeyOnlyIfNeeded:YES];

[tapted, 2016/10/11 03:55:44]

ah-ha! So If I comment out this line, then crbug.com/459306 easily reproduces
for me again with this patched in (maybe worth trying out so you have an idea of
the badness).

So perhaps so long as we don't put textfields into the notification I think this
will hold up -- we're essentially just adding an alternative workaround. I'm
pretty sure we don't put textfields in - but we _do_ have buttons. However, said
buttons are not keyboard accessible. But if we want them to be, then this bug
might crop up again. (and it's possible a way to resolve the issue then is to
have canBecomeKeyWindow report YES rather than NO). But, maybe we'll have native
notifications by then.

The CL description still needs to be updated - please remove any mention of
apps.

I also wouldn't use a tricky repro as a justification for risk taking. However,
since we've narrowed it down to this `setBecomesKeyOnlyIfNeeded` line, then we
should talk about that instead.

     [window
         setCollectionBehavior:NSWindowCollectionBehaviorCanJoinAllSpaces |
-                              NSWindowCollectionBehaviorIgnoresCycle |
                               NSWindowCollectionBehaviorFullScreenAuxiliary];
 
     [window setHasShadow:YES];