Bug 1610613 - Make Picture-in-Picture window persist across workspaces on macOS. r=spohl

Differential Revision: https://phabricator.services.mozilla.com/D69932

--HG--
extra : moz-landing-system : lando

widget/cocoa/nsCocoaWindow.mm

@@ -496,6 +496,9 @@ nsresult nsCocoaWindow::CreateNativeWindow(const NSRect& aRect, nsBorderStyle aB
 
   if (mAlwaysOnTop) {
     [mWindow setLevel:NSFloatingWindowLevel];
+    NSWindowCollectionBehavior newBehavior = [mWindow collectionBehavior];
+    newBehavior |= NSWindowCollectionBehaviorCanJoinAllSpaces;
+    [mWindow setCollectionBehavior:newBehavior];
   }
 
   [mWindow setContentMinSize:NSMakeSize(60, 60)];