Previously, `Window::Notifications::Default::Manager` would not start
hiding notifications that are fading in when other notifications should
hide. This would lead to some notifications never hiding, e.g., when the
cursor passes through the notification too quickly and there was not
enough time for the notification to fade in completely.
Also renamed `Widget::isShowing` -> `Widget::isFadingIn` for clarity.
Fixes#28811.
Nikolai Nechaev
7f6221b409