[WelcomeUX] Overlay moved up in OSX insider

[chrmarti]

Testing #18091

My guess is that the custom title bar affects it. Placing it a little lower in the DOM might fix this.

[chrmarti]

@bgashler1 I'm fixing this by making the overlay smaller if there is a custom titlebar, that's easiest for now. We could change that to instead move all affected arrows and labels down, such that, the titlebar is blurred by the overlay too.