in src/dotnet/JetBrains.PresentationAssistant/FadingWpfPopupWindow.cs [18:26]
public FadingWpfPopupWindow(LifetimeDefinition lifetimeDefinition, IPopupWindowContext context,
PopupWindowMutex mutex, PopupWindowManager popupWindowManager,
Window window, double opacity, HideFlags hideFlags = HideFlags.None)
: base(lifetimeDefinition, context, mutex, popupWindowManager, window, hideFlags)
{
this.window = window;
this.opacity = opacity;
window.AllowsTransparency = true;
}