The best thing you could do for your UMG regardless of whether you use a graphics window or not would be to add in a tick event a HUD voucher, this will require a HUD that in turn will require a driver, to blueprint terms it could be solved as follows.
Base = Canvas Panel / Root Component
BP: HUD Matinee with Custom HUD posted by anonymous | blueprintUE | PasteBin For Unreal Engine 4
Image:
Text:
Event Tick (Your UMG) → Set render opacity
Target: Base
Opacity: Get player controller → Get HUD → Show HUD → Opacity (Transform bool to float)
EDIT:
alternative without taking the Base.
Use: Set Visible