How to do it...

1. Create a new GameModeBase class called ToggleHUDGameMode:

2. Add the following UPROPERTY and function definitions to the ToggleHUDGameMode.h file:

#pragma once

#include "CoreMinimal.h"
#include "GameFramework/GameModeBase.h"
#include "SlateBasics.h"
#include "ToggleHUDGameMode.generated.h"

UCLASS()
class CHAPTER_14_API AToggleHUDGameMode : public AGameModeBase
{
 GENERATED_BODY()
 
public:
 UPROPERTY()
 FTimerHandle HUDToggleTimer;

 TSharedPtr<SVerticalBox> widget;

 virtual void BeginPlay() override;
 virtual void EndPlay(const EEndPlayReason::Type EndPlayReason) override;
};

3. Implement BeginPlay with the following code in the method body:

void AToggleHUDGameMode::BeginPlay()
{
    Super::BeginPlay();
    widget = SNew(SVerticalBox)
        + SVerticalBox::Slot()
        .HAlign(HAlign_Center)
        .VAlign(VAlign_Center)
        [
            SNew(SButton)
            .Content()
        [
            SNew(STextBlock)
            .Text(FText::FromString(TEXT("Test button")))
        ]
        ];

    auto player = GetWorld()->GetFirstLocalPlayerFromController();

    GEngine->GameViewport->AddViewportWidgetForPlayer(player, widget.ToSharedRef(), 1);

    auto lambda = FTimerDelegate::CreateLambda
    ([this]
    {
        if (this->widget->GetVisibility().IsVisible())
        {
            this->widget->SetVisibility(EVisibility::Hidden);

        }
        else
        {
            this->widget->SetVisibility(EVisibility::Visible);
        }
    });

    GetWorld()->GetTimerManager().SetTimer(HUDToggleTimer, lambda, 5, true);
}

4. Implement EndPlay:

void AToggleHUDGameMode::EndPlay(const EEndPlayReason::Type EndPlayReason)
{
    Super::EndPlay(EndPlayReason);
    GetWorld()->GetTimerManager().ClearTimer(HUDToggleTimer);
}

5. Compile your code and start the editor.
6. Within the Editor, open World Settings from the toolbar:

7. Inside World Settings, override the level's Game Mode to be our AToggleHUDGameMode:

8. Play the level and verify that the UI toggles its visibility every five seconds.