SAssignNew(ContainerHBox,SHorizontalBox)
+ SHorizontalBox::Slot().VAlign(VAlign_Fill).HAlign(HAlign_Fill)
Sorry I didnt notice this… you add empty slot to your Container at construction time so after you call ContainerHBox->AddSlot() you have 2 slots in your container, not one. During construction +SHorizontalBox::Slot() works same as AddSlot() later on. So make it like this:
TSharedPtr<SHorizontalBox> ContainerHBox;
Menu->AddSlot().AutoHeight().VAlign(VAlign_Fill).HAlign(HAlign_Fill)
SAssignNew(ContainerHBox,SHorizontalBox)
];
ContainerHBox->AddSlot().VAlign(VAlign_Fill).HAlign(HAlign_Fill)
SNew(STextBlock)
.Text(LOCTEXT("TextExample", "TextExample").ToString())
.Font(FSlateFontInfo(FPaths::EngineContentDir() / TEXT("Slate/Fonts/Roboto-Bold.ttf"), 16))
];