mat-menu disappears after opening on mobile Safari

#### Bug, feature request, or proposal:
Bug

#### What is the expected behavior?
Menus open and stay open until dismissed

#### What is the current behavior?
Menus open for a split second and then disappear

#### What are the steps to reproduce?
This is only on 7.0.0-beta.0, I need a starter stackblitz for that version. Any basic menu should reproduce it.

#### What is the use-case or motivation for changing an existing behavior?

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
7.0.0-beta.0

#### Is there anything else we should know?
The issue goes away if you use lazy rendering, ie. `ng-template` with `matMenuContent`.

Crea el StackBlitz inicial solicitado usando Angular Material 7.0.0-beta.0 y reproduce el problema con un mat-menu básico en Mobile Safari. Compara el comportamiento con el renderizado diferido mediante ng-template y matMenuContent. La tarea está terminada cuando el menú se abre y permanece abierto hasta que se cierra, y se ha capturado la reproducción o la prueba relevante.

Escrito por el modelo de indexación a partir del texto del issue.