how to keep autocomplete panel open when click outside the panel without select option?

#### Bug, feature request, or proposal:
this is a feature request

#### What is the expected behavior?
i want to keep autocomplete panel open when click outside the panel without select an option

#### What is the current behavior?
the panel is closed when click outside

#### What are the steps to reproduce?
https://stackblitz.com/angular/ynbmnnrjolq

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

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
i work with angular 7.x

#### Is there anything else we should know?

Start with the linked StackBlitz reproduction and its Angular 7.x autocomplete setup. Trace the behavior that closes the panel after an outside click, then define the change so the panel remains open when no option is selected. Done means the reproduced interaction keeps the panel open without selecting an option.

Written by the indexing model from the issue text.