title: Menu is not opening on keydown.enter when used inside mat-expansion panel header in angular 6
body: <p>I am using the mat-menu with mat-expansion-panel where there is an expansion panel, i placed the mat-icon at the end of header of expansion panel, on click of this icon it will open a menu item. on click of header of expansion panel it opens the panel description . This is the desired behaviour, this is working using mouse click.</p>
  
  <p>But this is not same when i navigate to icon by keydown.enter(using tab), it was collapsing the panel when i tab into icon and press enter, somehow i fixed it with event.stopPropagation() and by returning false. But it is not opening the menu item on press of enter on the icon. Here is the stackblitz link <a href="https://stackblitz.com/edit/angular-oyz5pr" rel="nofollow noreferrer">menuwithExpansionPanel</a>. Please help me in resolving this. Thanks in advance.</p>