Hi,

https://stackblitz.com/edit/angular-dadm5ywe?file=src%2Fapp%2Fapp.component.ts

Please select the top-most button, then press TAB until the focus is on the button in the last grid cell.
Then press tab again => the focus moves to the last cell itself.
Press tab again => the focus moves back to the last button. 
=> here we got into an endless loop, you cannot navigate to the button below the grid.

Interestingly, you can leave the grid if you navigate backwards with shift+tab.

best regards,

Michael