keyboard: assert that page is still visible
keyboard: assert that page is still visible

Otherwise you get an extremely confusing exception because
`page.selection` is `None` when the page isn't visible; and
`networkx.shortest_path(source=None)` returns a dict (of all paths from
every node in the graph) instead of a list, which causes our code later
on to break.

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester