[hover preview] Popover not dismissed when changing docs with keyboard

[redmunds]

1. Open 2 docs in the working set
2. Move mouse to where hover preview shows popover
3. Without moving mouse, Ctrl-Tab to switch to other doc

Result:
Popover is not dismissed even though mouse position in second doc does not invoke popover.

Expected:
Popover dismissed (or updated) when contents under it change.

[gruehle]

Reviewed--assigned to @redmunds

[peterflynn]

Confirmed fixed