Comment 4 for bug 1077260

It seems the search field loses focus to some kind of tab when typing.

Clicking the search field:
At-spi focus event: focused=True, name=Search, role=ATSPI_ROLE_TEXT, editable=True
-> Keyboard shows

Typing one letter and this tab gets the focus:
At-spi focus event: focused=True, name=, role=ATSPI_ROLE_PAGE_TAB, editable=False
-> Keyboard hides, not an editable text entry

Then just moving the pointer to the search field, or tapping on the Nexus, focuses search again:
At-spi focus event: focused=True, name=Search, role=ATSPI_ROLE_TEXT, editable=True
-> Keyboard shows

I can't say much about this tab, except that the role ATSPI_ROLE_PAGE_TAB is the same one Onboard sees when clicking the tab bar in gnome-terminal.