Fix tab bar button's focus handling lagging behind

This fixes https://github.com/zed-industries/community/issues/2442 for
me.

Previously, the tab bar buttons would stay when the chat panel was
opened (and received focus) until something else was drawn (or caused a
render?)

With this change, the tab bar buttons are only shown if the pane was
focus.

I'm not sure about the side-effects of this, but the toolbar still seems
to work fine.