Fix tab bar button's focus handling lagging behind (#4247)

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.

### Before


https://github.com/zed-industries/zed/assets/1185253/24b56e51-467b-4a09-909f-72ee6f76f32c

### After



https://github.com/zed-industries/zed/assets/1185253/18865f49-d00e-453a-8a38-aee15f06beb8



### Release Notes:

- (Added|Fixed|Improved) ...
([#<public_issue_number_if_exists>](https://github.com/zed-industries/community/issues/<public_issue_number_if_exists>)).