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>)).

crates/workspace/src/pane.rs

@@ -342,7 +342,16 @@ impl Pane {
     }
 
     pub fn has_focus(&self, cx: &WindowContext) -> bool {
+        // We not only check whether our focus handle contains focus, but also
+        // whether the active_item might have focus, because we might have just activated an item
+        // but that hasn't rendered yet.
+        // So before the next render, we might have transferred focus
+        // to the item and `focus_handle.contains_focus` returns false because the `active_item`
+        // is not hooked up to us in the dispatch tree.
         self.focus_handle.contains_focused(cx)
+            || self
+                .active_item()
+                .map_or(false, |item| item.focus_handle(cx).contains_focused(cx))
     }
 
     fn focus_in(&mut self, cx: &mut ViewContext<Self>) {
@@ -1470,7 +1479,7 @@ impl Pane {
                         ),
                 )
             })
-            .when(self.was_focused || self.has_focus(cx), |tab_bar| {
+            .when(self.has_focus(cx), |tab_bar| {
                 tab_bar.end_child({
                     let render_tab_buttons = self.render_tab_bar_buttons.clone();
                     render_tab_buttons(self, cx)