Improve multibuffer hints (#14690)

This PR improves the multibuffer hints added in #14668 to fix a few
issues.

The original implementation relied on bailing out early in `render` by
returning an `Empty` element. However, this had the unintended
side-effect that when initially opening a multibuffer (such as the
project search) there would be additional whitespace increasing the
height of the toolbar due to the empty element.

The reason we were doing this in the first place was because the hints
weren't updating when the item's breadcrumbs changed.

We're able to address this properly by using a subscription to the
item's events and recompute the visibility of the hint when the active
item's breadcrumbs change.

This also has the benefit of making the hints re-appear right away when
running the `welcome: reset hints` command with a multibuffer open.

Release Notes:

- N/A

crates/welcome/src/multibuffer_hint.rs

@@ -3,14 +3,15 @@ use std::sync::atomic::{AtomicUsize, Ordering};
 use std::sync::OnceLock;
 
 use db::kvp::KEY_VALUE_STORE;
-use gpui::{AppContext, Empty, EntityId, EventEmitter};
+use gpui::{AppContext, EntityId, EventEmitter, Subscription};
 use ui::{prelude::*, ButtonLike, IconButtonShape, Tooltip};
-use workspace::item::ItemHandle;
+use workspace::item::{ItemEvent, ItemHandle};
 use workspace::{ToolbarItemEvent, ToolbarItemLocation, ToolbarItemView};
 
 pub struct MultibufferHint {
     shown_on: HashSet<EntityId>,
     active_item: Option<Box<dyn ItemHandle>>,
+    subscription: Option<Subscription>,
 }
 
 const NUMBER_OF_HINTS: usize = 10;
@@ -22,6 +23,7 @@ impl MultibufferHint {
         Self {
             shown_on: Default::default(),
             active_item: None,
+            subscription: None,
         }
     }
 }
@@ -60,6 +62,29 @@ impl MultibufferHint {
     fn dismiss(&mut self, cx: &mut AppContext) {
         Self::set_count(NUMBER_OF_HINTS, cx)
     }
+
+    /// Determines the toolbar location for this [`MultibufferHint`].
+    fn determine_toolbar_location(&mut self, cx: &mut ViewContext<Self>) -> ToolbarItemLocation {
+        if Self::shown_count() >= NUMBER_OF_HINTS {
+            return ToolbarItemLocation::Hidden;
+        }
+
+        let Some(active_pane_item) = self.active_item.as_ref() else {
+            return ToolbarItemLocation::Hidden;
+        };
+
+        if active_pane_item.is_singleton(cx)
+            || active_pane_item.breadcrumbs(cx.theme(), cx).is_none()
+        {
+            return ToolbarItemLocation::Hidden;
+        }
+
+        if self.shown_on.insert(active_pane_item.item_id()) {
+            Self::increment_count(cx);
+        }
+
+        ToolbarItemLocation::Secondary
+    }
 }
 
 impl EventEmitter<ToolbarItemEvent> for MultibufferHint {}
@@ -70,37 +95,34 @@ impl ToolbarItemView for MultibufferHint {
         active_pane_item: Option<&dyn ItemHandle>,
         cx: &mut ViewContext<Self>,
     ) -> ToolbarItemLocation {
-        if Self::shown_count() > NUMBER_OF_HINTS {
-            return ToolbarItemLocation::Hidden;
-        }
+        cx.notify();
+        self.active_item = active_pane_item.map(|item| item.boxed_clone());
 
         let Some(active_pane_item) = active_pane_item else {
             return ToolbarItemLocation::Hidden;
         };
 
-        if active_pane_item.is_singleton(cx) {
-            return ToolbarItemLocation::Hidden;
-        }
+        let this = cx.view().downgrade();
+        self.subscription = Some(active_pane_item.subscribe_to_item_events(
+            cx,
+            Box::new(move |event, cx| {
+                if let ItemEvent::UpdateBreadcrumbs = event {
+                    this.update(cx, |this, cx| {
+                        cx.notify();
+                        let location = this.determine_toolbar_location(cx);
+                        cx.emit(ToolbarItemEvent::ChangeLocation(location))
+                    })
+                    .ok();
+                }
+            }),
+        ));
 
-        if self.shown_on.insert(active_pane_item.item_id()) {
-            Self::increment_count(cx)
-        }
-
-        self.active_item = Some(active_pane_item.boxed_clone());
-        ToolbarItemLocation::Secondary
+        self.determine_toolbar_location(cx)
     }
 }
 
 impl Render for MultibufferHint {
     fn render(&mut self, cx: &mut ViewContext<Self>) -> impl IntoElement {
-        let Some(active_item) = self.active_item.as_ref() else {
-            return Empty.into_any_element();
-        };
-
-        if active_item.breadcrumbs(cx.theme(), cx).is_none() {
-            return Empty.into_any_element();
-        }
-
         h_flex()
             .px_2()
             .justify_between()