Enable review comments in branch diff view (#46951)

<img width="611" height="102" alt="Screenshot 2026-01-15 at 4 18 30 PM"
src="https://github.com/user-attachments/assets/ba422cbb-4607-47c7-94c7-77752d7b4479"
/>

When viewing a branch diff (e.g., 'Changes since origin/main'), the
toolbar was hidden because `ProjectDiffToolbar` filters for
`DiffBase::Head` only. This meant users couldn't send review comments to
the Agent panel from the branch diff view.

Now there's a new `BranchDiffToolbar` which:
- Only activates for branch diff views (`DiffBase::Merge`)
- Shows only the 'Send Review to Agent' button (not stage/unstage
controls)
- Only appears when there are review comments (same behavior as the
existing toolbar)

(No release notes because this is still feature-flagged.)

Release Notes:

- N/A

crates/git_ui/src/project_diff.rs

@@ -1446,20 +1446,104 @@ impl Render for ProjectDiffToolbar {
             // "Send Review to Agent" button (only shown when there are review comments)
             .when(review_count > 0, |el| {
                 el.child(vertical_divider()).child(
-                    Button::new(
-                        "send-review",
-                        format!("Send Review to Agent ({})", review_count),
-                    )
-                    .icon(IconName::ZedAssistant)
-                    .icon_position(IconPosition::Start)
-                    .tooltip(Tooltip::for_action_title_in(
-                        "Send all review comments to the Agent panel",
-                        &SendReviewToAgent,
-                        &focus_handle,
-                    ))
-                    .on_click(cx.listener(|this, _, window, cx| {
-                        this.dispatch_action(&SendReviewToAgent, window, cx)
-                    })),
+                    render_send_review_to_agent_button(review_count, &focus_handle).on_click(
+                        cx.listener(|this, _, window, cx| {
+                            this.dispatch_action(&SendReviewToAgent, window, cx)
+                        }),
+                    ),
+                )
+            })
+    }
+}
+
+fn render_send_review_to_agent_button(review_count: usize, focus_handle: &FocusHandle) -> Button {
+    Button::new(
+        "send-review",
+        format!("Send Review to Agent ({})", review_count),
+    )
+    .icon(IconName::ZedAssistant)
+    .icon_position(IconPosition::Start)
+    .tooltip(Tooltip::for_action_title_in(
+        "Send all review comments to the Agent panel",
+        &SendReviewToAgent,
+        focus_handle,
+    ))
+}
+
+pub struct BranchDiffToolbar {
+    project_diff: Option<WeakEntity<ProjectDiff>>,
+}
+
+impl BranchDiffToolbar {
+    pub fn new(_: &mut Context<Self>) -> Self {
+        Self { project_diff: None }
+    }
+
+    fn project_diff(&self, _: &App) -> Option<Entity<ProjectDiff>> {
+        self.project_diff.as_ref()?.upgrade()
+    }
+
+    fn dispatch_action(&self, action: &dyn Action, window: &mut Window, cx: &mut Context<Self>) {
+        if let Some(project_diff) = self.project_diff(cx) {
+            project_diff.focus_handle(cx).focus(window, cx);
+        }
+        let action = action.boxed_clone();
+        cx.defer(move |cx| {
+            cx.dispatch_action(action.as_ref());
+        })
+    }
+}
+
+impl EventEmitter<ToolbarItemEvent> for BranchDiffToolbar {}
+
+impl ToolbarItemView for BranchDiffToolbar {
+    fn set_active_pane_item(
+        &mut self,
+        active_pane_item: Option<&dyn ItemHandle>,
+        _: &mut Window,
+        cx: &mut Context<Self>,
+    ) -> ToolbarItemLocation {
+        self.project_diff = active_pane_item
+            .and_then(|item| item.act_as::<ProjectDiff>(cx))
+            .filter(|item| matches!(item.read(cx).diff_base(cx), DiffBase::Merge { .. }))
+            .map(|entity| entity.downgrade());
+        if self.project_diff.is_some() {
+            ToolbarItemLocation::PrimaryRight
+        } else {
+            ToolbarItemLocation::Hidden
+        }
+    }
+
+    fn pane_focus_update(
+        &mut self,
+        _pane_focused: bool,
+        _window: &mut Window,
+        _cx: &mut Context<Self>,
+    ) {
+    }
+}
+
+impl Render for BranchDiffToolbar {
+    fn render(&mut self, _: &mut Window, cx: &mut Context<Self>) -> impl IntoElement {
+        let Some(project_diff) = self.project_diff(cx) else {
+            return div();
+        };
+        let focus_handle = project_diff.focus_handle(cx);
+        let review_count = project_diff.read(cx).total_review_comment_count();
+
+        h_group_xl()
+            .my_neg_1()
+            .py_1()
+            .items_center()
+            .flex_wrap()
+            .justify_end()
+            .when(review_count > 0, |el| {
+                el.child(
+                    render_send_review_to_agent_button(review_count, &focus_handle).on_click(
+                        cx.listener(|this, _, window, cx| {
+                            this.dispatch_action(&SendReviewToAgent, window, cx)
+                        }),
+                    ),
                 )
             })
     }

crates/zed/src/zed.rs

@@ -32,7 +32,7 @@ use futures::future::Either;
 use futures::{StreamExt, channel::mpsc, select_biased};
 use git_ui::commit_view::CommitViewToolbar;
 use git_ui::git_panel::GitPanel;
-use git_ui::project_diff::ProjectDiffToolbar;
+use git_ui::project_diff::{BranchDiffToolbar, ProjectDiffToolbar};
 use gpui::{
     Action, App, AppContext as _, AsyncWindowContext, Context, DismissEvent, Element, Entity,
     Focusable, KeyBinding, ParentElement, PathPromptOptions, PromptLevel, ReadGlobal, SharedString,
@@ -1233,6 +1233,8 @@ fn initialize_pane(
             toolbar.add_item(migration_banner, window, cx);
             let project_diff_toolbar = cx.new(|cx| ProjectDiffToolbar::new(workspace, cx));
             toolbar.add_item(project_diff_toolbar, window, cx);
+            let branch_diff_toolbar = cx.new(BranchDiffToolbar::new);
+            toolbar.add_item(branch_diff_toolbar, window, cx);
             let commit_view_toolbar = cx.new(|_| CommitViewToolbar::new());
             toolbar.add_item(commit_view_toolbar, window, cx);
             let agent_diff_toolbar = cx.new(AgentDiffToolbar::new);