ui: Add buttons to switch diff view when diffing buffers (#64529)

# Objective

- This PR enables users to switch between unified/split diff views when
comparing buffers.
- Fixes #64419 .

## Solution

- The unified/split toggle (`DiffStyleControls`) is normally surfaced
automatically by `BufferSearchBar`, which detects any active item
exposing a `SplittableEditor`. That auto-detection is gated to
multi-buffer views, since it's paired with an expand/collapse-all-hunks
control that only makes sense across multiple files. `FileDiffView`
(used by `zed --diff <old> <new>`) is a singleton buffer, so it's
intentionally excluded from that generic path.
- Git's own single-file diff view,
[`SoloDiffView`](https://github.com/zed-industries/zed/blob/f2e33599724432f329f9e23b353a8fb28cd6ece3/crates/git_ui/src/solo_diff_view.rs#L52),
hits the same limitation and works around it with a small, dedicated
`ToolbarItemView` (`SoloDiffStyleToolbar`) that renders
`DiffStyleControls` directly, independent of `BufferSearchBar`.
- This PR follows the same pattern: a new `FileDiffStyleToolbar` (in
`git_ui_core::file_diff_view`) activates whenever a `FileDiffView` is
the focused pane item, and renders just the unified/split toggle, no
expand/collapse or file-status controls, since neither concept applies
to a plain two-file diff. It's registered in `initialize_pane` alongside
the existing git toolbars.


## Testing

- Did you test these changes? If so, how?
Ran `cargo test -p git_ui_core`, all passed. I've also attached the
results of manual testing in the Showcase section below.
- Are there any parts that need more testing?
I think more tests can be added.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

## Showcase

Adds a unified/split toggle to the diff view opened via `zed --diff
<old> <new>`, matching the toggle already available in git's diff views.

<details>
  <summary>Click to view before/after</summary>

**Before**:
<img width="1920" height="1080" alt="screenshot_20260921_020610"
src="https://github.com/user-attachments/assets/92cbcf75-0cac-4555-8d2d-ce927064515f"
/>

**After**:
<img width="1920" height="1080" alt="screenshot_20260921_020355"
src="https://github.com/user-attachments/assets/56f9ebc6-11df-41cc-a44d-99bfa8caa130"
/>

</details>

---

Release Notes:

- Added a unified/split toggle to the diff view opened via `zed --diff
<old> <new>` command.

---------

Co-authored-by: Kirill Bulatov <mail4score@gmail.com>
Co-authored-by: Kirill Bulatov <kirill@zed.dev>

crates/git_ui_core/src/file_diff_view.rs

@@ -3,13 +3,13 @@
 use anyhow::Result;
 use buffer_diff::BufferDiff;
 use editor::{
-    Editor, EditorEvent, EditorSettings, HiddenUnstagedDiffHunkRenderer, MultiBuffer,
-    SplittableEditor,
+    DiffStyleControls, Editor, EditorEvent, EditorSettings, HiddenUnstagedDiffHunkRenderer,
+    MultiBuffer, SplittableEditor,
 };
 use futures::{FutureExt, select_biased};
 use gpui::{
-    App, AppContext as _, AsyncApp, Context, Entity, EventEmitter, FocusHandle, Focusable, Font,
-    IntoElement, Render, Task, WeakEntity, Window,
+    App, AppContext as _, AsyncApp, Context, Empty, Entity, EventEmitter, FocusHandle, Focusable,
+    Font, IntoElement, Render, Task, WeakEntity, Window,
 };
 use language::{Buffer, HighlightedText, Point};
 use project::{Project, ProjectPath};
@@ -21,10 +21,11 @@ use std::{
     sync::Arc,
     time::Duration,
 };
-use ui::{Color, Icon, IconName, SharedString};
+use ui::{Color, Icon, IconName, SharedString, prelude::*};
 use util::paths::PathExt as _;
 use workspace::{
-    Item, ItemHandle as _, ItemNavHistory, ToolbarItemLocation, Workspace,
+    Item, ItemHandle, ItemNavHistory, ToolbarItemEvent, ToolbarItemLocation, ToolbarItemView,
+    Workspace,
     item::{ItemEvent, SaveOptions},
     searchable::SearchableItemHandle,
 };
@@ -128,6 +129,22 @@ impl FileDiffView {
             splittable
         });
 
+        let include_root = project.read(cx).visible_worktrees(cx).count() > 1;
+        let breadcrumb_path = |buffer: &Entity<Buffer>| {
+            buffer
+                .read(cx)
+                .snapshot()
+                .resolve_file_path(include_root, cx)
+                .unwrap_or_else(|| MultiBuffer::DEFAULT_TITLE.to_string())
+        };
+        let old_path = breadcrumb_path(&old_buffer);
+        let new_path = breadcrumb_path(&new_buffer);
+        editor.update(cx, |editor, cx| {
+            editor.rhs_editor().update(cx, |editor, _| {
+                editor.set_breadcrumb_header(format!("{old_path} ↔ {new_path}"));
+            });
+        });
+
         let (buffer_changes_tx, mut buffer_changes_rx) = watch::channel(());
 
         // The buffers' languages may load after the diff was built, e.g. when
@@ -394,6 +411,57 @@ impl Render for FileDiffView {
     }
 }
 
+pub struct FileDiffStyleToolbar {
+    file_diff: Option<WeakEntity<FileDiffView>>,
+}
+
+impl FileDiffStyleToolbar {
+    pub fn new(_: &mut Context<Self>) -> Self {
+        Self { file_diff: None }
+    }
+
+    fn file_diff(&self) -> Option<Entity<FileDiffView>> {
+        self.file_diff.as_ref()?.upgrade()
+    }
+}
+
+impl EventEmitter<ToolbarItemEvent> for FileDiffStyleToolbar {}
+
+impl ToolbarItemView for FileDiffStyleToolbar {
+    fn set_active_pane_item(
+        &mut self,
+        active_pane_item: Option<&dyn ItemHandle>,
+        _: &mut Window,
+        cx: &mut Context<Self>,
+    ) -> ToolbarItemLocation {
+        self.file_diff = active_pane_item
+            .and_then(|item| item.act_as::<FileDiffView>(cx))
+            .map(|entity| entity.downgrade());
+
+        if self.file_diff.is_some() {
+            ToolbarItemLocation::PrimaryLeft
+        } else {
+            ToolbarItemLocation::Hidden
+        }
+    }
+}
+
+impl Render for FileDiffStyleToolbar {
+    fn render(&mut self, _: &mut Window, cx: &mut Context<Self>) -> impl IntoElement {
+        let Some(file_diff) = self.file_diff() else {
+            return Empty.into_any_element();
+        };
+
+        let editor = file_diff.read(cx).editor.clone();
+
+        h_flex()
+            .pl_0p5()
+            .gap_1()
+            .child(DiffStyleControls::new(editor))
+            .into_any_element()
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -560,7 +628,34 @@ mod tests {
                     path!("test/new_file.txt")
                 )
             );
-        })
+            let (breadcrumbs, _) = diff_view.breadcrumbs(cx).unwrap();
+            assert_eq!(
+                breadcrumbs
+                    .iter()
+                    .map(|crumb| crumb.text.as_ref())
+                    .collect::<Vec<_>>(),
+                vec!["old_file.txt ↔ new_file.txt"]
+            );
+        });
+
+        let toolbar = cx.new(FileDiffStyleToolbar::new);
+        let location = toolbar.update_in(cx, |toolbar, window, cx| {
+            toolbar.set_active_pane_item(Some(&diff_view), window, cx)
+        });
+        assert_eq!(location, ToolbarItemLocation::PrimaryLeft);
+        assert_eq!(
+            toolbar.read_with(cx, |toolbar, _| toolbar.file_diff()),
+            Some(diff_view.clone())
+        );
+
+        let location = toolbar.update_in(cx, |toolbar, window, cx| {
+            toolbar.set_active_pane_item(None, window, cx)
+        });
+        assert_eq!(location, ToolbarItemLocation::Hidden);
+        assert_eq!(
+            toolbar.read_with(cx, |toolbar, _| toolbar.file_diff()),
+            None
+        );
     }
 
     #[gpui::test]

crates/zed/src/zed.rs

@@ -38,6 +38,7 @@ use git_ui::project_diff::ProjectDiffToolbar;
 use git_ui::solo_diff_view::{SoloDiffGitToolbar, SoloDiffStyleToolbar};
 use git_ui::staged_diff::StagedDiffToolbar;
 use git_ui::unstaged_diff::UnstagedDiffToolbar;
+use git_ui_core::file_diff_view::FileDiffStyleToolbar;
 use gpui::{
     Action, App, AppContext as _, AsyncWindowContext, ClipboardItem, Context, DismissEvent,
     Element, Entity, FocusHandle, Focusable, Image, ImageFormat, KeyBinding, ParentElement,
@@ -1466,6 +1467,8 @@ fn initialize_pane(
             toolbar.add_item(multibuffer_hint, window, cx);
             let solo_diff_style_toolbar = cx.new(SoloDiffStyleToolbar::new);
             toolbar.add_item(solo_diff_style_toolbar, window, cx);
+            let file_diff_style_toolbar = cx.new(FileDiffStyleToolbar::new);
+            toolbar.add_item(file_diff_style_toolbar, window, cx);
             let breadcrumbs = cx.new(|_| Breadcrumbs::new());
             toolbar.add_item(breadcrumbs, window, cx);
             let buffer_search_bar = cx.new(|cx| {

script/bundle-mac

@@ -4,6 +4,8 @@ set -euo pipefail
 source script/lib/blob-store.sh
 
 build_flag="--release"
+zed_features=""
+remote_server_features=""
 target_dir="release"
 open_result=false
 local_install=false
@@ -34,6 +36,8 @@ do
         d)
             export CARGO_INCREMENTAL=true
             build_flag="";
+            zed_features="--features util/debug-embed"
+            remote_server_features="--features debug-embed"
             target_dir="debug"
             ;;
         i) local_install=true;;
@@ -87,10 +91,10 @@ script/generate-licenses
 rustup target add $target_triple
 
 echo "Compiling zed binaries"
-cargo --config .cargo/bundle-config.toml build ${build_flag} --package zed --package cli --target $target_triple
+cargo --config .cargo/bundle-config.toml build ${build_flag} ${zed_features} --package zed --package cli --target $target_triple
 # Build remote_server in separate invocation to prevent feature unification from other crates
 # from influencing dynamic libraries required by it.
-cargo --config .cargo/bundle-config.toml build ${build_flag} --package remote_server --target $target_triple
+cargo --config .cargo/bundle-config.toml build ${build_flag} ${remote_server_features} --package remote_server --target $target_triple
 
 echo "Creating application bundle"
 pushd crates/zed