Add the ability to follow the agent as it makes edits (#29839)

Nathan here: I also tacked on a bunch of UI refinement.

Release Notes:

- Introduced the ability to follow the agent around as it reads and
edits files.

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/project/src/project.rs

@@ -68,9 +68,9 @@ use gpui::{
 };
 use itertools::Itertools;
 use language::{
-    Buffer, BufferEvent, Capability, CodeLabel, Language, LanguageName, LanguageRegistry,
-    PointUtf16, ToOffset, ToPointUtf16, Toolchain, ToolchainList, Transaction, Unclipped,
-    language_settings::InlayHintKind, proto::split_operations,
+    Buffer, BufferEvent, Capability, CodeLabel, CursorShape, Language, LanguageName,
+    LanguageRegistry, PointUtf16, ToOffset, ToPointUtf16, Toolchain, ToolchainList, Transaction,
+    Unclipped, language_settings::InlayHintKind, proto::split_operations,
 };
 use lsp::{
     CodeActionKind, CompletionContext, CompletionItemKind, DocumentHighlightKind, InsertTextMode,
@@ -138,7 +138,7 @@ const MAX_PROJECT_SEARCH_HISTORY_SIZE: usize = 500;
 const MAX_SEARCH_RESULT_FILES: usize = 5_000;
 const MAX_SEARCH_RESULT_RANGES: usize = 10_000;
 
-pub trait ProjectItem {
+pub trait ProjectItem: 'static {
     fn try_open(
         project: &Entity<Project>,
         path: &ProjectPath,
@@ -197,6 +197,13 @@ pub struct Project {
     environment: Entity<ProjectEnvironment>,
     settings_observer: Entity<SettingsObserver>,
     toolchain_store: Option<Entity<ToolchainStore>>,
+    agent_location: Option<AgentLocation>,
+}
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct AgentLocation {
+    pub buffer: WeakEntity<Buffer>,
+    pub position: Anchor,
 }
 
 #[derive(Default)]
@@ -304,8 +311,11 @@ pub enum Event {
     RevealInProjectPanel(ProjectEntryId),
     SnippetEdit(BufferId, Vec<(lsp::Range, Snippet)>),
     ExpandedAllForEntry(WorktreeId, ProjectEntryId),
+    AgentLocationChanged,
 }
 
+pub struct AgentLocationChanged;
+
 pub enum DebugAdapterClientState {
     Starting(Task<Option<Arc<DebugAdapterClient>>>),
     Running(Arc<DebugAdapterClient>),
@@ -986,6 +996,8 @@ impl Project {
                 search_excluded_history: Self::new_search_history(),
 
                 toolchain_store: Some(toolchain_store),
+
+                agent_location: None,
             }
         })
     }
@@ -1142,6 +1154,7 @@ impl Project {
                 search_excluded_history: Self::new_search_history(),
 
                 toolchain_store: Some(toolchain_store),
+                agent_location: None,
             };
 
             // ssh -> local machine handlers
@@ -1381,6 +1394,7 @@ impl Project {
                 environment,
                 remotely_created_models: Arc::new(Mutex::new(RemotelyCreatedModels::default())),
                 toolchain_store: None,
+                agent_location: None,
             };
             this.set_role(role, cx);
             for worktree in worktrees {
@@ -4875,6 +4889,46 @@ impl Project {
     pub fn status_for_buffer_id(&self, buffer_id: BufferId, cx: &App) -> Option<FileStatus> {
         self.git_store.read(cx).status_for_buffer_id(buffer_id, cx)
     }
+
+    pub fn set_agent_location(
+        &mut self,
+        new_location: Option<AgentLocation>,
+        cx: &mut Context<Self>,
+    ) {
+        if let Some(old_location) = self.agent_location.as_ref() {
+            old_location
+                .buffer
+                .update(cx, |buffer, cx| buffer.remove_agent_selections(cx))
+                .ok();
+        }
+
+        if let Some(location) = new_location.as_ref() {
+            location
+                .buffer
+                .update(cx, |buffer, cx| {
+                    buffer.set_agent_selections(
+                        Arc::from([language::Selection {
+                            id: 0,
+                            start: location.position,
+                            end: location.position,
+                            reversed: false,
+                            goal: language::SelectionGoal::None,
+                        }]),
+                        false,
+                        CursorShape::Hollow,
+                        cx,
+                    )
+                })
+                .ok();
+        }
+
+        self.agent_location = new_location;
+        cx.emit(Event::AgentLocationChanged);
+    }
+
+    pub fn agent_location(&self) -> Option<AgentLocation> {
+        self.agent_location.clone()
+    }
 }
 
 pub struct PathMatchCandidateSet {