acp: Hide loading diff animation for external agents and update in place (#36699)

The loading diff animation can be jarring for external agents because
they stream the diff at the same time the tool call is pushed, so it's
only displayed while we're asynchronously calculating the diff. We'll
now only show it for the native agent.

Also, we'll now only update the diff when it changes, which avoids
unnecessarily hiding it for a few frames.

Release Notes:

- N/A

Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>

crates/acp_thread/src/acp_thread.rs

@@ -238,10 +238,21 @@ impl ToolCall {
         }
 
         if let Some(content) = content {
-            self.content = content
-                .into_iter()
-                .map(|chunk| ToolCallContent::from_acp(chunk, language_registry.clone(), cx))
-                .collect();
+            let new_content_len = content.len();
+            let mut content = content.into_iter();
+
+            // Reuse existing content if we can
+            for (old, new) in self.content.iter_mut().zip(content.by_ref()) {
+                old.update_from_acp(new, language_registry.clone(), cx);
+            }
+            for new in content {
+                self.content.push(ToolCallContent::from_acp(
+                    new,
+                    language_registry.clone(),
+                    cx,
+                ))
+            }
+            self.content.truncate(new_content_len);
         }
 
         if let Some(locations) = locations {
@@ -551,6 +562,28 @@ impl ToolCallContent {
         }
     }
 
+    pub fn update_from_acp(
+        &mut self,
+        new: acp::ToolCallContent,
+        language_registry: Arc<LanguageRegistry>,
+        cx: &mut App,
+    ) {
+        let needs_update = match (&self, &new) {
+            (Self::Diff(old_diff), acp::ToolCallContent::Diff { diff: new_diff }) => {
+                old_diff.read(cx).needs_update(
+                    new_diff.old_text.as_deref().unwrap_or(""),
+                    &new_diff.new_text,
+                    cx,
+                )
+            }
+            _ => true,
+        };
+
+        if needs_update {
+            *self = Self::from_acp(new, language_registry, cx);
+        }
+    }
+
     pub fn to_markdown(&self, cx: &App) -> String {
         match self {
             Self::ContentBlock(content) => content.to_markdown(cx).to_string(),