Reduce render allocations and redundant work in core components

crates/editor/src/diff_view.rs

@@ -237,6 +237,8 @@ pub fn diff_rows(old: &str, new: &str, language: Language) -> Vec<DiffRow> {
 pub struct DiffViewState {
     id: ElementId,
     rows: Vec<DiffRow>,
+    /// The row whose width drives horizontal scroll extent, `rows`-derived.
+    widest_row: Option<usize>,
     options: DiffViewOptions,
     /// Drives both scroll axes of the virtualized row list.
     handle: UniformListScrollHandle,
@@ -251,13 +253,32 @@ impl DiffViewState {
         O: AsRef<str>,
         N: AsRef<str>,
     {
-        Self {
+        let mut state = Self {
             id: id.into(),
-            rows: diff_rows(old.as_ref(), new.as_ref(), language),
+            rows: Vec::new(),
+            widest_row: None,
             options: DiffViewOptions::default(),
             handle: UniformListScrollHandle::new(),
             a11y_label: None,
-        }
+        };
+        state.set_rows(old, new, language);
+        state
+    }
+
+    /// Replaces the rows, recomputing `widest_row` with them.
+    fn set_rows<O, N>(&mut self, old: O, new: N, language: Language)
+    where
+        O: AsRef<str>,
+        N: AsRef<str>,
+    {
+        self.rows = diff_rows(old.as_ref(), new.as_ref(), language);
+        self.widest_row = self
+            .rows
+            .iter()
+            .enumerate()
+            .filter(|(_, row)| row.kind != DiffRowKind::Hunk)
+            .max_by_key(|(_, row)| row.char_count())
+            .map(|(ix, _)| ix);
     }
 
     /// Sets how changed rows are marked.
@@ -284,7 +305,7 @@ impl DiffViewState {
         O: AsRef<str>,
         N: AsRef<str>,
     {
-        self.rows = diff_rows(old.as_ref(), new.as_ref(), language);
+        self.set_rows(old, new, language);
         self.handle.scroll_to_item(0, ScrollStrategy::Top);
         // The replaced versions rarely share the old width; start at the left.
         let offset = ScrollbarHandle::offset(&self.handle);
@@ -328,14 +349,7 @@ impl Render for DiffViewState {
         let state = cx.entity();
         let row_count = self.rows.len();
         let options = self.options;
-
-        let widest_row = self
-            .rows
-            .iter()
-            .enumerate()
-            .filter(|(_, row)| row.kind != DiffRowKind::Hunk)
-            .max_by_key(|(_, row)| row.char_count())
-            .map(|(ix, _)| ix);
+        let widest_row = self.widest_row;
 
         let mut rows = uniform_list((id.clone(), "rows"), row_count, {
             move |range, _window, cx| {