Fix crash in manipulate_text on multibuffers (#57165)

In `Editor::manipulate_text`, we computed selection boundaries for the
updated text assuming the requested edit would be applied exactly. This
is not always true. As a result, we could produce an invalid selection
range and panic.

This change replaces manual selection boundary computation with anchors.
It also skips edits when `new_text == old_text`.

Closes FR-10.

Release Notes:

- N/A

crates/editor/src/editor.rs

@@ -6771,7 +6771,6 @@ impl Editor {
 
         let mut new_selections = Vec::new();
         let mut edits = Vec::new();
-        let mut selection_adjustment = 0isize;
 
         for selection in self.selections.all_adjusted(&self.display_snapshot(cx)) {
             let selection_is_empty = selection.is_empty();
@@ -6786,23 +6785,24 @@ impl Editor {
                 )
             };
 
-            let text = buffer.text_for_range(start..end).collect::<String>();
-            let old_length = text.len() as isize;
-            let text = callback(&text);
+            let old_text = buffer.text_for_range(start..end).collect::<String>();
+            let new_text = callback(&old_text);
 
             new_selections.push(Selection {
-                start: MultiBufferOffset((start.0 as isize - selection_adjustment) as usize),
-                end: MultiBufferOffset(
-                    ((start.0 + text.len()) as isize - selection_adjustment) as usize,
-                ),
+                start: buffer.anchor_before(start),
+                end: buffer.anchor_after(end),
                 goal: SelectionGoal::None,
                 id: selection.id,
                 reversed: selection.reversed,
             });
 
-            selection_adjustment += old_length - text.len() as isize;
+            if new_text != old_text {
+                edits.push((start..end, new_text));
+            }
+        }
 
-            edits.push((start..end, text));
+        if edits.is_empty() {
+            return;
         }
 
         self.transact(window, cx, |this, window, cx| {

crates/editor/src/editor_tests.rs

@@ -6898,6 +6898,61 @@ async fn test_convert_to_base64(cx: &mut TestAppContext) {
     "});
 }
 
+#[gpui::test]
+fn test_manipulate_text_handles_cross_excerpt_edit_that_applies_differently(
+    cx: &mut TestAppContext,
+) {
+    init_test(cx, |_| {});
+
+    let buffer_1 = cx.new(|cx| {
+        let mut buffer = Buffer::local("ab", cx);
+        // The selected multibuffer range starts in this excerpt, but edits to
+        // it are skipped because the underlying buffer is read-only.
+        buffer.set_capability(language::Capability::ReadOnly, cx);
+        buffer
+    });
+    let buffer_2 = cx.new(|cx| Buffer::local("cd", cx));
+    let multibuffer = cx.new(|cx| {
+        let mut multibuffer = MultiBuffer::new(ReadWrite);
+        multibuffer.set_excerpts_for_path(
+            PathKey::sorted(0),
+            buffer_1.clone(),
+            [Point::new(0, 0)..Point::new(0, 2)],
+            0,
+            cx,
+        );
+        multibuffer.set_excerpts_for_path(
+            PathKey::sorted(1),
+            buffer_2.clone(),
+            [Point::new(0, 0)..Point::new(0, 2)],
+            0,
+            cx,
+        );
+        multibuffer
+    });
+
+    cx.add_window(|window, cx| {
+        let mut editor = build_editor(multibuffer, window, cx);
+        let len = editor.buffer().read(cx).len(cx);
+        editor.change_selections(SelectionEffects::no_scroll(), window, cx, |selections| {
+            selections.select_ranges([MultiBufferOffset(0)..len])
+        });
+
+        // No-op transformations should not be sent through `MultiBuffer::edit`.
+        editor.manipulate_text(window, cx, |text| text.to_string());
+        assert_eq!(buffer_1.read(cx).text(), "ab");
+        assert_eq!(buffer_2.read(cx).text(), "cd");
+
+        // A real replacement can apply differently than requested; selection
+        // remapping should follow the actual edit instead of predicted offsets.
+        editor.manipulate_text(window, cx, |_| "replacement".to_string());
+        assert_eq!(buffer_1.read(cx).text(), "ab");
+        assert_eq!(buffer_2.read(cx).text(), "");
+
+        editor
+    });
+}
+
 #[gpui::test]
 async fn test_manipulate_text(cx: &mut TestAppContext) {
     init_test(cx, |_| {});