Z-2303 editor: fix comment extension on the start of line (#2595)

Release Notes:

- Improved comment extension for cases where cursor was placed before
start of comment marker

crates/editor/src/editor.rs

@@ -2219,11 +2219,23 @@ impl Editor {
                                     buffer
                                         .buffer_line_for_row(start_point.row)
                                         .is_some_and(|(snapshot, range)| {
-                                            snapshot
+                                            let mut index_of_first_non_whitespace = 0;
+                                            let line_starts_with_comment = snapshot
                                                 .chars_for_range(range)
-                                                .skip_while(|c| c.is_whitespace())
+                                                .skip_while(|c| {
+                                                    let should_skip = c.is_whitespace();
+                                                    if should_skip {
+                                                        index_of_first_non_whitespace += 1;
+                                                    }
+                                                    should_skip
+                                                })
                                                 .take(delimiter.len())
-                                                .eq(delimiter.chars())
+                                                .eq(delimiter.chars());
+                                            let cursor_is_placed_after_comment_marker =
+                                                index_of_first_non_whitespace + delimiter.len()
+                                                    <= start_point.column as usize;
+                                            line_starts_with_comment
+                                                && cursor_is_placed_after_comment_marker
                                         })
                                         .then(|| delimiter.clone())
                                 } else {

crates/editor/src/editor_tests.rs

@@ -1744,6 +1744,15 @@ async fn test_newline_comments(cx: &mut gpui::TestAppContext) {
         // Foo
         //ˇ
     "});
+    // Ensure that if cursor is before the comment start, we do not actually insert a comment prefix.
+    cx.set_state(indoc! {"
+        ˇ// Foo
+    "});
+    cx.update_editor(|e, cx| e.newline(&Newline, cx));
+    cx.assert_editor_state(indoc! {"
+
+        ˇ// Foo
+    "});
 }
 
 #[gpui::test]