fix(formatter): keep a comment before } inside a JSX spread child (#28103)

`jsx/spread/child.js` is in the "Not idempotent" list of both Prettier
conformance reports. Its own-line comment before `}` survives the first
pass, but on the second pass it moves out of the spread child:

```jsx
// pass 1 (same as Prettier)
<div>
  {
    ...a
    //comment
  }
</div>;

// pass 2
<div>
  {
    ...a
  }
//comment
</div>;
```

`JSXExpressionContainer` prints the comments before `}` inside its
braces
(`FormatTrailingComments::Comments(comments_before(self.span.end))`), so
`{a //comment}` is stable; `JSXSpreadChild` printed only the leading
comments and the expression, so once the child sits on its own lines the
comment is left for the parent to print after `}`. This adds the same
trailing-comment print to `JSXSpreadChild`. (`JSXSpreadAttribute`
already keeps it.)

Tests:
- New fixture `js/jsx/spread-child-trailing-comment.jsx`; it fails on
`main`, and its output is identical to Prettier 3.9.10 (the pinned
version) at both option sets and stable on a second pass.
- `jsx/spread/child.js` drops out of the "Not idempotent" lists in
`conformance__prettier-js.snap` and `conformance__prettier-ts.snap`; no
other conformance change.
- `cargo test -p oxc_formatter --all-features`, `cargo fmt --check`,
`just lint -p oxc_formatter` (clippy with warnings denied) and `typos`
pass.

AI disclosure: this PR (code, tests and description) was written with
Claude Code (Claude Opus 5.5); the verification above is what was run.

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: leaysgur <6259812+leaysgur@users.noreply.github.com>

crates/oxc_formatter/src/print/jsx/mod.rs

@@ -336,23 +336,7 @@ impl<'a> FormatWrite<'a> for AstNode<'_, 'a, JSXAttribute<'a>> {
 
 impl<'a> FormatWrite<'a> for AstNode<'_, 'a, JSXSpreadAttribute<'a>> {
     fn write(&self, f: &mut JsFormatter<'_, 'a>) {
-        let comments = f.context().comments();
-        let has_comment = comments.has_comment_before(self.argument.span().start)
-            || comments.has_comment_in_range(self.argument.span().end, self.span.end);
-        let format_inner = format_with(|f| {
-            write!(f, [format_leading_comments(self.argument.span()), "..."]);
-            self.argument().fmt(f);
-        });
-
-        write!(f, ["{"]);
-
-        if has_comment {
-            write!(f, [soft_block_indent(&format_inner)]);
-        } else {
-            write!(f, [format_inner]);
-        }
-
-        write!(f, "}");
+        write_jsx_spread(self.span, self.argument(), f);
     }
 }
 
@@ -364,26 +348,39 @@ impl<'a> FormatWrite<'a> for AstNode<'_, 'a, JSXIdentifier<'a>> {
 
 impl<'a> FormatWrite<'a> for AstNode<'_, 'a, JSXSpreadChild<'a>> {
     fn write(&self, f: &mut JsFormatter<'_, 'a>) {
-        let comments = f.context().comments();
-        let has_comment = comments.has_comment_before(self.expression.span().start)
-            || comments.has_comment_in_range(self.expression.span().end, self.span.end);
-        let format_inner = format_with(|f| {
-            write!(f, [format_leading_comments(self.expression.span()), "..."]);
-            self.expression().fmt(f);
-        });
-
-        write!(f, "{");
-
-        if has_comment {
-            write!(f, [soft_block_indent(&format_inner)]);
-        } else {
-            write!(f, [format_inner]);
-        }
-
-        write!(f, "}");
+        write_jsx_spread(self.span, self.expression(), f);
     }
 }
 
+/// Writes `{...argument}` for a spread attribute or child.
+fn write_jsx_spread<'a>(
+    span: Span,
+    argument: AstNode<'_, 'a, Expression<'a>>,
+    f: &mut JsFormatter<'_, 'a>,
+) {
+    let argument_span = argument.span();
+    let comments = f.context().comments();
+    let has_comment = comments.has_comment_before(argument_span.start)
+        || comments.has_comment_in_range(argument_span.end, span.end);
+    let format_inner = format_with(|f| {
+        write!(f, [format_leading_comments(argument_span), "..."]);
+        argument.fmt(f);
+        // Keep the comments before `}` inside the braces, like `JSXExpressionContainer`
+        let comments = f.context().comments().comments_before(span.end);
+        write!(f, [FormatTrailingComments::Comments(comments)]);
+    });
+
+    write!(f, "{");
+
+    if has_comment {
+        write!(f, [soft_block_indent(&format_inner)]);
+    } else {
+        write!(f, [format_inner]);
+    }
+
+    write!(f, "}");
+}
+
 impl<'a> FormatWrite<'a> for AstNode<'_, 'a, JSXText<'a>> {
     fn write(&self, f: &mut JsFormatter<'_, 'a>) {
         write!(f, text(self.value().as_str()));

crates/oxc_formatter/tests/fixtures/js/jsx/spread-trailing-comment.jsx

@@ -0,0 +1,21 @@
+// A comment before `}` stays inside the braces of a spread child or attribute.
+<div>
+  {
+    ...a
+    //comment
+  }
+</div>;
+
+<div>
+  {
+    ...a
+    /* comment */
+  }
+</div>;
+
+<div
+  {
+    ...a
+    //comment
+  }
+/>;

crates/oxc_formatter/tests/fixtures/js/jsx/spread-trailing-comment.jsx.snap

@@ -0,0 +1,78 @@
+---
+source: crates/oxc_formatter/tests/fixtures/mod.rs
+---
+==================== Input ====================
+// A comment before `}` stays inside the braces of a spread child or attribute.
+<div>
+  {
+    ...a
+    //comment
+  }
+</div>;
+
+<div>
+  {
+    ...a
+    /* comment */
+  }
+</div>;
+
+<div
+  {
+    ...a
+    //comment
+  }
+/>;
+
+==================== Output ====================
+------------------
+{ printWidth: 80 }
+------------------
+// A comment before `}` stays inside the braces of a spread child or attribute.
+<div>
+  {
+    ...a
+    //comment
+  }
+</div>;
+
+<div>
+  {
+    ...a
+    /* comment */
+  }
+</div>;
+
+<div
+  {
+    ...a
+    //comment
+  }
+/>;
+
+-------------------
+{ printWidth: 100 }
+-------------------
+// A comment before `}` stays inside the braces of a spread child or attribute.
+<div>
+  {
+    ...a
+    //comment
+  }
+</div>;
+
+<div>
+  {
+    ...a
+    /* comment */
+  }
+</div>;
+
+<div
+  {
+    ...a
+    //comment
+  }
+/>;
+
+===================== End =====================

crates/oxc_formatter/tests/snapshots/conformance__prettier-js.snap

@@ -93,4 +93,3 @@ js compatibility: 1033/1123 (91.99%), 23 files skipped
 - js/for/continue-and-break-comment-without-blocks.js
 - js/function/issue-12967.js
 - jsx/comments/in-attributes.js
-- jsx/spread/child.js

crates/oxc_formatter/tests/snapshots/conformance__prettier-ts.snap

@@ -73,6 +73,5 @@ ts compatibility: 754/819 (92.06%), 16 files skipped
 # Not idempotent
 
 - jsx/comments/in-attributes.js
-- jsx/spread/child.js
 - typescript/as/comments/17407.ts
 - typescript/method-chain/object/issue-17239.ts