Using @examples from scribble/example with #:lang can mess up indentation

Here is an example document that illustrates the issue:

#lang scribble/manual

@(require scribble/example)

@examples[#:lang racket
(require racket/gui/base
         ffi/unsafe
         ffi/unsafe/define)
(define cairo-lib
  (ffi-lib #f))
(define-ffi-definer define-cairo
                    (ffi-lib #f))
(define-cairo cairo-create
  (_fun _pointer -> _pointer)
  #:c-id cairo_create)
]

If you produce HTML output from this, the indentation of the cairo-lib definition and the cairo-create definition are both off. The indented lines are shown with no indentation. Removing the #:lang racket argument restores the correct indentation behavior.

Reproduce the example document using scribble/example with #:lang racket and compare its HTML output with the version without #:lang. Start by tracing how the #:lang argument processes the indented definitions, then verify that the cairo-lib and cairo-create bodies retain their indentation in the generated HTML.