`at-exp' changes indentation in whole document

CURRENT BEHAVIOR

In #lang racket, {braces} work the same as (parentheses) and expressions using either are indented the same:

#lang racket

(define foo
  `{(a)
    (b)})

(define foo-paren
  `((a)
    (b)))

In #lang at-exp, the @-expressions with {braces} are indented differently:

#lang at-exp racket

@foo{bar
 bar}

And that difference applies even to non-@-expressions with {braces}:

#lang at-exp racket

(define foo
  `{(a)
 (b)})

(define foo-paren
  `((a)
    (b)))

EXPECTED BEHAVIOR

Using #lang at-exp does not affect the indentation of non-@-expressions:

#lang at-exp racket

@foo{bar
 bar}

(define foo
  `{(a)
    (b)})

(define foo-paren
  `((a)
    (b)))

RELATED DISCUSSION

https://github.com/mbutterick/pollen/issues/124

Do you think this makes sense? If so, how can I help turning this idea into code?

No implementation files or tests are named. Start by comparing the indentation examples for #lang racket and #lang at-exp racket, then review the related Pollen discussion to understand the intended scope. Done means non-@ brace expressions retain their normal indentation under at-exp while @-expressions keep their expected indentation.