backtrace: fix empty backtrace on subsequent activation

racket / drracket

DrRacket, IDE for Racket

http://www.racket-lang.org/

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Closed sorawee closed 3 years ago

sorawee commented 3 years ago

Backtrace display mutates viewable stacks. Therefore, we should copy them first so that subsequent activations still produce correct result.

Fixes #461

rfindler commented 3 years ago

Thanks!