I tried this in 5.1 and it worked

Debugger class>>

morphicOpenOn: process context: context label: title contents: 
contentsStringOrNil fullView: full
     "Open a notifier in response to an error, halt, or notify. A 
notifier view just shows a short view of the sender stack and provides a 
menu that lets the user open a full debugger."

     | errorWasInUIProcess debugger |
     ErrorRecursion ifTrue: [
         "self assert: process == Project current uiProcess -- 
DOCUMENTATION ONLY"
         ErrorRecursion := false.
         "^ Project current handleFatalDrawingError: title"]. 
<============commented this out
...


On 1/24/18 6:31 PM, Eliot Miranda wrote:
> Hi All,
>
>     I'm comparing two close copies of a class in attempting a complex 
> refactoring, comparing the original code with the close copy.  In 
> doing this I need to open up two debuggers to step through the two 
> computations simultaneously.  If I try it this way, where the two JITs 
> break at a particular bytecode:
>
> [:m :opts|
>  [StackToRegisterMappingCogit cog: m options: opts] fork.
>  Processor yield.
>  AltStackToRegisterMappingCogit cog: m options: opts]
> value: TabbedPalette class>>#unload
> value: #(ObjectMemory Spur64BitCoMemoryManager
> debugBytecodePointers #(59)
> compilationTrace 0).
>
> I get an emergency evaluator :-(, and if I type restart I only ever 
> get one debugger.  What's the right way to do this?
>
> To boil down the example I need this to work:
>
>     [1 halt] fork.
>     Processor yield.
>     2 halt
>
> i.e. I want to be able to get two debuggers, one open on 1 halt, and 
> the other on 2 halt.
> _,,,^..^,,,_
> best, Eliot
>
>

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