How does the SUnit debug view, highlight lower down in the stack?

I’ve been working on improving some of my testing techniques and the tools used to do that, and it highlighted something where I’m not clear on how it works. 

Sometime the debugger highlights a failure lower down in the stack (not just the stack frame directly below the signaller).

How does it do it? I can’t see what it does to achieve this - however curiously it differs between types of assert.

I.e. with: self assert: 2 =3 (it just shows the frame below)

Yet: self assert: 2 equals: 4 (shows two frames below, and actually directly shows where in the test failed - with no further clicks which is brilliant)

The photos (at the end) demonstrate what I mean - and I can see there is a defaultAction in TestFailure that pops open the debugger:

defaultAction

Processor activeProcess

debug: self signalerContext

title: self description

And if I try and do a similar thing for one of my own exceptions - but wind further down the stack manually (which TestFailure does’t do) like:

defaultAction

| triggerContext |

triggerContext := self signalerContext findContextSuchThat:

[ :context | context method selector = #doesNotUnderstand: ].

"Trigger the debugger where the failure happened"

Processor activeProcess

debug: (triggerContext ifNil: [ self signalerContext ])

title: self messageText

This gives me a debugger on the line of the failure (which I like), but the stack above this failure (which in my case was many frames deep) is gone (presumably as contexts only know their parent, and I’ve backed up through the parent chain). This is a bit confusing to understand how you got the error.

So I’m left wondering how #assert:equals: ends up highlighting further down. I’m missing some concept - and wondering if anyone knows? It’s not urgent - but a smalltalk’y curiosity about how our environment works.