The cited OpenerView solution used Trace / TraceOriginal to generate its content. This allowed the definition of show in that response to be defined succinctly, but had the disadvantage of discarding some of the trace information. TraceScan provides more information since it calls a user-specified function at the start and end of every evaluation.

Two functions are defined below that try to format the TraceScan information in (somewhat) readable form.

traceView2 shows each expression as it is evaluated, along with the subevaluations ("steps") that lead to the result of that evaluation. "Drill-down" is provided by OpenerView. The function generates output that looks like this:

traceView2[(a + 1) + 2] 

As one drills deeper into the view, it rapidly crawls off the right-hand side of the page. traceView4 provides an alternative view that does not exhibit the crawling behaviour at the expense of showing much less context for any given evaluation:

Choose your poison ;)

The definitions of the functions follow...

traceView2

ClearAll@traceView2 traceView2[expr_] := Module[{steps = {}, stack = {}, pre, post, show, dynamic}, pre[e_] := (stack = {steps, stack}; steps = {}) ; post[e_, r_] := ( steps = First@stack ~Join~ {show[e, HoldForm[r], steps]} ; stack = stack[[2]] ) ; SetAttributes[post, HoldAllComplete] ; show[e_, r_, steps_] := Grid[ steps /. { {} -> {{"Expr ", Row[{e, " ", Style["inert", {Italic, Small}]}]}} , _ -> { {"Expr ", e} , {"Steps", steps /. { {} -> Style["no definitions apply", Italic] , _ :> OpenerView[{Length@steps, dynamic@Column[steps]}]} } , {"Result", r} } } , Alignment -> Left , Frame -> All , Background -> {{LightCyan}, None} ] ; TraceScan[pre, expr, ___, post] ; Deploy @ Pane[steps[[1]] /. dynamic -> Dynamic, ImageSize -> 10000] ] SetAttributes[traceView2, {HoldAllComplete}] 

traceView4

ClearAll@traceView4 traceView4[expr_] := Module[{steps = {}, stack = {}, pre, post}, pre[e_] := (stack = {steps, stack}; steps = {}) ; post[e_, r_] := ( steps = First@stack ~Join~ {{e, steps, HoldForm[r]}} ; stack = stack[[2]] ) ; SetAttributes[post, HoldAllComplete] ; TraceScan[pre, expr, ___, post] ; DynamicModule[{focus, show, substep, enter, exit} , focus = steps ; substep[{e_, {}, _}, _] := {Null, e, Style["inert", {Italic, Small}]} ; substep[{e_, _, r_}, p_] := { Button[Style["show", Small], enter[p]] , e , Style[Row[{"-> ", r}], Small] } ; enter[{p_}] := PrependTo[focus, focus[[1, 2, p]]] ; exit[] := focus = Drop[focus, 1] ; show[{e_, s_, r_}] := Column[ { Grid[ { {"Expression", Column@Reverse@focus[[All, 1]]} , { Column[ { "Steps" , focus /. { {_} :> Sequence[] , _ :> Button["Back", exit[], ImageSize -> Automatic] } } ] , Grid[MapIndexed[substep, s], Alignment -> Left] } , {"Result", Column@focus[[All, 3]]} } , Alignment -> Left, Frame -> All, Background -> {{LightCyan}} ] } ] ; Dynamic @ show @ focus[[1]] ] ] SetAttributes[traceView4, {HoldAllComplete}]