Ttdfjt

The MWE below yields the desired results. But, is there a better way to get this alignment instead of the raise=-0.3ex:

Without the raise=-0.3ex, the item number is not aligned with the text:

Code:

documentclassarticle
usepackageenumitem
usepackage[export]adjustbox

fboxsep=0pt

newadjustboxenvMyAdjustboxvalign=t, raise=-0.3ex%

begindocument
 beginenumerate
 item 
 beginMyAdjustbox
 fbox%
 beginminipage[t]linewidth
 Some text that takes up several lines, so we need to adjust the
 item number to align with the top baseline.
 endminipage%
 %
 endMyAdjustbox%
 endenumerate%
enddocument

I think that there are two separate issues. The main one arises from the use of a minipage environment.

• The first row in the material inside the fbox has no material taller than uppercase letters. Because that material is encased in a minipage, the unused vertical whitespace needed for symbols such as ( and ) is removed. In contrast, the enumeration symbol is not encased in a minipage, and hence its baseline is chosen without removing the implicit strut. If one inserts a strut in the first line of the fbox, most of the need for vertical adjustment is removed.



• 
  

  To fully align the baselines of the "1." particle before the fbox and the material inside the minipage, one also needs to make an adjustment (pun intended) for the thickness of the rule (given by the parameter fboxrule; default value: arrayrulewidth, usually 0.4pt) that surrounds the fbox. And, since the value of fboxsep is nonzero in general, I suggest using the following code

  
  
  

  newadjustboxenvMyAdjustboxvalign=t, raise=fboxrule+fboxsep
  

  
  
  

  in lieu of

  
  
  

  newadjustboxenvMyAdjustboxvalign=t, raise=-0.3ex
  

  
  

With these two adjustments (yet another pun -- ouch!), I get this screenshot:

documentclassarticle
usepackageenumitem
usepackage[export]adjustbox

fboxsep=0pt

newadjustboxenvMyAdjustboxvalign=t, raise=fboxrule+fboxsep

begindocument
 beginenumerate
 item 
 beginMyAdjustbox
 fbox%
 beginminipagelinewidthstrut%
 Some text that takes up several lines, so we need 
 to adjust the item number to align with the top 
 baseline.
 endminipage%
 %
 endMyAdjustbox%
 endenumerate%
enddocument

The option valign=t of adjustbox doesn't retrieve the inner top baseline as the [t] option of minipage or tabular do. adjustbox calculates a height which takes the current text font size into account: by default valign=t sets the height to the height of the current strutbox.

This means that depending on the text on the first line it can be too high or too low relative to the number. You can "repair" the first case with the code from Mico but for the second case there is no easy fix.

In my opinion adjustbox is the wrong environment for such boxes. It is useful for aligning pictures which have no intrinsic baseline, but not for text boxes.

documentclassarticle
usepackageenumitem
usepackage[export]adjustbox

newadjustboxenvMyAdjustboxvalign=t

begindocument
 beginenumerate
 item
 beginMyAdjustbox%too high
 beginminipage[t]linewidth
 aaaa
 endminipage%
 endMyAdjustbox
 item beginMyAdjustbox%too low
 beginminipage[t]linewidth
 $intlimits_1^3 f(x) $
 endminipage%
 endMyAdjustbox 
 endenumerate
enddocument