Why is leavevmode required in this example?

Question

In the following mwe, If I comment out leavevmode in the redefinition of the equation* environment, the proof title disappears
documentclass{article}
usepackage{amsmath}
usepackage{amsthm}

% from https://tex.stackexchange.com/a/59955/172923
makeatletter
newcommand{settowidthofalign}[2]{%
  setboxz@=vbox{
    begin{align*}
    #2
    ifmeasuring@elsegloballetgot@maxcolwdmaxcolumn@widthsfi
    end{align*}
  }%
  begingroup
  defor{+}edefx{endgroup#1=dimexprgot@maxcolwdrelax}x}
makeatother

ExplSyntaxOn

dim_new:N l_eqn_width_dim

cs_set_eq:cc { oldequation* }{ equation* }
cs_set_eq:cc { endoldequation* }{ endequation* }

RenewDocumentEnvironment{ equation* }{ +b }
  {
    %leavevmode
    settowidthofalign{ l_eqn_width_dim } { #1 }
    begin{oldequation*} #1 end{oldequation*}
  }{}

ExplSyntaxOff

begin{document}
begin{proof}
  begin{equation*}
    a + b = c
  end{equation*}
end{proof}
end{document}

without leavevmode:

with leavevmode:

What is the cause of this behaviour?

egreg · Accepted Answer

When you say begin{proof}, the label “Proof” is obtained with item[...] and it's a standard feature of trivlist (that's used for implementing proof) that the typesetting of the label is delayed when the first text following it is scanned and typeset.
In your case this happens when begin{align*} inside the vbox is executed, so the label is lost together with the box.
You need leavevmode at the beginning of settowidthofalign. Just for safety: starting a proof with a display is not the best thing to do anyway.

Why is leavevmode required in this example?

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