tcolorbox doesn't print offset correctly? [closed]

Question

Edit: It turned out to be my PDF viewer.

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So I made this little tcolorbox and the distance between each subsequent pair of rules does not match for all rules:

The code I have is this:

\documentclass{article}
\usepackage[skins]{tcolorbox}
\definecolor{bg}{HTML}{F8FAFF}
\definecolor{col1}{HTML}{80B2FF} %4 Outer right borderline
\definecolor{col2}{HTML}{99C2FF} %3
\definecolor{col3}{HTML}{B2D1FF} %2
\definecolor{col4}{HTML}{CCE0FF} %1 Outer left borderline
\newdimen\BLW % BorderLine Width
\BLW\dimexpr 5pt\relax
\newdimen\BLS % BorderLine Sep
\BLS\dimexpr 0pt\relax
\newdimen\BLLa % ``Gap'' between first and second line
\BLLa\dimexpr -\BLW-\BLS-(\dimexpr 3\BLW/4)/2\relax\relax
\newdimen\BLLb % between second and third line
\BLLb\dimexpr -\BLW-\BLS-(\dimexpr 3\BLW/4)-\BLS-(\dimexpr 9\BLW/16)/2 \relax\relax\relax
\newdimen\BLLc % between third and fourth line
\BLLc\dimexpr -\BLW-\BLS-(\dimexpr 3\BLW/4) -\BLS-(\dimexpr 9\BLW/16)-\BLS-(\dimexpr 27\BLW/64)/2 \relax\relax\relax\relax

\begin{document}
\begin{tcolorbox}[
  width=\linewidth-6pt,
  frame hidden,
  colback=bg,
  freelance,
  arc=0pt,
  boxrule=0pt,
  frame code={
    \draw[col1,line width=\BLW]  % right
      ([xshift=-\BLW/2]frame.north west) -- ([xshift=-\BLW/2]frame.south west);
    \draw[col2,line width=3\BLW/4] 
      ([xshift=\BLLa]frame.north west) -- ([xshift=\BLLa]frame.south west);
    \draw[col3,line width=9\BLW/16] 
      ([xshift=\BLLb]frame.north west) -- ([xshift=\BLLb]frame.south west);
    \draw[col4,line width=27\BLW/64] 
      ([xshift=\BLLc]frame.north west) -- ([xshift=\BLLc]frame.south west);
  }
]
some stuff
\end{tcolorbox}

BLW: \the\BLW\relax

BLS: \the\BLS\relax

$\frac{3}{4}$ BLW: \the\dimexpr3\BLW/4\relax

$\frac{9}{16}$ BLW: \the\dimexpr9\BLW/16\relax

$\frac{27}{64}$ BLW: \the\dimexpr27\BLW/64\relax

BLLa: \the\BLLa\relax

BLLb: \the\BLLb\relax

BLLc: \the\BLLc\relax

\end{document}

As you can see the \dimexpr values are in fact correct, while the offset of the borderlines/rules is not. (This offset should be equal to the \BLS, but instead it's all over the place.)

To understand how I calculated the placement of the rules to create a static offset about \BLS, consider this picture:

Printed values with the \the command show correct values, but the tcolorbox

Answer 1

Your assertion about \dimexpr is not demonstrated by your example. The \dimexpr calculations seem accurate, although you have used a rather verbose style with redundant nested \dimexpr which could be simplified to

\documentclass{article}

\newdimen\BLW % BorderLine Width
\BLW\dimexpr 5pt\relax
\newdimen\BLS % BorderLine Sep
\BLS\dimexpr 1pt\relax
\newdimen\BLLa % ``Gap'' between first and second line
\BLLa\dimexpr -\BLW/2-\BLS-(3\BLW/4)/2\relax


\newdimen\BLLb % between second and third line
\BLLb\dimexpr -\BLW/2-\BLS-(3\BLW/4)-\BLS-(9\BLW/16)/2 \relax


\newdimen\BLLc % between third and fourth line
\BLLc\dimexpr -\BLW/2-\BLS-(3\BLW/4) -\BLS-(9\BLW/16)-\BLS-(27\BLW/64)/2 \relax


\stop

But presumably your error is using arithmetic expressions in the width key, which is not interpreted by \dimexpr (and if it were interpreted the ^ would be a syntax error)

Your example isn't very minimal and you give no indication of the intended result so I can't suggest a change to that.