How to add finishing to a zig-zag line in tikzpicture

Question

Consider the following MWE
documentclass{article}
usepackage{tikz}
usetikzlibrary{decorations.pathmorphing}
begin{document}
    begin{center}
        begin{tikzpicture}
        draw[thick] (3,-0.5)--(3,-0.8)--(0.2,-0.8)--(0.2,0.8)--(3,0.8)--(3,0.5);
        draw[thick,decoration={zigzag,segment length=2mm}] decorate{(3,-0.5)--(3,0.5)};
        end{tikzpicture}
    end{center}
end{document}

This is the output shown by this code:
Note that the point where the zigzag and the straight lines combine doesn't have nice finishing, as shown in this figure.
How to achieve the correct finishing??

AndréC · Answer

Your figure is composed of two paths and is therefore not closed, which is why there is this discontinuity.
The idea is to make a single closed path and using the options pre=lineto,pre length=5pt,post=lineto,post length=5pt just give the coordinates of the four vertices of the rectangle.

documentclass[tikz,border=5mm]{standalone}
usetikzlibrary{decorations.pathmorphing}
begin{document}

begin{tikzpicture}
draw[thick](3,-0.8)--(0.2,-0.8)--(0.2,0.8)--(3,0.8) 
        decorate[decoration={zigzag,segment length=2mm,pre=lineto,pre length=5pt,post=lineto,post length=5pt}] {--cycle};
end{tikzpicture}

end{document}

domperor · Answer

Splitting a draw command into two is not a good idea. Try like this:
begin{tikzpicture}
draw[thick] (3,-0.8)--(0.2,-0.8)--(0.2,0.8)--(3,0.8)--(3,0.5) decorate [decoration={zigzag,segment length=2mm}] { -- (3,-0.5)} -- cycle;
end{tikzpicture}

Result (zoomed in) :

How to add finishing to a zig-zag line in tikzpicture

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