I agree with changing the capstyle. As for the overlap at the edges, likely it's a zorder issue? #18216/ #17247 likely matter too.

Just another note: The mean (as opposed to median lines) do not seem to have the same issue:

data = [[1, 2, 3, 4, 7]]
fig, ax = plt.subplots()
ax.boxplot(data,
           showmeans=True,
           meanline=True,
           meanprops={"linewidth": 8})

gives me

where the length of the green mean line is fine. (The line is broken because that is the default mean line style.)

Edit: The mean line still overlaps the bounding box, but at least does not extend beyond it.

A symptomatic workaround for the overlapping issue would be to change

zdelta = 0.1

in this line to

zdelta = - 0.1

That resolves it for me.

As far as I can see, zdelta is only used for drawing mean and median lines, so changing that should be safe.

There are different cap styles for solid lines and dashed lines.

This is also a case where the re-scaling of the dash pattern with linewidth is not an improvement...

Zooming in gives that the zorder for the mean line indeed should be changed as well (as it is clear that there are different cap styles).

Should be a first good issue: change the default cap style and the zorder. Problems may involve testing.

Problems may involve testing.

I would like to make a small contribution in order to fix this issue. The solution has already been hinted above. This would be my first contribution and I am not quite clear on what is expected when testing the fix.

There are different cap styles for solid lines and dashed lines.

Where are these defined? Maybe changing the default cap style would help.

There are different cap styles for solid lines and dashed lines.

Where are these defined? Maybe changing the default cap style would help.

Nevermind, found it in lines.py:299