Bug summary

Using the Qt5Agg or most other backends, when drawing a filled marker with alpha < 1 the marker edge line overlaps with the marker face patch thus naturally creating a (IMHO nice) effect where the semi-transparent filled marker is surrounded by a less transparent edge line. This is visible in a few examples in the gallery, for example:

https://matplotlib.org/stable/gallery/shapes_and_collections/scatter.html#sphx-glr-gallery-shapes-and-collections-scatter-py
https://matplotlib.org/stable/gallery/lines_bars_and_markers/scatter_symbol.html#sphx-glr-gallery-lines-bars-and-markers-scatter-symbol-py

However, when saving one of these plots in PDF format, this overlap between edge line and marker patch is gone.

I tried to find a way to obtain the darker edge effect in a PDF either by specifying parameters to the plt.scatter() function or by overlapping multiple scatter plots with different parameters, but so far I haven't found a work-around.

Code for reproduction

import numpy as np
import matplotlib.pyplot as plt
np.random.seed(19680801)
N = 50
x = np.random.rand(N)
y = np.random.rand(N)
colors = np.random.rand(N)
area = (30 * np.random.rand(N))**2
plt.figure()
plt.scatter(x, y, s=area, c=colors, alpha=0.5)
plt.show()
plt.savefig('test.pdf')
plt.savefig('test.png')

Actual outcome

PDF as rendered by Microsoft Edge. Adobe Acrobat reader renders it in the same way.

Expected outcome

The same plot saved in PNG format.

Operating system

Windows

Matplotlib Version

3.4.2

Matplotlib Backend

No response

Python version

3.8.8

Jupyter version

No response

Other libraries

No response

Installation

conda

Conda channel

No response