This is likely addressed by #15656.

What is the display scaling factor you are using? (probably something between 1 and 2)?

exactly, if there was some change from 3.0.3 version which worked correctly on Kubuntu 19

currently, on updated Kubuntu 20.04 even previously running correctly 3.0.3 version is no longer working

@chesnokov-yuriy If you change your scaling does it get better? I agree with @timhoffm this is likely due to Qt growing the ability to do fractional scaling under us.

@tacaswell let me try any different scaling and investigate #15656

Kubuntu 19 + matplotlib 3.0.3 command line plot.py: high quality
Kubuntu 19 + matplotlib 3.0.3 Spyder IDE plot: high quality
Kubuntu 19 + matplotlib 3.0.3 Jupyter notebook plot: high quality

on Kubuntu 20.04 situation became different somehow

Kubuntu 19 + matplotlib 3.0.3 command line plot.py: low quality
Kubuntu 19 + matplotlib 3.2.1 command line plot.py: low quality
Kubuntu 19 + matplotlib 3.2.1 Spyder IDE plot: high quality
Kubuntu 19 + matplotlib 3.2.1 Jupyter notebook plot: high quality only after %config InlineBackend.figure_format = 'retina'
Kubuntu 19 + matplotlib 3.2.1 IPython plot: low quality even after %config InlineBackend.figure_format = 'retina'

Only Spyder IDE produces high quality plots both inline and in QtAgg5 UI window even without any 'retina' settings

@chesnokov-yuriy do you know which Qt version each of the call uses?

Remarks:

• The effect is definitively dependent on the backend. In the original post, you should a window from the Qt backend.
• The inline backend in jupyter notebook is another piece of cake (though it might as well struggle with fractional magnifications).
• The InlineBackend config does not have an effect for IPython, because IPython cannot use the inline backend. It will likely also use the Qt backend.
• For spyder: do you pop up the plot in a separate window or is the plot integrated in the
  Spyder IDE window?

@timhoffm I do not remember which Qt was in Kubuntu 19.10 or 19.04 and which Plasma if only looked up from release specifications. Kubuntu 20.04 Qt is 5.12.8 Kde Plasma 5.18.4

Spyder 4.1.3 provides high quality plots in either inline or Qt5Agg without any additional settings

as IPython QtConsole is in low quality

@chesnokov-yuriy If you change your scaling does it get better?

display configuration settings global scale 100%: high quality plot

display configuration settings global scale 125%: low quality same as 150% sa reported before

@chesnokov-yuriy do you know which Qt version each of the call uses?

Kubuntu 19.10 Qt 5.12.4 Python 3.7
Kubuntu 20.04 Qt 5.12.8 Python 3.8

display configuration settings global scale 100%: high quality plot

That tracks really well with @timhoffm 's theory that this is related to us not handling the fractional scaling correctly.

qapp.py file

from matplotlib.backends.qt_compat import QtWidgets

qApp = QtWidgets.QApplication([])

print('  logical dpi:', qApp.desktop().logicalDpiX())
print(' physical dpi:', qApp.desktop().physicalDpiX())
print('        width:', qApp.desktop().screenGeometry().width())
print('       height:', qApp.desktop().screenGeometry().height())

bash shell environment

$ printenv | grep QT_
QT_ACCESSIBILITY=1
QT_SCREEN_SCALE_FACTORS=HDMI-1=1.5;HDMI-2=1.5;DP-1=1.5;
QT_AUTO_SCREEN_SCALE_FACTOR=0

display configuration: 150% scale with 3840x2160

$ python3 qapp.py
  logical dpi: 96
 physical dpi: 108
        width: 2560
       height: 1440

adding QT_SCALE_FACTOR environment variable set to 0.666667 which is 100/150%

$ QT_SCALE_FACTOR=0.666667 python3 qapp.py
  logical dpi: 96
 physical dpi: 163
        width: 3840
       height: 2160

using the same QT_SCALE_FACTOR set before plot.py call results in high quality plot with tiny toolbar and status bar, which looks much better though than default large margins

$ QT_SCALE_FACTOR=0.666667 python3 plot.py

This issue should not block tagging the release candidate.

Is this fixed by #15656?

Yes, it should be.