Bug report

Bug summary

Quality of qt rendered figures is drastically reduced when window is moved out of focus and then back in focus.

Code for reproduction

import matplotlib.pypot as plt
plt.plot([1, 2, 3, 4], [5, 5.5, 6.5, 8.5], alpha=0.5)

Actual outcome

Switch to different window and back:

At this stage, calling plt.draw() fixes it till the window goes out of focus. Zooming in on the figure also returns it to the earlier quality. Note that transparency is lost.

Expected outcome

Rendering doesn't change.

Matplotlib version

• Operating system: Ubuntu 16.04
• Matplotlib version: 3.0.2
• Matplotlib backend (print(matplotlib.get_backend())): Qt5Agg
• Python version: 3.6.7
• Jupyter version (if applicable): console 6.0.0
• Other libraries: pyqt 5.6.0 py36h8210e8a_7 conda-forge

everything installed from conda-forge using conda.