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On one of my machines (Lenovo T420 notebook) I have a problem with
plasmashell freezing often when a notification is shown. The
notification frame shows up, but no content and then the complete
desktop freezes. Killing and restarting plasmashell cures the problem
for a short time - til one of the the next notifications wants to
display something. It doesn't happen for all notifications, though.
Using tools like notify-send the problem can not be reproduced, but
notifications stemming from plasma-nm and plasma-workspace itself (like
the one for a removed plasmoid) always lead to the freeze.

The system originally was openSuSE Leap 42.2, but because of the problem
I upgraded step by step to
- the latest kernel from the kernel stable repo,
- the latest X server plus everyting from the X11:Xorg repo
- and the latest plasma workspace including everything from the
  KDE:Framework5 repo.
The problem remained unchanged, though.

A few more facts:

- The X server is xf86-video-intel.
- Switching off the compositor doesn't help either.
- Everything worked with Leap 42.1
- No according entry in the journal or .xsession-errors* can be found.
- And a curiosity: After removing .config/plasmarc and restarting
  plasmashell the problem vanishes for a few hours or even a day, before
  it comes back.
- Logging off or restarting the machine doesn't help in case the
  problems exists at that time.

In found a few bug reports for plasma freezes, but none of them seems to
fit exactly.


Reproducible: Sometimes

Steps to Reproduce:
1. unlock widgets
2. remove widget
3. a notification tries to popup
4. plasmashell freezes

1. wait for a plasma-nm notification
2. plasmashell freezes
Actual Results:  
plasmashell freezes

Expected Results:  
plasmashell should not freeze, the notification shoold popup

see details