The title aptly describes my adventure. I had installed openSUSE Leap 15.2 the week it was released. My workflow was happily married to it till I encountered a bug in Dolphin.

openSUSE Leap's status when I jumped off:

Operating System: openSUSE Leap 15.2
KDE Plasma Version: 5.18.5
KDE Frameworks Version: 5.71.0
Qt Version: 5.12.7
Kernel Version: 5.3.18-lp152.33-default
OS Type: 64-bit
Processors: 8 × Intel® Core™ i7-4770 CPU @ 3.40GHz
Memory: 31.3 GiB of RAM
Graphics Processor: Mesa DRI Intel® HD Graphics 4600

I nuked my hard drive and installed openSUSE Tumbleweed in the hope that the someone might have buried the Dolphin bug six-feet under the newer KDE Plasma releases. However, I was surprised when the bug eerily smiled at me in Tumbleweed.

Operating System: openSUSE Tumbleweed 20200727, openSUSE Leap 15.2
KDE Plasma Version: 5.19.3, 5.18.5
KDE Frameworks Version: 5.72.0. 5.71.0
Qt Version: 5.15.0, 5.12.7
Kernel Version: 5.7.9-1-default, 5.3.18-lp152.33-default
OS Type: 64-bit
Processors: 8 × Intel® Core™ i7-4770 CPU @ 3.40GHz
Memory: 31.3 GiB of RAM
Graphics Processor: Mesa DRI Intel® HD Graphics 4600

As a good citizen, I immediately made  it known to the authorities that a bug had been spotted. I filed an official bug report in KDE Bugzilla - 424902.

THE DOLPHIN BUG

SUMMARY

When working with PDF and Djvu files, if previews for files is enabled in Dolphin Preferences, too many instances of thumbnail.so are launched. They eventually flood the system as evidenced by the KSysGuard screenshot attached. It becomes impossible to launch any other application unless the thumbnail.so instances are **killed** or a reboot is performed. When previews are disabled, this issue does not occur.

STEPS TO REPRODUCE

  1. Launch Dolphin without changing the default settings (Preferences).
  2. Start working with PDF and Djvu files.
  3. Wait for a long time. In my experiment, I had to wait for 4-5 hours.

OBSERVED RESULT

Given enough time, thumbnail.so instances start to flood the system. It becomes impossible to start any application. I was cleaning up my ebooks library when I faced this issue for the first time.

The below error message is displayed when an attempt is made to launch an application from the terminal (I used Konsole):

Maximum number of clients reached
qt.qpa.xcb: could not connect to display :0 qt.qpa.plugin: 
Could not load the Qt platform plugin "xcb" in "" even though it was found. 
Failed to create wl_display (No such file or directory) qt.qpa.plugin:
Could not load the Qt platform plugin "wayland" in "" even though it was found. This application failed to start because no Qt platform plugin could be initialized. 
Reinstalling the application may fix this problem. Available platform plugins are: 
wayland-org.kde.kwin.qpa, eglfs, linuxfb, minimal, minimalegl, offscreen, vnc, wayland-egl, wayland, wayland-xcomposite-egl, wayland-xcomposite-glx, xcb. 

EXPECTED RESULT

User should be able to use the system for as many hours as they please without the system getting inundated with thumbnail.so instances.

VIRTUALBOX 6.1.10 AND LINUX KERNEL 5.8 INCOMPATIBILITIES

I remained on openSUSE Tumbleweed till Linux kernel 5.8 arrived. VirtualBox stopped functioning. Linux kernel 5.8 and VirtualBox 6.1.10 are incompatible. It became impossible to exist in Tumbleweed land. I moved back to openSUSE Leap 15.2.