Created attachment 130558 [details]
KSysGuard showing thumbnail.so instances

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 today's 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 reachedMaximum number of clients reachedqt.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.

SOFTWARE/OS VERSIONS
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

ADDITIONAL INFORMATION
I faced this issue originally on openSUSE Leap 15.2. Hence, this is not a new issue.
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