Comment 11 for bug 1451564

In the upstream bugtracker someone mentioned that this bug may be caused by a specific packaging problem / a missing library:

https://bugs.kde.org/show_bug.cgi?id=343418#c16

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Wolfgang Bauer 2016-04-19 22:34:28 UTC

FYI, I found out by chance what the likely reason for your problem is:
You probably miss the file /usr/lib/kde4/plugins/gui_platform/libkde.so (the path might be slightly different in Kubuntu).
This is needed so that Qt4 (and KDE4) applications actually use the KDE(4) settings.
Without it, only Qt4's settings are respected. You could use qtconfig to set the style e.g. but there's no option for single/double click.

This file is provided by the KDE4 desktop/workspace package (kde-workspace or similar). In openSUSE it's part of the subpackage kdebase4-workspace-libs which is still included and installed by default (it can be installed alongside Plasma5, unlike the actual KDE4 desktop).
No idea whether it's available in Kubuntu, or in which package it would be contained.

But maybe somebody could add this information to the Kubuntu bug report, I don't have an account.
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In fact this library is missing on my system and apt-file also does not know about it at all:

$ ls -l /usr/lib/kde4/plugins/gui_platform/libkde.so
ls: Zugriff auf /usr/lib/kde4/plugins/gui_platform/libkde.so nicht möglich: Datei oder Verzeichnis nicht gefunden
$ apt-file search libkde.so
$