Description:

Merging 2 files in ebook edition don't work in the plasma session.
In gnome session no problem.

On an other DD I have a gentoo system and same problem. This work with gnome but no with plasma.
I have open a gentoo bug too
https://bugs.gentoo.org/show_bug.cgi?id=602228

Additional info:
Calibre 2.75.0-1

* config and/or log files etc.

calibre 2.75 embedded-python: False is64bit: True
Linux-4.8.13-1-ARCH-x86_64-with-glibc2.2.5 Linux ('64bit', '')
('Linux', '4.8.13-1-ARCH', '#1 SMP PREEMPT Fri Dec 9 07:24:34 CET 2016')
Python 2.7.13
Linux: ('', '', '')
Traceback (most recent call last):
File "/usr/lib/calibre/calibre/gui2/tweak_book/boss.py", line 70, in ans
return func(*args, **kwargs)
File "/usr/lib/calibre/calibre/gui2/tweak_book/boss.py", line 1256, in merge_requested
merge(current_container(), category, names, master)
File "/usr/lib/calibre/calibre/ebooks/oeb/polish/split.py", line 495, in merge
raise AbortError('The master file (%s) must be one of the files being merged' % master)
AbortError: The master file (test_002.html) must be one of the files being merged

Steps to reproduce:
Open calibre edit book in a plasma session and try to merge 2 files.

Downloading and installing calibre from the calibre web site work fine in plasma.