Can't change pages with two simultaneous windows open

[dRaiNe]

How to reproduce:

1. Open first .epub file (the file is stored in NTFS Windows 7 drive). So far navigation using arrow keys (left and right for previous/next page) works.
2. Open different .epub file (again stored in NTFS drive in my case).
3. Switch focus to the first window. Its navigation using keys no longer works, but in the second window it's working properly.

Ubuntu 16.04 x64.

Terminal output from the broken window:
Chapter count: 201
Loaded: /tmp/easy-ebook-viewer-cache-user/index_split_000_split_048.html
Loaded: /tmp/easy-ebook-viewer-cache-user/index_split_000_split_048.html
Loaded: /tmp/easy-ebook-viewer-cache-user/index_split_000_split_049.html
Loaded: /tmp/easy-ebook-viewer-cache-user/index_split_000_split_048.html
Could not read: /tmp/easy-ebook-viewer-cache-user/index_split_000_split_049.html
Could not read: /tmp/easy-ebook-viewer-cache-user/index_split_000_split_048.html
Could not read: /tmp/easy-ebook-viewer-cache-user/index_split_000_split_049.html
Could not read: /tmp/easy-ebook-viewer-cache-user/index_split_000_split_050.html
Could not read: /tmp/easy-ebook-viewer-cache-user/index_split_000_split_051.html

Debugging:

• The reason why it can't read the files above is they don't exist, because the content of
  /tmp/easy-ebook-viewer-cache-user is overwritten by loading another book in separate window. Proposed solution: don't create temp files in root, but add separate directory for each book.
• It seems to me the app deletes content of /tmp/easy-ebook-viewer-cache-user on startup. So it should delete it only if there isn't any process of this app currently running (eg. from terminal or directly launched from icon), as there might be another window open and we can't delete its /tmp/ cache.

Note:

• I love this software especially in the dark theme. It's so lightweight and cool. Only feature I miss is search.