First some info: the real bug was reported to Red Hat at <https://bugzilla.redhat.com/show_bug.cgi?id=1067366>. Then we received a bug report in Debian for the very same issue.

The Red Hat bug has a patch... which has not been submitted to gerrit I'll try to track down the coder to see if I can make him push it to gerrit.

Now, the bug. As decribed in the link above:

Screen Keyboard accessibility only works in gnome-like applications. It means, with enabled Screen Keyboard option when I focus an input field in

Unknown macro: {gnome-terminal,xchat,gnome-shell,evince,...}

, I get on-screen keyboard and everything works fine. Otherwise, when I focus an input field in non-gnome app like

Unknown macro: {...,skype,probably all KDE apps,...}

, I get no on-screen keyboard. I understand that is graphical toolkit relevant and I understand it probably requires bigger changes to support Qt application such as KDE apps are, but it should be possible to use it for firefox and thunderbird as they are default, at least.

Then the proposed patch states (no code ahead, so should be fine):

The type should be "iiii" but becuase a QVariant<QRect> is passed to
QDBusMessage it is "(iiii)". Without this patch, Caribou (the GNOME
on-screen keyboard) could not be used to input text in the text fields
of Qt applications.

I'll try to see what I can do from the original's patch side, but maybe that's enough for you to determine the bug.