Qt5 Bug: Modifier keys ignored in xlib library (send_event routine)

While trying to fix a problem in Autokey-py3 I came upon this rather serious bug. Since I've switched from Kubuntu 14.04-32 to 16.04-64 Autokey is unable to paste uppercase characters [1].

The problem can be tracked down from Autokey to the python-xlib library to Qt5. This led to the conclusion that xlib, here the send_event routine, doesn't handle modifiers correctly.

This finding is backed by
1) The virtual keyboard xvkbd shows the same defect when started with "-xsendevent" and used as a client for a Qt5 application.
2) The program crikey, a small c utility to send keyboard events [2].

Under Qt5 all three programs are unable to produce uppercase letters and characters that require the shift modifier key, e.g. '!"§'.

The problem is rather serious since it renders these programs more or less useless under Qt5.

Moreover these applications are affected by the bug:
1) Qt5-KDE-Applications (e.g. Kate) under Plasma5
2) Qt5-KDE-Applications (e.g. Konsole) under Non-Plasma Window Managers like xfce (Gtk+)

Not affected are:
1) Qt4-KDE-Applications (e.g. Okular at the moment)
2) Gtk-Applications (e.g. Thunderbird)
3) Chromium
4) LibreOffice

The bug is present in all version of Qt5 - Kubuntu 16.04 LTS (Qt 5.5.1 and KDE Plasma 5.5.5 up to 5.6.5) and the latest version of Neon (Qt 5.7.0 and KDE Plasma 5.8.3) - neon-useredition-20161114-0947-amd64.

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1: https://github.com/autokey-py3/autokey-py3/issues/20
2: http://www.shallowsky.com/software/crikey/