All recent gtk+ Versions have a bug, that causes xinput button events
(e.g. from a tablet) to not work correctly any more on linux systems
(don't know about others; I could observe it on various debian
versions, but the exact environment does not seem to matter, it only
depends on the gtk+ version). Normal UI usage (menus, dialogs ...) is
ok, only programs that actually handle "extended input devices"
(gimp/inkscape) are affected: Button events on the canvas are only
recognized, if 2 buttons are down at the same time. The button that is
handled by the program in this case is always the button that is
pushed later:

With a 2-button stylus for example, drawing (button 1, normally just
by touching the tablet with the stylus) only works, if another button
is pressed before the button hits the tablet. The other way around,
normally the context menu can be accessed by pressing button 3 (the
rear of the rocker switch) without touching the tablet; this now only
works if I 1st push the stylus on the tablet and then press the
button. As soon as 1 of the buttons is released, no button is handled
anymore.

While in practice this problem will mostly occur with devices like
graphic tablets, it is not restricted to such devices: When I
configure my regular mouse as a "extended input device" in the
corresponding dialog in gimp or inkscape, the same behavior occurs
with the normal mouse.

I am not familiar enough with the xinput system to actually fix the
problem, but at least I could pretty much narrow it down: While it is
still present in gtk version 2.20.1 (obviously not too many people try
to use graphics tablets on linux ..) it was introduced between version
2.17.2 and 2.17.3, obviously with the pretty large commit by Alexander
Larsson on 2009-05-29 titled "Initial version of input support"