Como consecuencia de este problema (que no aparece el popup menu en un control treeview) he probado unas cuantas cosas, y resulta que sólo funciona mal si el treeview está dentro de un toolpanel o en un sidepanel.
Me pasa con LMDE gambas3.5.4 y con
[System]
gambas=3.8.4
OperatingSystem=Linux
Kernel=3.16.0-38-generic
Architecture=x86_64
Distribution=Linux Mint 17.3 Rosa
Desktop=GNOME
Theme=Gtk
Language=es_ES.UTF-8
Memory=7877M
[Libraries]
Cairo=libcairo.so.2.11301.0
Curl=libcurl.so.4.3.0
DBus=libdbus-1.so.3.7.6
GStreamer=libgstreamer-0.10.so.0.30.0
GStreamer=libgstreamer-1.0.so.0.204.0
GTK+2=libgtk-x11-2.0.so.0.2400.23
GTK+3=libgtk-3.so.0.1000.8
OpenGL=libGL.so.1.2.0
Poppler=libpoppler.so.44.0.0
QT4=libQtCore.so.4.8.6
QT5=libQt5Core.so.5.2.1
SDL=libSDL-1.2.so.0.11.4
[Environment]
CINNAMON_VERSION=2.8.6
CLUTTER_BACKEND=x11
DBUS_SESSION_BUS_ADDRESS=unix:abstract=/tmp/dbus-PwUkc8Unzq,guid=78a0955971ab69518a103a7856c0878a
DEFAULTS_PATH=/usr/share/gconf/cinnamon.default.path
DESKTOP_SESSION=cinnamon
DISPLAY=:0
GB_GUI=gb.qt4
GDMSESSION=cinnamon
GDM_XSERVER_LOCATION=local
GIO_LAUNCHED_DESKTOP_FILE=/usr/share/applications/gambas3.desktop
GIO_LAUNCHED_DESKTOP_FILE_PID=5285
GJS_DEBUG_OUTPUT=stderr
GJS_DEBUG_TOPICS=JS ERROR;JS LOG
GNOME_DESKTOP_SESSION_ID=this-is-deprecated
GNOME_KEYRING_CONTROL=/run/user/1000/keyring-GgykCV
GNOME_KEYRING_PID=2052
GPG_AGENT_INFO=/run/user/1000/keyring-GgykCV/gpg:0:1
HOME=<home>
LANG=es_ES.UTF-8
LC_ADDRESS=es_ES.UTF-8
LC_IDENTIFICATION=es_ES.UTF-8
LC_MEASUREMENT=es_ES.UTF-8
LC_MONETARY=es_ES.UTF-8
LC_NAME=es_ES.UTF-8
LC_NUMERIC=es_ES.UTF-8
LC_PAPER=es_ES.UTF-8
LC_TELEPHONE=es_ES.UTF-8
LC_TIME=es_ES.UTF-8
LOGNAME=<user>
MANDATORY_PATH=/usr/share/gconf/cinnamon.mandatory.path
MDMSESSION=cinnamon
MDM_LANG=es_ES.UTF-8
MDM_XSERVER_LOCATION=local
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games
PWD=<home>
QT_STYLE_OVERRIDE=gtk
SESSION_MANAGER=local/<hostname>:@/tmp/.ICE-unix/2062,unix/<hostname>:/tmp/.ICE-unix/2062
SHELL=/bin/bash
SSH_AGENT_PID=2216
SSH_AUTH_SOCK=/run/user/1000/keyring-GgykCV/ssh
TEXTDOMAIN=im-config
TEXTDOMAINDIR=/usr/share/locale/
TZ=:/etc/localtime
USER=<user>
USERNAME=<user>
WINDOWPATH=8
XAUTHORITY=<home>/.Xauthority
XDG_CONFIG_DIRS=/etc/xdg/xdg-cinnamon:/etc/xdg
XDG_CURRENT_DESKTOP=X-Cinnamon
XDG_DATA_DIRS=/usr/share/cinnamon:/usr/share/gnome:/usr/local/share/:/usr/share/:/usr/share/mdm/
XDG_RUNTIME_DIR=/run/user/1000
XDG_SEAT=seat0
XDG_SESSION_COOKIE=f225bc4a377b0e81d4dfb41c546fde72-1455458182.830869-1817573774
XDG_SESSION_DESKTOP=cinnamon
XDG_SESSION_ID=c2
XDG_VTNR=8