[LAU] RFV: LV2 Gtk2 UIs crashing Qt5 QTractor and MusE at same place

Hi list. Requesting verification of a bug: After upgrading from *buntu 14.04 LTS to 16.x, the LV2 Gtk2 UIs such as Calf and others crash upon opening them in both Qt5 QTractor and Qt5 MusE-3, at these similar calls: QTractor: In qtractorLv2Plugin.cpp QWidget *pQtContainer = QWidget::createWindowContainer(pQtWindow, pQtWidget); MusE: In lv2host.cpp: ewWin = QWidget::createWindowContainer(x11QtWindow); The crash ultimately happens deep inside the Qt5 libraries at the same place in both apps : QXcbWindow::setParent() I tried building Calf from source release, and tried different video cards. NG. But... If I bypass createWindowContainer() in MusE the UIs open and work fine! (Just that now of course they are outside of the container window.) It's Qt 5.5.1 I read about a bug fix and revert where they assumed always a valid screen but then had to revert, but that's supposed to be fixed now. I'm confused. Is it just me getting these crashes? Thanks. Tim.