Strange painting issue

Hi all, I've got a perplexing issue whereby a Qt app (qtractor) instantiates a plugin with a GTK UI - the UI of the plugin always comes up black. However running qtractor with the same plugin under XFCE does not exhibit this problem. Qtractor uses the suil library to instantiate the plugin UI's - it bascially reads the meta-data of a plugin and calls the appropriate suil sub-library to handle instantiation. The host shouldn't care. However, there is another app called jalv, a command line LV2 host which also uses the suil library and it doesn't exhibit this problem. I have desktop effects turned off etc. Any ideas what might be the problem? Note I am using the prop. nvidia driver. regards, Brendan