Not sure if this is a bug, but USE=xcb adds -xrender to QtGUI build options. And QT_NO_XRENDER is defined in headers. This is also causes lack of shadows in Breeze Qt5 style.
Yep it's a bug. I missed that define somehow... it should be added to QT5_GENTOO_CONFIG in the qtgui ebuild.
This is fixed in 5.6.0 and later. Thanks for reporting. https://gitweb.gentoo.org/proj/qt.git/commit/?id=c97b2ee08b83c40203e47fa83447b7c5c2ad543d
QT5_GENTOO_CONFIG was not fixed in the main tree ebuild.
Plus there aren't exactly many folk subscribed to this bug report!! Without a package revision bump how are people going to know that the package has been altered? The 5.6.0 version is already in the main tree...
Yeah the fix fell through the cracks during the move from qt overlay to main tree. Sorry about that.
https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=d84dd6a1c07d1c7a84cc2dc35ef02cff2e8b07c8 Thanks again.