Sample app with reproducible test case attached.
This bug manifests as a QComboBox which opens a popup list containing items that cannot be selected. The popup list opens when you click on the QComboBox, and the items in it appear to be enabled, but clicking on them neither selects them nor closes the popup list (you have to click elsewhere in the window to dismiss it). Additionally, it appears that singleShot QTimers are not getting executed when this happens. The app remains in this state until it is quit.
While this bug has been reproducible on every deployment I've tested so far without exception, it is not reliably reproducible: the bug doesn't manifest every single time the app is launched. Anywhere from approximately one-in-three to one-in-fifteen launches exhibit the bug on the machines that I've been able to test so far.
This bug appears to be correlated with calling QOpenGLWidget::update, which is performed in the sample app by connecting QOpenGLWidget::frameSwapped to QOpenGLWidget::update. I have not observed this bug if update() is not being called. I am not aware of any workarounds or other related variables.