I'm afraid there's another regression on the EGLFS plugin that was not present in Qt 5.4.x.
I have tested Qt 5.5.1 and 5.6.0 beta on a Raspberry Pi.
It doesn't happen with the linuxfb plugin.

After a number of times a QWindow, QDialog or QMenu is displayed, the EGLFS plugin renders them as a black rectangle.
It looks like a memory leak, cause it doesn't happen for at least 10 times.
Unfortunately there is no way to recover when this happens apart for restarting the application.

At the moment the only way to show the issue is with the attached pictures (sorry for the bad quality)

I have reproduced it with the 'standarddialogs' example in qtbase/examples/widgets/dialogs/standarddialogs.
Click on QInputDialog::getMultiLineText() 50-60 times and it will render a black window.
Attached a picture showing it after clicking 53 times.
I believe this really is related to a wrong memory consumption as in my application it happens a lot sooner than 50 times.