[REG 4 -> 5] In MDI having QGLWidget leaves artifacts when scrolling

Having a QGLWidget to an MdiArea where scrollbars are enabled leaves artifacts when you scroll using Scrollbars.

Attached example reproduces it.
In this example move one of the windows out of bounds and scroll to see artifacts.

Steps to reproduce:

• Launch qtbug38999, scroll horizontally. Note the subwindows are not moved at all.
• Same happens for a native Non-GL-widget (options -no-gl -n).
• Compare to QScrollArea: Works fine (option -s)