The issue reproduces only on Windows 7 when aero is disabled and single-threaded render loop is enabled.
It reproduces at least with AMD gpus. On Intel gpu it can't be reproduced.
The issue can't be seen with Qt 5.7.0.

Steps to reproduce the issue:
1. Disable aero on Windows 7
2. Restart computer
3. Launch attached test application
4. Resize the window e.g. maximise

Expected outcome:
Window content is refreshed after the resize

Actual outcome:
Window content is only refreshed after moving the window

Tested with:
The latest AMD Gpu drivers (16.50.3201), WX7100 Radeon pro and Firepro.