Handlebar of the splitter is invisble

I created a simple GUI using Designer, which contains splitters. The splitter functionality works, but the handle bar of the splitter is invisible. So when I move the mouse, where I know the bar should be, I can see that the mouse cursor changes to indicate that I can move the bar, and I can also drag it, but the bar itself is never painted.
I also tried to look into the code, because I thought that the bug may be in the QSplitterPrivate::insertWidget function not calling show(), but this is not the case so it must be somewhere else.
I'm using the Qt open source 5.2.1 build with Mingw w64 with the following configure command:
configure -prefix d:\opt\qt-5.2.1 -opensource -nomake tests -confirm-license

This problem not only exists when I run my own code. It can also be shown when I design the GUI with Designer and press CTRL-R to preview the window.

Steps to reproduce:
1. Create new QFrame
2. Drag Push Button to frame
3. Drag another Push Button to frame
4. Select both buttons (CTRL-Mouse)
5. Choose splitter layout (doesn't matter if horizontal or vertical)
6. Press CTRL-R to preview.

Attached is a screenshot to show the problem.