setTitlebarWidget for qdockwidget makes floating window resize handle to about 1 pixel size

When you add a titlebar for qdockwidget, the resize handle for the floating window becomes 1 pixel size, so it becomes very difficult to resize the floating window.

How to reproduce:
1. Open the Main Window example in Qt creator.
2. Build and run the project.
3. Click pink icon in Blue Dock Widget to float the widget.
4. Hover mouse over left corner of the floating widget and notice how the resize handle size is about 1 pixel size.

As a comparison float any other widget in the example and notice how the resize handle is much bigger and easier to grab.