Floating dock-widget resize border size

[Steve Lorimer <steve lorimer gmail com>]

I am developing an app using Qt 5.5

I have a main window with a dock, and a dockable window which can be dragged out/popped out, floating in its own separate window. (QDockWidget)

The resize border width of the main window is perfectly adequate, so resizing the main window using the mouse is very easy.

I've marked the resize border width in red here: http://i.stack.imgur.com/1aM5x.png

When I pop out my docked window, so that it's now floating, the resize border width drops down to a single pixel, as seen here: http://i.stack.imgur.com/f5R65.png.

As such it is almost impossible to resize a floating dock window.

I am running Ubuntu Gnome 14.04, so using Gnome 3 with (I think?) Adwaita theme.

I have tested this on default Ubuntu 14.04, so using Unity, and this is not an issue there (the resize border width is several pixels, so much easier to "find" with the mouse)

How can I configure Gnome 3 / Adwaita to increase this border width?

TIA

Steve