Currently CSD gtk apps (client side decorations) like gnome-disk-utility cannot be resized with mouse, when the breeze gtk theme is used. A simple solution to that is to add a 2px border which is easily clickable. This is what adapta gtk theme does. It has no borders for CSD apps when mutter is used, but it has a 2px border when the window manager cannot resize the CSD app without it (openbox, xfwm4, kwin, etc...). I don't know much about gtk themes but maybe you find useful the bug I had submitted for adapta when I needed it for lxde: https://github.com/adapta-project/adapta-gtk-theme/issues/500
*** This bug has been marked as a duplicate of bug 390550 ***