overlay-scrollbar-mediated window resizing is offset from cursor position

(I'm using unity-2d). For comparison:

1. Open a new terminal window with ctrl+alt+t. It only has one line of content, so no scrollbar. Move it so the right edge "snaps" to the right edge of the screen. You should still be able to see the rounded corner on the right side of it's title bar

2. Resize the window, using the right hand edge: Make it slightly smaller, then bigger again. Note that when resizing this way, you're naturally prevented from making the window edge go outside the screen boundary.

Ok. Now hold down the enter key, to generate lots of lines, and trigger the scrollbar. Now have another go. You'll notice that when you move your cursor to the right edge, the scroll bar appears. If you click and drag to the left, the scroll bar recognizes that you're trying to resize the window, and effectively gets out of the way.

But this time, the window edge isn't quite lined up with mouse cursor. You can see this, because you're now able to resize the window so that it goes outside the screen boundary. I.e. you can no longer see that rounded corner of the title bar.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: overlay-scrollbar 0.2.16-0ubuntu1
ProcVersionSignature: Ubuntu 3.2.0-26.41-generic 3.2.19
Uname: Linux 3.2.0-26-generic x86_64
ApportVersion: 2.0.1-0ubuntu8
Architecture: amd64
Date: Tue Jul 17 22:44:33 2012
PackageArchitecture: all
ProcEnviron:
 LANGUAGE=en_GB:en
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_GB.UTF-8
 SHELL=/bin/bash
SourcePackage: overlay-scrollbar
UpgradeStatus: No upgrade log present (probably fresh install)