Comment 7 for bug 771450

@Andrea: I do agree with Alex that this behaviour is inconsistent and far from ideal. If it is really the intended design, it should at least be communicated more clearly: For example, in the design blog entry[1], it is stated that "Scrolling via thumb" has second priority (after "scrolling via mouse wheel (or dragging content on touch devices)") -- your statement "let the user use the mouse wheel or the keys" seems to suggest that using the keys has higher priority.

Furthermore, after the fix you suggest, anyone testing using the Desktop tracker[2], following the test guidelines for the scrollbars[3] would have to file a bug like the OP did: Point 3 of the testing procedure clearly states:
"verify that scrollbar "thumbs" do appear when approaching the scrollbars: the position of the thumbs should be outside, on the right, of the window containing the content pane, except when the window is close to the screen border, in which case it should be appear inside the content pane"

[1] http://design.canonical.com/2011/03/introducing-overlay-scrollbars-in-unity/
[2] http://desktop.qa.ubuntu.com/
[3] https://wiki.ubuntu.com/Ayatana/ScrollBars