Comment 1 for bug 287546

The slider is a stock gtk.HPaned handle, so I am tempted to say this is really a bug in the GTK theme Xubuntu uses.

I will note however, that the slider widget is being removed in 9.04 and the partition resizing functionality is being merged into the "After" SegmentedBar widget. As the latter is implemented with Cairo, a much more obvious slider can be drawn that will look the same regardless of which GTK theme is used.