Pre-requisites:
 * two workspaces, one above the other;
 * workspace wrapping when dragging a window is enabled;
 * a window positioned at the top edge of a screen.

Move the window horizontally, using the keyboard shortcuts. Watch as it is
moved vertically by the minimum amount into the workspace above.

(A bisection search showed that this problem became apparent in changeset 22939, but the underlying problem seems to have been around for somewhat longer.)