I have a problem with the way the mouse pointer is constrained on my

Essentially when constraining in the horizontal plane, things are pretty
much as I expect; at the lhs of the screen, the mouse abuts the edge, and
I can move it no further, on the rhs, it more or less disappears leaving
one visible pixel.

In the vertical plane it seems to work slightly differently: at the
bottom, the "point" of the pointer sticks several pixels into the screen
at its furthest extent, at the top, it is is allowed to move several
pixels past the screen edge.

This is a problem as, under fluxbox, I want to be able to "throw" the
mouse at the top of the screen, click and have a tab selected. As the
hotspot of the mouse seems to be off the screen, the click is discarded
and so I have to aim carefully at the right zone.

The mouse is also constrained this way in KDE, so it isn't fluxbox

I've setup hotkeys so I can switch with the keyboard which is how I
tend to work anyway, but it's beginning to bug me that I can't do this

Any suggestions?