balloon expressions and changing workspace

[Charles Campbell]

Hello:

I can use a balloon expression (onlookers: see :help 'bexpr') that
creates a small balloon text -- but it persists when I change workspaces
(Scientific Linux and I also use mate: yum -y groupinstall mate-desktop)
(ie. ctrl-alt-arrow).  When its displayed on the wrong workspace, its
annoying and sometimes covering up something I need to see.  Personally,
I think vim ought to remove any balloon texts when the workspace is
changed automatically; but nonetheless, it ought to be possible for me
to clear them.  I've tried using FocusLost (the only autocmd event that
fires on a workspace change), but I can't seem to clear the balloon. 
I've tried having the autocmd set nobeval and having a test in the
balloon expression function do a if &beval == 0 | let mesg= "" | endif
(and the function returns mesg).

No luck, the balloon insists on being displayed in the wrong workspace. 
Any known workarounds?

Chip Campbell