Delphi TListBox contents overflow when selecting (Win7, 32bit, themed)

Question

I've a TListBox on a Windows form with 966 elements in it. When I click a button on my form, a subset of these strings are selected (roughly 200 of them).

If I now unfocus my application by clicking somewhere on the task bar, the entries from my TListBox bleed upward, so as they are visible above the boundaries of the TListBox. They are thankfully bleeding out behind the TGroupBox component which is directly above them - however this still looks extrememly unpolished.

I'm able to reproduce this only on one Win 7 x64 machine using the Aero theme (I've another Win 7 x64 laptop which doesn't exhibit the same problem with the same theme setting).

I've tried issuing a PostMessage( Self.Handle, WM_PAINT, 0, 0) directly after changing the select status, preceded by a ListBox.Refresh. This doesn't help.

Any help would be much appreciated. Please advise if you need more details.

Cheers, Duncan