Hack to make tank selection work on Mac

This is an ugly hack as it partially breaks the ability to abort a selection
(so if you click outside of the combobox, the last value you hovered over is
selected). But this seems much better than not being able to select a different
tank at all.

Fixes #275

(and by 'Fixes' I mean "hacks to sort of make work"... but the trac automation
is rather inflexible in which words it understands...)

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>