In fluxbox-1.0_rc2-r2, merged with USE="nls truetype", the pop-up that displays the window geometry while resizing isn't always wide enough to fit all the characters it's trying to display. With USE="-nls truetype" everything seems OK. The artwiz font 'snap' as well as 'Bitstream Vera Sans:size=10:bold' should show this behavior.
So report this upstream, not exactly a Gentoo issue.
Upstream has fixed this in 1.0-rc3. Please reopen if you find this is not the case.