width of title-pane

On Windows, title-panes are exactly as wide as the string they're
enclosing. On Linux, they aren't, there's a tiny bit of horizontal
space around the string. Can I change that? I need this for a graph
pane where (for the nodes) I fit different title panes tightly
together with a row layout and the space between them is quite
disturbing.

Thanks,
Edi.