Matchbox suporta janelas modais, mas não suporta a não-maximização de janelas de nível superior por design, simplesmente porque vai contra o seu objetivo principal.
Dialogs
Matchbox attempts to accommodate application dialog windows. Rather than being resized to fill the display like their parent application windows, their requested size is usually honored, (if within the display), and they are not statically positioned, allowing the user to drag them about the display.