When a TextLayout is constructed with a map and no font, the font substitution mechanism tries to find a font that is able to display each character. Some scripts, though, require layout processing, and fonts may support the normative characters in the script, but not support the required layout for the script. So the font substitution mechanism needs to be script-aware.

This can be illustrated by using an empty map to create a TextLayout do draw the lam alef characters. On solaris the font Akhbar MT contains the normative arabic characters, but not the presentation forms (at least, not that the current font implementation can find) and so can not be used to display arabic. Font substitution should not find this font for arabic unless no other font exists that can do a better job.