[XeTeX] Uneven line spacing

[Meho R.]

Recently I've had this problem. I got uneven line spacing in arabic
text. It seems that the basis for the spacing isn't baseline (in fact,
I have no idea what is the basis in this case), which produce pretty
ugly result with different spacing between some lines (especially prone
to this is the last line; look at the attached sample .jpg file).
First I thought that the problem here may be vocals, but without them
the difference between lines is even more obvious 

Is
there a command that explicitely set baseline (or any other fixed
point) as the basis for spacing even if as a result some overlapping
occurs? Or can the spacing be fixed, not variable as it seems to be
here?



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