Dear Behdad,

I see from the code that the USE shaper doesn't zero marks. But that the USE 
spec implies that they are:

the width of the base character must be added back using the <dist> feature. 
This is necessary because OT processing cancels the width associated with a 
mark. It is necessary to cancel the width of a non-spacing mark because it is 
not clear where to apply the width of a non-spacing mark during OpenType 
processing.

I'm hoping that the spacing marks proposal will answer that final question. But 
it does imply that marks need to have their advances zeroed.

I don't quite follow how not zeroing marks works. If I attach acute with 
advance of 100 to an a with advance 200, I assume I end up with a total advance 
of 300? I think the spacing mark proposal helps sort out the overlap problem 
which is really tricky to resolve otherwise even for those shapers that don't 
zero their marks.

Yours,
Martin
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