back classes and ahead classes unchangeable when lookup first created

[RolfNoyer]

When first creating a contextual chaining substitution lookup subtable with the options “By Classes” and “Simple” (Dialog), even if you uncheck the box “Same as Match Classes” in for Ahead Classes or for Back Classes, it is not possible to add any classes.

Workaround is to do the following steps in this order only:

1. uncheck the boxes
2. close the lookup (after creating a fake rule which you will later change, since otherwise you cannot close the lookup at all)
3. reopen lookup
4. create back classes or ahead classes

If you do (2) before (1), however, you can never create any back classes or ahead classes and you have to delete the lookup and start over again. The order must be strictly (1), (2), (3) or the workaround will fail.

Using FontForge 20160402 on Mac Laptop running OS 10.11.6

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