Overall this looks like a positive and useful change, but offhand I don't remember the details of anchor state preservation. So: In the paths that call _SCNLTrans, and when the glyph isn't multilayer (which should be the typical case), can you test if (in the UI, for example), you perform a non-linear transformation and then Undo (ctrl-z) it, does the anchor point change also reverse, or not?

If it doesn't, that stuff is probably taken care of in SCPreserveState and you'll have to confirm that and then refactor a bit so that SCPreserveState is called whether multilayer is true or not.

I just checked this and Undo / Redo work as expected when the non-linear transformation is applied by opening the Non Linear Transformation dialog from the menu of the Font View. This way NonLinearDlg calls _SFNLTrans, which calls _SCNLTrans.

Ah, sure enough, SCPreserveLayer handles the anchor points if the layer is ly_fore.