G3 [3032] 10.13.6 – Cursify is not working as is should. Instead of slanting it is rotating, more or less.
As far as I can see, Glyphs 2 would produce the same result. That is just how the algorithm works (not that this couldn’t be improved).
I have rarely used slanting and cursify so my experience is very limited but had never seen it do that. I’m not even sure what the word “cursify” means as I cannot find a definition for it.
I just tried it again and G2 does not rotate the horizontals at the baseline and cap height.
I also discovered that G2 does a better job regarding weight shift on round glyphs so I will continue to use it for that purpose alone. It’s not something I need to do often.
The rotation on the baseline only happens with this kind of serifs.
It does not happen in G2, and as an assist algorithm for optical correction it should not happen with any style. Horizontal is horizontal, no correction needed.
Can you send me one of your glyphs to test this?
Sent just now.
I get the same result in Glyphs 2:
It depends on the stem settings (and it should say so if there are none, I’ll fix that).
This is mine from the latest G2. It makes no difference what my origin is either. I’m using 10.13.6 if that makes any difference.
Do you have any stems define for that master?
In the G2 file, no; in the G3 file, yes.
Add some (meaningful) stems in the G2 file.
OK, I did and now my results are the same as yours.
After thinking about it a little, perhaps the value of the algorithm lies only in slanting rounded glyphs alhough slanting any glyph will result in weight shift which needs optical correction.
There is this manual ‘algorithm’ that relies on half slanting and half rotating. And even the algorithm is quite different it produces similar results in this case. 
I didn’t look at that code in a long time. I’ll have a look when I find some time.
Revisiting cursify vs slant, I did some tests today using a round glyph 750 units in size with a 100 unit stroke. A few things I found:
- Cursify gives better results than either Slant or RMX Tuner Slant.
- The tool-tip for Right Slant says “…Option key to Cursivy…” but it does not work. The Shift key is the modifier for this. That dates to a post where you revealed the modifier Shift key but added you hadn’t told anyone yet, so this is just an oversight.
- The tool tip for Left Slant also says to use the Option key, but it doesn’t work and neither does the Shift key.
- Both tool-tips for Slant use the spelling “Cursivy” whereas the Transformation Palette selector has always used “Cursify”.
Regarding my original post here about the bottom of the stems being on a slant, unless it is an easy fix I now don’t see any value in spending much time fixing it considering how often serifs like that are likely part of a design.