Before:
After:
File:
extend-flips-blend.3dm (154.8 KB)
Try _HistoryPurge
I think you’re missing the point.
History should just work.
In fact, I can’t think of a simpler example where it should just work…
3 Likes
Got it… Something must be wrong.
PS: maybe change your title to “Extend surface breaks blend with history”…
1 Like
I believe this is fixed in the WIP. I will check.
-Pascal
Nope, I just confirmed… it’s very much not fixed.
I have 7 blends in a fairly simple design that I’m trying to do rapid design iteration on… and it is not very rapid at all… and yes, it affects ALL of them! Repeatedly! Even if you re-do one, and then extend again, the same thing happens! 
1 Like
Hi Robert -
I’ve added this thread to RH-70644 ExtendSrf: changes edge indices
-wim
2 Likes
This is adjacent, not sure if it deserves its own thread or not but before trim:
And after trim:
No reason for any of the unaffected edge IDs to change (and just to pre-empt the inevitable technical answer: Yes, I know a trim curve goes all around even untrimmed edges in Rhino but as an end user I simply don’t care… that’s a technical debt McNeel engineers have to clean up).
mutual-trim-blend.3dm (170.1 KB)
I’ve also noticed this:
The edge indices issue with ExtendSrf will be better in upcoming 8.13. There is still an issue with this particular file, which I logged here:
RH-83958 ExtendSrf twists or pinches BlendSrf
RH-70644 is fixed in Rhino 8 Service Release 13
Looks like this in Rhino 8 todays build and Rhino 9 WIP
That particular issue is still open (RH-83958), the one fixed is RH-70644
I noticed extend surf took my blend curve and shifted it to the opposite end of the surface. !
In the latest 8 build
Hi Sach -
Please post the 3dm file.
-wim
I’ve lost track of when it happened. I’ll try and send it when I next come across it.