I have created this custom window - on the right.
How does one keep the center diamond shape of the custom sash to be resizable without distorting as in the left window? Do I need to create a custom sash for each window size?
Yes, you will need a new custom window for every size.
They are not parametric like a scripted window could be.
They will stretch all parts proportionally - even the top, bottom and side frames will change width.
ERRRG. Not the answer I was hoping for but thanks. Just a couple of extra steps that will help hone the process.