Yeah, the undo button is really something that misses me a lot…
I’m not a big fan of the resize function either. Would have been much better if we just could input our final dimensions, instead of having to calculate our own percentages.
For me those two things are the only big flaws of Estlcam… I don’t really understand why, given all the incredible work that have been done on this software, I thought that these two “basic” things would have been easy to do for such skilled programmer. But I don’t know, maybe it is indeed very hard to do?
It’s hard to predict the why, I haven’t seen any reasonings about it. But I can try 
The undo is actually harder than it looks. You have to have every action save the previous state to a list, and the state needs to be complete. The undo/redo buttons need to be able to completely restore the state. Then you’ll need some logic for iterating through the list, and that list is going to increase memory consumption, so you might have to optimize storage of big objects, like pockets. I’ve done it, and it’s not trivial, and it gets it’s fingers all over your code. Depending on the structure of the code, it might be harder. Plus, it’s just not as interesting as the milling features. Plus, more buttons, and more text to translate.
W.r.t. rescale not going to final dimensions. I think that’s more of a philosophical reason. “There should be one and only one obvious way to do something”. I think EstlCAM follows that design idiom very well. And it’s why it’s the easiest to use, while doing almost everything.
Or, he could have completely different reasons…
I’ve figured out the resize and move commands. You first need to click the move button, then select the parts, then click the parts on the screen. You should be able to move/scale. Once you’re all done click the “select” icon and it will calculate and update. I thought there’d be some predictability to it and there is. It’s much easier now. I’d still like the ability to scale the toolpaths and drawing at the same time.