Hi many times I have observed that while doing editing to any post with multiple edits, if anywhere we have gone to the wrong edit and if we go for undo (CTRL+Z ) then all changes including the correct edits also get lost .
can't we fix this ?
This is dependent on browser, and perhaps 100 other things. It's not the site that does it but your computer. It has to do with the fact that the text area element is hooked into via jquery. In short, your browser is undoing some, jquarry is undoing some, and your OS might be undoing some.
In order to be more specific we would need to know exact browser, OS window manager, blah blah blah versions to track it all down. But at it's core your have "native" ctrl+z only if your willing to loose ctrl+i etc. Which obviously the site owners are not willing to give up.