When doing a Unified Transform, if the transform is cancelled for any reason, the specific part of undo history relating to the Unified Transform is removed (in other words, the undo history is apparently rolled back to the step just before the Unified Transform was commenced, and all steps taken during the transform stage are removed).

None of this makes any logical sense, in that normally a rollback in history will not remove advance steps unless the user then embarks on new steps after rolling back. Similarly, there are no other actions one can think of in the software that will arbitrarily roll back more than one step in the undo history.

This is of material concern because of how easy it is to inadvertently terminate a Unified Transform, which can be by accidentally clicking the wrong menu entry (if one was attempting to click the Undo item on the Edit menu to go back a transform step, and for example accidentally moved across to the Select All menu item), or even just by pressing the wrong key on the keyboard inadvertently.

Terminating a unified transform should leave the transform steps in the undo history and make it possible to easily return to an accidentally terminated uniform transform (Undo Cancel Unified Transform) because the undo steps should remain in the history just like every other step or action, or at the very least, the Undo Cancel step should be available.

But Unified Transform is a single operation - the steps you see before hitting the Transform button are just the results of applying the transform matrix to that point acting to show you a preview. When you use the Transform button the operation is actually performed and put into the history. You wouldn't expect intermediate steps in the Curves tool (for instance) to appear in the history as individual operations.

Well,

There are lots of ways of responding to this question. But the biggest concern for me is how easy it is to accidentally start or cancel a transform that is being prepared. I don't use many things in the Toolbox so I don't know how this compares with other tools. I use UT a lot and for big layer transforms that can take quite a few minutes work to prepare, overlaying aerial photos over other aerial photos, because no two aerial photos ever quite line up perfectly where they overlap, but also because of other design limitations in the Unified Transform tool.

There seems to be the design assumption that if a user presses a key or selects a menu item, they must want the transform to begin or cancel, which is inconvenient; I only want the transform to start or cancel when I click the button with my mouse or press a shortcut key specific to that action, and all conflicting menu or keyboard shortcut operations to be ignored.