Thanks, this one seems better, granularity-wise. Not perfect though:
1. When I first home on a hot spot with the cross and press the LMB to drag the red bullet, I can't do it: it won't move. I have to first release the LMB and then press it again to start dragging. These two events go on the UR stack as two separate actions, so that when I unwind the stack to its beginning, the very last action (immediately before the Undo button goes gray) doesn't move the bullet anywhere as it's already at its origin where dragging started. This "empty" LMB down/up may however be somehow (although with a considerable stretch) classified as a distinct "hot-spot selection" event, and as such may have a (weak) reason to live.
2. But the above "empty" LMB down/up also occurs any time I release the LMB and then press it again to resume dragging to a new location. When unwound by one action, the stack brings me to the previous location, then yet one more action unwound, the bullet just stays where it is, and then unwound for the third time, the stack moves finally to the yet farther previous location.
This glitch is unjustifiable because the hot spot (and bullet position) was already selected and hasn't yet changed. Which means the stack is still almost double-oversized against what it should be.
3. This unwanted phenomenon doesn't occur when dragging the bullet by arrow key auto-repeat, and that in fact is good.
Note: the stack still isn't brancheable and stores the entire sequence of actions not partially cleared regardless of my unwinding it to some previous state and taking a different route for dragging the bullet from that midpoint. Thus, when I start to Redo the actions again, I'm still brought to all the locations I used to visit despite my having taken an alternative midway action that should've cancelled the locations visited after that midpoint had been visited for the first time.