Digital Light & Color

1. Load an image
2. start a transformation, do something (I used Brightness and then Filter)
3. go to Amount square, choose New Mask, Curve: shape it, clear and reshape it multiple times, also using different channels, inverting masks, etc., do as many steps as possible, Apply and close the mask dialog
4. close the Mask dialog
5. go to dialog Amount square and choose Edit -> the more changes you did to the Mask curve the longer the Mask progress bar oscillates before the mask dialog opens
6. optionally you can repeatedly do some more changes to the curve to make the cumulative effect more pronounced
7. click the dialog OK
8. dblclick the Output to reopen the transformation dialog -> this time the whole process of opening the transformation dialog takes from seconds to tens of seconds making the whole PC stop responding
This way it is possible to bring the whole PC to a several minutes halt.

Is looks as if instead of applying just the final mask curve, all the steps and curves (including all those cleared) preceeding the final one, are applied in succession one after another.

It does save all the intermediate steps. That way, when you edit the mask you can undo the steps used to create the mask one at a time. If you want to avoid this, just reset the mask before making changes.

OK, so Clear then just only removes the mask coloured overlay, but is supposed to keep the whole history. But it does not seem to work properly at this moment anyway. See the separate post.

One last comment.
If you build gradually after many changes a complex mask even with some repeated pronouncend feathering, you will not want to reset it to get rid of its unneeded history and start afresh. If the mask carries all its history, you may not even reopen the output.
Not an easy situation, I see.