Pause until sheet disappears

Mac interface includes a "sheet" which is a panel that slides down from the window frame. In my situation, I have launched a long task (but of variable time). The sheet appears, offering a chance to cancel the action. If I ignore this (the normal situation) it eventually disappears which signifies that the action is completed.
What I want is to be able to detect that the sheet is no longer present which is my signal that the work has been performed and the macro can proceed.
I think I could use a picture of the "sheet" and use the Pause Until Conditions are Met action to wait for an image of the sheet is no longer visible but looking for changes in images are not quite as reliable in my experience as detecting other more concrete changes of state.
I wondered if there was some "condition" like "There is no sheet present in front window" which can be accessed in Keyboard Maestro and kill the pause when that condition is detected. Or some other way to approach this issue.

Well, I noticed that there was a button in the "sheet" which was Cancel. I used in the Pause Until All Conditions Met the condition that Button with the name "Cancel" is not enabled. That seems to work for me. When the sheet "disappeared " (signalling the work was done) the Cancel button disappeared and KM detected that and let the Macro proceed.
If anyone has an idea that they think better or an idea that would cover the situtation when the sheet did not have this button, I would like to hear about it. But, anyway, for now my problems seems to be solved.

Yes, this is generally a good solution. An alternative is based on a found image.

Keep in mind in both of these, the button may appear or disappear at the start or end of the animation, so you may want an additional pause for the length of the animation after the Pause Until.