Undo does not work

By pressing undo, the video goes black, like there is nothing in the video, and you can't go redo, can't undo, you are stuck. then, you refresh the entire page (browser refresh) and then video goes white, like there is no video at all. Like with undo, but the first undo makes the video screen go black, the refresh makes it white, like there is no video. Total mess, you must start again, switch off everything and start anew and you do not know where the program will land, what did you do by pressing several times undo. It takes time, energy, etc. Frustrating. irritating.