If you close the Preview Rows dialog using the "X", the preview will keep running with no way to cancel (ever).
If that preview transform result window is closed; there should be a way to open it again like there was in Kettle OR it should stop the preview.
This was done using hop without a project configured if that helps. Just generate 10k rows and preview the first 1000 and then use the :X: to close the preview window. That makes it where you have to exit hop to recover.