I determined that to cause it to happen in Waterfox (on Ubuntu), if I click and drag the same node several times then the Drop Flow popup flashes up and disappears, leaving the node selected and dragged about by the mouse even with the mouse button release. Clicking the mouse releases it, but if I then move it again then often the popup appears again and the page locks up (with the message up). There isn't anything unusual in the developer console. However, as I said, I can't get it to happen on FF so it may be a completely different issue. The previous thread is How to disable 'drop flow here' feature?.
@GiovanniG are you certain you are using node red 1.3.5? Do you see that at the bottom of the dropdown menu?
