Bug#1015827:

[Frédéric Mesplède]

In fact I've just realised that the window was now reduced to a one pixel
wide vertical line, practically invisible to the eye. I noticed a white
line over the left panel and when I moved my cursor over it, it changed to
resizing mode, so I concluded that it was the Form Navigator window and
resized it to normal. The button works correctly. Sorry for disturbing you.