Impress : the video get over all the rest of the slide ? How make it keep its size?

Two year ago every things worked well. I could insert videos in slides and the whole slide was seen while the video played. But now it does not work any more. The video invade all the sceen. The rest of the slide disappear under the video. I use on this computer 6.07.3 and ubuntu 18.04.2. The computer is Lenovo Yoga. The problem came during one updating a few months ago and was not solved by the following.
What can I do ? Is there some setting liable to miss ?
Thank for your help

Try removing libreoffice-gtk3 and install libreoffice-gtk2

my findings:

update: it might be missing package with gtk3, solving by installing apt install gstreamer1.0-gtk3

It works well ! Thank you very much

It works well! thank you very much.