Graphic problems (becoming apparent) with Libreoffice calc

[porinoco]

Hello everybody, this is my first post here so I hope to do everything right. I am not really a Mint newbie: have been using it for 9 years with my old laptop which recently died. I got another one (Dell Inspiron 15 3000) with Ubuntu 16.04 pre-installed. I soon installed Linux Mint 18.3 XFCE but I have this weird problem that never happened to me.

When I insert some text/number in a cell in Libreoffice Calc here is what I got (while I am still writing in the cell):


I select another cell and the numbers then appear normal:


With text it is not so bad but still you can see the beginning gets bold-y, you can feel there is still something not right:


I am not a newbie but I don't have much technical knowledge so I would be really thankful if somebody could point where the problem might be. I don't even know if it's more a software or a hardware issue... I am also a bit frustrated because it is a fresh installation on a new laptop and I also read very good reviews on this Mint distro.

Thanks a lot
Riccardo