RPi 7in touchscreen click not working properly

[sylvanomtl]

I have installes the RPi touchscreen (official board). It seems to work more or less fine under MATE
here are my two observations:
1) The left click works only on the top level menus: when selecting "Applications" "Places" "System", and the first submenu only. I hardly can select the second sebmenu. The click is not enable on the other items of the desktop display

2) While writing these lines, the menu bar has moved .

Is there any way to fix this ?

[sylvanomtl]

[sylvanomtl]

I was worried my screen was not working properly, so I tried a Raspbina distribution. The touchscreen is working perfectly fine, and I can click on the menus.

So my first conclusions are something is missing either in DietPi or in Mate.
as a beginer I cannot go further. As I mess up with my distribution, I am afraid I need to redo the whole system.

next time, I will create a restoration point.