waveshare touchscreen - always need to click twice ?!

I am running DietPI , latest version , on a Odroid c2 :

root@DietPi:/DietPi# uname -a
Linux DietPi 3.16.57+ #1 SMP PREEMPT Sun Aug 19 15:27:56 CEST 2018 aarch64 GNU/Linux
root@DietPi:/DietPi# lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description: Debian GNU/Linux 9.11 (stretch)
Release: 9.11
Codename: stretch

The touchscreen from Waveshare, 10.1" , makes me some worries. It works, but I have to click TWICE on any button so as to click on the button.
At the first click, only the mouse cursor is being moved to the button. Only at the second click, the actual button click function is executed.

This DOES NOT happen on a Ubuntu 18.04.3 LTS image on the same Odroid C2. There, everything works fine.

Can you give me any clue of just what to do to get the touch screen to work properly ?