Comment 4 for bug 1470825

This combination worked (for single press only, no multitouch obviously):

Preferences->Keyboard->Advanced->"Touch input"="none"
Preferences->Keyboard->Advanced->"Input event source"="GTK"

Setting "Touch Input" back to "Multitouch" I get this error:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/Onboard/TouchInput.py", line 538, in _on_button_press_event
    self._can_handle_pointer_event(event),
  File "/usr/lib/python3/dist-packages/Onboard/TouchInput.py", line 526, in _can_handle_pointer_event
    not device.touch_active
AttributeError: 'X11DeviceXI2' object has no attribute 'touch_active'

For KWin vs KDE, I meant to edit my original statement, I tried it with KDE and then Gnome and had the same behaviour. My setup is KWin standalone.

I am also very familiar with that SYNA7500 thread haha. The SYNA7500 is an i2c device, not USB. I have to use this kernel for the moment because I do have another device which uses a different touchscreen (ELAN), that has the same behaviour. However I know that this device worked fine in the past, so I think this points to something that changed in the touchscreen driver for the kernel build that I am using.

Thanks for the quick feedback, I think I need to look at the touch driver as well.