[Interest] QDialog and Ubuntu 18.04

[maitai]

Hello all

I am having strange QDialog behavior under Ubuntu 18.04. The application 
is a QWidget app compiled either with qt 5.14.1 or qt 5.12.3 that does 
not matter

Here the behavior is the following:

If I use UNITY desktop moving (dragging) a QDialog works, but instead of 
moving straight away it display an ugly red rectangle and the dialog 
moves there when I release the mouse.
If I use UBUNTU desktop (which is in fact Gnome 3 I believe), then it 
does not work at all: trying to move the dialog does nothing, and when 
you release the mouse the main application (QMainWindow) disappears.

Some users have reported other bad behaviors, for instance just not 
moving at all, or bad behavior also with UNITY desktop

I have tried playing with QT_QPA_PLATFORMTHEME environnement variable 
but it seems to do nothing.

The same app compiled with qt 5.11.3 running on Ubuntu 14.04 behaves 
normally (QDialogs are moving like on Windows, i.e just moving)

So my question is: is there a way to have a regular normal QDialog 
dragging on Ubuntu 18.04 (= no fancy decoration, just move the box) ?

Thanks in advance
Philippe Lelong