User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:65.0) Gecko/20100101 Firefox/65.0
Steps to reproduce:
I use Thunderbird and Lightning for years, on Windows either Linux ; but here is the problem I have on Linux since Ubuntu 16.04 (and now 18.04 - 64 bits) :
there is no minimize maximize button in the popup lightning reminder title bar ; only the close button.
So, it's impossible to minimize the popup reminder alarm in the task bar.
When I right-click in the title bar, Minimize and Maximize are not allowed.
And in the Thunderbird screen, this popup is always on top ; it's impossible to pass it behind the main Thunderbird screen.
These buttons are available on Windows system, but no more on Ubuntu. (I tried with a blank profile, without any else add-ons or themes, but it's the same).