XPS 9730 : linux power : resume from standby and power plug

On a machine running Linux Fedora 28, resuming from standby sometimes does not work. This affects either opening the screen lid or pressing the power button.

In https://www.dell.com/community/Linux-General/XPS-13-9370-Standby-issues-resume-and-indicator/m-p/6095568/highlight/true#M16162 I had already described that this can be circumvented with plugging in the power supply.

Oddly enough, it also seems to help to plug the USB-C-A adaptor into the power plug of the laptop, to close some circuits that allow to resume from standby easily.

After sending it to sleep recently while playing music, I have also found out that the machine resumes from standby when being closed without a reason: suddenly there was the music playing from my backpack again.

Instructions how to reproduce this behaviour would be much appreciated.