Since I started using Plasma 5, I am noticing that panel flashes from time to time.
It seems that task manager widget flashes because of new ghost entry which sometimes appears for some msec and then dissapears. So that ghost entry makes panel "flash" if taskbar is full with opened apps.
Look the video: https://www.dropbox.com/s/iznqz20ukq4v3 ... 4.mp4?dl=0

This happens on Manjaro, everything latest stable (kernel, drivers, qt 5.5, plasma 5.5... etc), intel gpu.

Is this a known bug (and sorry if I am posting the thread in wrong section of the forum) ?

Wrong subforum, -> plasma5
Bug is https://bugs.kde.org/show_bug.cgi?id=332024

moved to Plasma 5 forum

it is a known bug : https://bugs.kde.org/show_bug.cgi?id=332024