I'm trying to investigate this issue for some time now. I'm using Xubuntu and I've noticed that Whisker Menu icon is blinking when I move my mouse cursor on the left and top screen edges, but it behaves normally when I drag it on the bottom and right edges. Not only panel plugins/elements are affected by this, but also other applications, for example Plank.
You can see the issue on this video: https://youtu.be/YXmFXFV_ONQ
Issue occurs on Ubuntu 18.04 (GNOME), Xubuntu, Ubuntu MATE and some other distributions like Manjaro Xfce. EDIT: Kubuntu is also affected.
More details (and videos showing this issue) are here: https://forum.xfce.org/viewtopic.php?pid=48257
Please help me to kill that nasty bug, it's been on my system for a year now, I've noticed it on Xubuntu 17.04. I don't know where exactly to submit this bug, I previously submitted it wrongly on the Xfce's Bugzilla, but this issue is not Xfce specific, now I know that.