Comment 26 for bug 1397095

This bug is even worse on Xenial than stated above. And, on my system I believe the issue is with Whiskermenu alone and not with the panel: I.e., I have read that it only happens if Whiskermenu is in the far left of the panel etc. etc.... I have also read that any external plugin icon will do it if in the far left position. Not the case on all my XFCE systems (including other distros).

The truth is no matter where you put Whiskermenu on an XFCE 4.12 panel along with "Window Buttons" the icon will go black if the buttons are dragged. No other icon has this issue even if in the supposed offending position. Hence my opionion that this is really now a Whiskermenu issue alone.

I have tested the above assertions on Manjoro, Ubuntu, and Debian running XFCE 4.12. It is identical across distros. Again... seems like a Whiskermenu bug to me as I can not get another other plugin icon to do it on Xenial regardless of panel position.

Here is a fix. It is a bit of a kludge, but it works until they actually "fix" it.

1) Create another panel and place it to the far left of your panel with window buttons.
2) Move the Whiskermenu icon to the new panel and make the panel as short as possible using the panel's preferences.
3) Adjust the length of your main panel to account for the length of the new panel and have them meet. You may notice a seperation line... or not. It depends on your wallpaper darkness and how transparent your panels are etc..

Now you can drag Window buttons in the main panel and the tiny panel with Whiskermenu will be unaffected.

Nothing else works.