Only external panel plugins placed left of the window buttons and close to the left screen seem to be affected by this glitch. Hovering over these panel items with the cursor restores the panel appearance.
xfce4-panel 4.10.0git-d296ca7 (Xfce 4.10)
Xubuntu 14.04 LTS Alpha 2
Related bug report (xfce4-whiskermenu-plugin):
This issue is triggered by the following gtk_widget_get_snapshot call:
Requirements for this glitch to occur when dragging a window button:
- panel placed on top
- show button labels
- any external panel plugin placed leftmost within the panel
Created attachment 5969
Do not use widget snapshot as tasklist draw icon
*** Bug 11567 has been marked as a duplicate of this bug. ***
*** Bug 11573 has been marked as a duplicate of this bug. ***
Too bad this fix did not make it into 4.12.0...
Works perfectly for me, nevertheles... Thanks!
*** Bug 11897 has been marked as a duplicate of this bug. ***
*** Bug 9096 has been marked as a duplicate of this bug. ***
What is the current status?
Thaddaeus Tintenfisch' patch works perfectly for me.
But it is neither committed to git nor released.
*** Bug 12001 has been marked as a duplicate of this bug. ***
Pushed the patch to git master:
8dc83f5 Do not use widget snapshot as tasklist draw icon
On Xubuntu 15.04 with XFCE 4.12, the bug is still active.
There is now no need of the snapshot widget to reproduce the bug.
To reproduce the bug, just boot the Xubuntu ISO as a live-CD into a VirtulBox, open a Thunar window then drag it.
The Whisker button disappears...
*** Bug 12275 has been marked as a duplicate of this bug. ***
Created attachment 6606
Video of bug (theora ogg)
Mint 17.3 Xfce 4.12 Confirm bug, added details video. Please report to developers. Thanks!
I also have this bug on Debian testing with XFCE 4.12. I would have liked to test Tintenfisch's patch, but I can't get xfce4-whiskermenu-plugin-1.5.2 compiled, because it depends on libxfce4panel-1.0, but I can only find libxfce4panel-2.0 in my repositories. Trying to compile libxfce4panel from source also doesn't work easily, because it depends on garcon-gtk2-1 which doesn't seem to be satisfied with libgarcon-1-0-dev which is the only one I an find in my repository. I get the feeling I first would need to do a Linux from Scratch to compile this, why is that?
Created attachment 6884
The black square in right top corner is Whisker Menu.
I have the same bug on my Xubuntu 16.04.1, xfce4-panel 4.12.0 and Whisker Menu.
This has been fixed  and version 4.12.1 is no longer suffering this issue.
Closing as per comment 17 (which is correct).