I've a dual head setup.
On the left-side display:
1* On the left, i've a verical panel with width=$W
2* On the very right side of the same display, there are other widgets 
+--------------+----+
|p|            |Widg|
|q|            |ets |
|a|            +----|
|n|                 |     
|e|                 |
|l|                 |
+-------------------+


When i login to the desktop the widgets from "*2" are moved slightly to the left, by $W.
+-----------+-------+
|p|         |Widg|  |
|q|         |ets |  |
|a|         +----|  |
|n|                 |     
|e|                 |
|l|                 |
+-------------------+



if i kill plasmashell and restart it, they return where to right position
I can mitigate the issue even editing /etc/xdg/autostart/plasmashell.desktop bu replacing:

Exec=plasmashell
by:
Exec=sleep 10 ; plasmashell