With gtk3 the desktop pager does confuse the constant width of the panel,
it is drawn to a constant width depending on what is currently active resulting
in its width being smaller or wider than fullscreen.

Remove it from the default configuration.

diff -Naur lxpanel-0.8.2.orig/data/default/panels/panel.in lxpanel-0.8.2/data/default/panels/panel.in
--- lxpanel-0.8.2.orig/data/default/panels/panel.in	2016-06-18 14:33:25.398929460 +0200
+++ lxpanel-0.8.2/data/default/panels/panel.in	2016-06-18 14:40:09.486171468 +0200
@@ -80,10 +80,6 @@
 }
 
 Plugin {
-    type = pager
-}
-
-Plugin {
     type = space
     Config {
         Size=4