Ok guys, time to panic or to enter in fury mode
It appeared in my tests that the problem come from :
<Element ID="ToolbarField" X="120" Y="746" Width="1024" Height="200">
Setting a "Y" value going outside the actual height of the screen seems to cause the bug.
On my own system, which has a 1200 height display, setting "Y" beyond 1000 make it bug. 1000 + 200 (toolbar's height from this example) = 1200
All works fine untill I test the same code on a smaller resolution of 980 pixels height. --> Welcome back Mr Bug !
This means people now have to edit both width and height values in the skindefinition.xml files to position the toolbar
I'm really fed up with this kind of crappy dev job...