My multi-monitor setup looks like this:

           +----------------+
___________|                |
|--PANEL--||                |
|         ||        2       | 
|    1    ||                |
+---------++----------------+

The main screen is monitor 1.

When windows are maximized the main monitor, they expand to cover the whole screen, regardless if there is a panel with an "always visible" setting on the top of the screen, pointing to a bug on kwin.

Its important to note that in this particular setup, the panel sizes are ignored if the panels are on the top or right of the screen, but they are taken into account if the panels are on the bottom or left.

Reproducible: Always

Steps to Reproduce:
1. Use more than one monitor and set them up like in the figure
2. Add a panel to the top or right of the main screen/monitor
3. Maximize a window on the main screen that contains the plasma panel.
Actual Results:  
The window maximizes/resizes to the maximum possible size, going below panels that are marked as "always visible".

Expected Results:  
Window maximizes/resizes to the size of the screen minus the size of panels.