When zoomed in, the white boxes appear to be a background layer that is used to provide translucency to plasma widgets.  However, the location / size of the background layer is not adjusted when zooming in.  Hence, they appear in the wrong location and obstruct the view of areas that should be visible.  The best way to see this is by looking at the screenshots I posted at the URLs below.  In the first screenshot in particular you can see that the white layer is in the shape of the Application Menu, but it is in the size / location it should be if the desktop were *not* zoomed.  These white layers block the view of other applications (e.g. bottom edge of the screen in the second screenshot).

Screenshots of the bug:

http://snag.gy/XahKw.jpg
http://snag.gy/pg8tA.jpg


Reproducible: Always

Steps to Reproduce:
1.  Press Meta and = at the same time to zoom.

Actual Results:  
You should see a white bar at the bottom of the screen.  The bar stays there when the mouse is moved.  If you go to the KDE Application Menu and open it, you will see white boxes appear in the shape of the menu items that are open.  These white boxes will be in the location and size they would be if the screen was not zoomed in.

Expected Results:  
No white boxes.

I have observed this issue in at least the last few versions of KDE plasma.  I can reproduce the bug 100% of the time on both a laptop and desktop machine.  I use openSUSE Tumbleweed on both.