Please describe the problem:
Say you are on workspace A.

If, in the workspace switcher, you move slightly a window of the workspace B but
keep it on the same workspace (B), the window will disappear from the workspace
switcher.

Changing workspace, from A to B or to C makes the window visible again.

The window is in fact only invisible because you can still take it and move it
to another workspace !!!  

You can even reproduce the bug with the window on your own workspace (A) and
made it invisible for the workspace switcher but you can see it in front of you.

Steps to reproduce:
1. Open a window on workspace A.
2. Move to workspace B.
3. In the switcher, move slightly the window on the workspace A


Actual results:


Expected results:


Does this happen every time?
Yes, always reproducible. During your move, the window cannot touch the border
line ! So use a big switcher with a little window.

Other information:
Ubuntu Breezy, freshly upgraded.

I use a 2 row * 4 column = 8 workspaces configuration.