this is a bug that was also there in earlier versions of plasma, I believe. when you click on the desktop and select "add widgets", the widget explorer opens, but then moving the mouse towards it will close it if you touch any window in between. this is probably related to how you set your focus policy - it is set to activate windows when touched by the cursor in my case - but is still not the desired behavior. Reproducible: Always
*** This bug has been marked as a duplicate of bug 336285 ***