This happens when multiple new actions shows up as flagged in the upper left hand corner (The red raindrop with a number). It will only happen with 3 or more. If you click the number to show the dialog, then click off on the page somewhere (not a link, or a link if you want), the dialog will go away and so will the red raindrop. However, page requests will become rather laggy for some time period. I am not sure how it evens back out - perhaps by re-visiting the inbox area. Eventually it does, but it gets very laggy when in that state.
Thanks for bringing this up. I cannot reproduce the issue on the current build of our sites. If the problem remains please let me know.