Share window closes when user selected

One of my users is having an issue with the share button.

when she selects a user, the share window closes and she needs to click elsewhere before she can click on share and re-open the window. This means adding several users to share with involves clicking in and out of the share window multiple times, then clicking back in to write message and send.

Attempted on both windows7 and xp machine - appears to be user specific on her two machines.