Share button can't be pushed because the Share dialog box too long

A user "Shared" a page using the Share button and added many users to the share. After completing the entry of his message, the user discovered that he could not complete the Share by clicking the Share button because the dialog box extended below the end of the page. Scrolling down to the bottom of the page showed that the dialog box disappeared off the page!

Clicking outside the dialog box closed the share so the message and user selection were lost.

Ideally, the dialog box should not extend past the end of the page.