Preview screen doesn't open until recipients list is built on mail compose screen
To replicate the issue:
- Create a new mailing and fill out everything but not the Recipients field.
- Now select a recipients group that is either very large or is based on a complex smart group. after selecting it -- click preview HTML/Text.
The preview window will not open until the recipient count query has completed. This can also be replicated by setting up a mailing with a large/complex group, saving it, then continuing it and immediately clicking preview. the window will not open until the query completes.