Link to matrix cell takes over page instead of being a pop-up in Assignments

When clicking the link to an associated matrix cell in Assignments, the information regarding the cell should be in a pop-up. Instead, it takes over the page and there is no way for the student to get back to the assignment other than to refresh and re-enter.

To reproduce:
Add student to both a portfolio site and course site
In the portfolio site you will need a published matrix
In the portfolio site on the manage matrices page, click 'Manage Site Associations' link
Associate the course site
In the course site, add an assignment
Click 'Create/Edit Matrix Links' for the assignment and associate it with one of the cells in the matrix

As the student, log into the course site
Click on Assignments, then the linked assignment
The following text displays:
This item is linked to one or more matrix cells. Click the cell name to learn more about the purpose of the cell.
Click the link

Instead of being a pop-up which can be closed after viewing, the cell information takes over the page