What version are you running?
3.0 RC1 (dev)
What's the URL of the page containing the problem?
https://reviews.reviewboard.org/r/9337/
What steps will reproduce the problem?

Create a review request and add a file to it. 
Once the file is displayed under the "Testing Done" field, click inside he field.
Click the "OK" button that appears below the "Testing Done" field.

What is the expected output? What do you see instead?
The expected result would be that step 3 would behave the same as when you click the "OK" button under the "Description" field: the button disappears and changes are saved.
What actually happens is: the changes are saved, and the field disappears instead of the buttons. 

Once this happens, clicking either "OK" or "Cancel" will make the text field reappear (and the buttons are hidden again), but it is overlapped by the file preview.
What operating system are you using? What browser?
OS: Ubuntu 16.04 LTS

Browser: Firefox 56.0 (64-bit)
Please provide any additional information below.