Selecting options in `ezselection` field type is limited to the first few because overflow doesn't appear.

Steps to reproduce:
1. Install clean version of eZ Platform Enterprise Edition 2.4
2. Add new Content Type with the following Field Types (in following order):

• `ezstring`
• `ezstring`
• `ezstring`
• `ezdate`
• `ezselection`
• `ezlandingpage`

3. Add at least 10 options to `ezselection` field type
4. Save the content type
5. From admin panel try to add new content based on created content type.
7. Switch to the content edit view (second icon in the top-left corner)

Result
When we try to select fields from `ezselection` field type, we cannot select the last ones.

Expected behavior
When an option list exceeds the page height. Overflow appears and it is able to choose all options.