Prerequisites:

Configure additional classes and attributes for RTE with: https://doc.ibexa.co/en/latest/content_management/rich_text/extend_online_editor/#configure-custom-data-attributes-and-classes

Steps to reproduce:

1. Create a content item with richtext field
2. In richtext field add some number of lines (maybe 1,5 screens)
3. Select text and open 'Custom attributes' settings
4. Scroll down

Actual result: 

'Custom attributes' settings box is overlapped by RTE toolbar.  'Custom attributes' settings box can go outside rich text field.

Expected result:

'Custom attributes' settings box is overlapped by RTE toolbar - which is correct.   'Custom attributes' settings box should not 'venture' outside richtext field (see ibx-6403-Edit article-1.png).