With List v3 enabled along with Async v2 related lists, the floating horizontal scrollbar (glide.ui.related_list.floating_scrollbars) does not appear.
Steps to Reproduce
Activate List v3.
For more information, see the product documentation topic Activate List v3.
Go to the incident table.
Add Audit History to the related list if it is not already there.
The Audit history usually must have more than 20 records for the floating scrollbars to appear.
In the Audit History related list, click the gear icon to personalize the list layout.
Add about 7 columns or more to create the necessity for the scrollbar to appear.
Note that although a floating horizontal scrollbar should appear to scroll through horizontally, you have to scroll to the very bottom of the related list to see that there are additional columns in the list.
Download the attached XML file (sys_ui_script_5e5ce7a09f0013005cec3b8eb42e7009.xml).
Go to any list, click on the list header, and choose Import XML.
Select the XML file you downloaded and click Upload.
This should resolve the issue. The horizontal floating scrollbar should be displayed across all List V3/ Async V2 related lists.
Related Problem: PRB1250034