STR
- Put the button into the overflow menu (i.e. that >> icon).
- Open it there and open it and select to "Pick a custom date/time".
What happens
The height is way too large and thus, you cannot click/choose the button to accept the date/time you've entered.


What should happen
I should see the buttons and be able to click them.
Workaround
You can zoom in and suddenly you will see some buttons at the bottom.
Or of course move it into the toolbar or so.
Potential solution
I know this menu is a hard weird thing and so on. Maybe you can just set a height as a workaround (for now)?
System
Firefox 90.0.2
Fedora 34, GNOME