Sometimes you want one part of your tab behind a section, but not every part. This can be the case when you want to hide some advanced settings for instance.
Do not give an error if just one part of the tab is divided into sections.
You can use toggles and show the content when the toggle is on. However, this not visually appealing.
I would like to second this feature request. There are times when you have generic fields that you want to be visible to the user at all times. Or in other situations you might have some plotting options like colours, sizes, etc that you want to hide because they are only relevant at certain times. This option would give the developers the opportunity to take the UI to a next level for the users.
This would be the preferred feature request over setting the default setting for sections to collapsed or not, as discussed in this feature request:
As in some cases, you want to have a canvas that has some fields under sections, and some not.