Gap in sidebar when boxes are collapsed

[timobrembeck]

Describe the Bug

When there is only one sidebar and some boxes in the upper part are collapsed, there is a bit of unused space between the two segments of the sidebar (Also see #1769 (comment)).

Steps to Reproduce

1. Go to page form
2. Adapt screen resolution until exactly one sidebar is shown
3. Collapse box in upper part of the sidebar
4. See spacing

Expected Behavior

The collapsing should cause the lower boxes to move upwards. Maybe we should define a grid that only has one column when there's only space for one sidebar and two columns otherwise, instead of wrapping the two columns around which causes the space?

Actual Behavior

There is spacing between them

Additional Information