Fix collapsing margin issue

[ykadosh]

When the content of a collapsible has margins, the height is calculated without the margin, causing a janky transition.
This is due to collapsing margins:

6.2. Collapsing margins

Certain adjoining margins combine to form a single margin. Those margins are said to “collapse.” Margins are adjoining if there are no nonempty content, padding or border areas or clearance to separate them.

One way to fix this is by adding overflow: auto to the container, but i'm worried about unwanted scrollbars appearing in certain cases.

Another solution is adding padding: 1px to .content, to prevent the margins from collapsing.
We can then add margin: -1px to .content-wrapper to remove the 1px spacing.