Fix horizontal scrolling in full-width collapsable insets

Fix horizontal scrolling in full-width collapsable insets

While a one paragraph large collapsable inset (containing for example a tabular) could be very wide and trigger horizontal scrolling, the code that makes collapsable insets wide when they contain several paragraphs would actually make them narrower in this case.

Typical example is a wide tabular and a caption in a table float, where horizontal scrolling would not trigger.

(cherry picked from commit 8c7ace4b on 2.3.0-staging)