Currently the sidebar width is limited to 400px (hard coded).
Some extensions might wish to have a larger size to fit their content into.
So the maximum sidebar width should be configurable.
Samuel Mehrbrodt committed a patch related to this issue.
It has been pushed to "master":
tdf#124255 Make maximum sidebar width configurable
It will be available in 6.3.0.
The patch should be included in the daily builds available at
https://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More
information about daily builds can be found at:
Affected users are encouraged to test the fix and report feedback.