Go to Impress - Tools - Spelling - Options and resize the dialog vertically.
You will see that there is a lot of space between "Writing aids" and "User defined dictionaries".
Instead, when resizing, only the listboxes should grow.
Created attachment 132330 [details]
Writing Aids dialog spacing on resize
Andreas Brandner committed a patch related to this issue.
It has been pushed to "master":
tdf#106946 Removed ability to resize Spellchecking-Options dialog
It will be available in 6.0.0.
The patch should be included in the daily builds available at
http://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.