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 Confirmed. +1
Andreas Brandner committed a patch related to this issue. It has been pushed to "master": http://cgit.freedesktop.org/libreoffice/core/commit/?id=417bcf6b3e46eb2b5b912fe1df35974a626eabe3 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: http://wiki.documentfoundation.org/Testing_Daily_Builds Affected users are encouraged to test the fix and report feedback.