Without my knowingly making changes to how my program selects text, selecting text no longer highlights in blue. Instead, when dragging or clicking, text disappears. Reverse dragging and clicking again makes it reappear.
Try resetting your profile: https://wiki.documentfoundation.org/UserProfile
When asking these types of questions it’s always good to include your OS and version of LibreOffice so that we can narrow it down a bit 