Activate PressAndHold for LibreOffice or OpenOffice
This feature is not supported by LibreOffice 5.3 according to this page.  It's the same for 5.4 according to my tests.  I did not try the dev version 6.0.




  EricP 2017-06-01 01:56:19 UTC Comment 8               
  
  A few years later now... Press And Hold still does not work in
  LibreOffice, even though LibreOffice is now built against an SDK
  requiring MacOS 10.8 or higher (see comment 3).  As indicated in
  bug 42437, comment 7, just an SDK update is not enough; adding this
  feature will require some developer time.
  
  It is, apparently, possible to "add" Press and Hold support to some
  applications simply by typing a Terminal command similar to this:
  
  defaults write org.libreoffice ApplePressAndHoldEnabled -bool true
  
  but this does NOT enable Press and Hold in LibreOffice 5.3.