When mouse hovers over lower right corner of a cell a plus sign appears when no keys pressed. Pressing any modifier keys has no effect. Can only copy contents not move them. Tried with two different computers, mice. This happens in both LibreOffice and OpenOffice.
It's needed to have the cell selected, please read: https://help.libreoffice.org/latest/en-US/text/scalc/guide/mark_cells.html?DbPAR=CALC#bm_id3153361