I worked in two Calc windows, and their toolbars and worksheets were shifting up and down by a couple of pixels whenever I switched between them. Quite annoying.
I noticed it would start happening after the LibreOffice Update bubble pops up in the top right corner, because it plants some invisible icon there that pushes the UI down.
To get rid of annoying UI movements, go to Tools / Options / LibreOffice / Online Update / uncheck Check for updates automatically