Clock with seconds moves panel items.
Xfce 4.16, potentially carried over to 4.17/18
When having a clock at the right of the panel and enabling seconds, as the width of the number changes, it pushes all panel items next to the clock across.
For example, as seconds reach 10, items move a few pixels to the left. As seconds turn to 11, with 1 being narrower than 0, panel items move a few pixels to the right, then left again as seconds become 12 etc.
My panel currently has from right to left, separator, clock, separator, notification plugin (external), status tray plugin (external), separator, workspace switcher. Even removing the external plugins and separators, the clock still pushes the workspace switcher across.