Area below panel is shifted left

When the panel option called "Don't reserve space on borders" is disabled and i click e.g. Whisker menu plugin the area below the panel is shifted left.

The wider the whisker menu popup is the greater the shift is (the more pixels the background is moved). Also happens with plugins like "clock" but not much because the popup is not so wide. Does not happen with "time and date" plugin.

Xfce 4.14 Linux Mint 20.1