Noticed while working on #8554 /Cc @jasmussen

Seems that sidebar panels added by plugin may inherit (depending on their implementation) styles from WordPress that could lead to unexpected results.

For example, when using the Ctrl + backtick keyboard shortcut to navigate through the main regions, the focus style applied around the sidebar may look wrong:

See the last sidebar panel that overlaps the focus style. This specific panel uses some markup from the WordPress meta boxes. After a quick investigation, seems two position: relative CSS declarations are responsible for this:

from Gutenberg:

.edit-post-meta-boxes-area {
    position: relative;
}

from WordPress

.postbox {
    position: relative;
    ...

Potentially, there could be other inherited CSS rules that could produce unexpected results. Soem special care should be taken in overriding these rules for the panels in the sidebar.