…extraAction` to example custom CSS

- likely not important to preserve the text-decoration CSS here

- include global banner if present, account for chrome visible or not

+ flatten conditional modifiers, to make overriding by other usages easier

+ remove offsets logic in _banner.scss, easier to handle it all in one place

note: all flyouts/collapsible navs should inherit automatically from the `--euiFixedHeadersOffset` var

+ delete unused `kibanaFullBodyMinHeight` mixin

- by obtaining the height value from the DOM/CSS var

- Remove deprecated EUI mixin (now handled by CSS below)

- Remove selector-specific overrides; update their usages to use the global CSS variable instead

- remove manual offset calculation and fall back to EuiPageSidebar's default inline styles instead, which automatically uses `--euiFixedHeadersOffset`

+ bonus: fix broken `:hover` effect on solution side navs by restoring missing className

+ continue dismantling `kbnAffordForHeader` mixin

+ remove `kbnStickyMenu` entirely - it looks like old code and it's not clear to me exactly how that behavior is being used, or if it should be using EuiPageSidebar intead - will ping Fleet team to ask

- # of fixed EuiHeaders should now be automatically detected and handled by EUI without needing any custom body logic

… var

+ some snapshot updates around the new header CSS var

- was relying on the old selectors to set the correct height - use the new EUI var instead and remove direct className reference

- className no longer has any direct styles applied, and as far as I can tell doesn't need any associated header-related flyout logic