Feature summary

• Add a design token, both in LESS and as a CSS variable, for the height of a skin's sticky header.
• The variable should be set to 0 by default, and always be available, also for skins that don't have sticky headers.

Use case(s) and benefits

• This would enable us to use that variable when calculating offsets that depend on the sticky header's height.
  • Example: For T407113, patch 1196876 wouldn't have to add skin-specific handling at all; instead, we could just set the value for top in wikibase.less#192 to ~'calc( var( --sticky-header-height, 0 ) + 10px )'.
• Some bugs whose solution would be easier if we had this: T407113, T362140, T290518, dewiki diff 256999907 (T367369), T42763, most likely several others too