Referring to source code of ClippableElement, but may occur at other places, too.

function OoUiMixinClippableElement() uses #$element within doc block.

This is obviously permitted if and only if a definition of $element is present in the same unit.

When generating doc, $element is unknown and cannot be linked, showing up as source string in formatted $clippable doc item e.g. in PopupWidget when expanded.

Seems to be a general issue with mixin since mixed-in does not know how to link within the current parent (one of many). Brave new world of inheritance.