Function adjust_anchor_for_user_select 

pub(crate) fn adjust_anchor_for_user_select(
    no_gc: &NoGC,
    anchor_candidate: RootedSelectionBoundary,
) -> Option<(RootedSelectionBoundary, Option<DomRoot<Node>>)>

Adjust the range boundary for the selection anchor (start of a drag gesture) based on user-select of the container node and of its ancestors.

Returns None if no new drag gesture should be started (user-select: none), or:

• The new anchor container
• The new anchor offset
• The node with user-select: contain that the selection must not leave, if any