Manuel Zahariev 00d43b39d1 LibWeb: Retain calculated Element::ordinal_value for lists ...

`Element::ordinal_value` is called for every `li` element in
a list (ul, ol, menu).

Before:
  `ordinal_value` iterates through all of the children of the list
  owner. It is called once for each element: complexity $O(n^2)$.

After:
  - Save the result of the first calculation in `m_ordinal_value`
  then return it in subsequent calls.
  - Tree modifications are intercepted and trigger invalidation
    of the first node's `m_ordinal_value`:
    - insert_before
    - append
    - remove
  Results in noticeable performance improvement rendering' large
  lists: from 20s to 4s for 20K elements.

2025-06-16 12:44:58 +01:00

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