Composed event dispatch with relatedTarget across subtrees

Visualizing the DOM spec's dispatching events algorithm. The walk re-retargets relatedTarget against every parent. When it crosses upward out of a shadow tree into a wider tree, the stored shadow-adjusted target updates. The walk terminates early when parent === relatedTarget (the retargeted one). Use the scenario picker below to see this play out across shadow, light DOM, and nested shadow trees.