Bug 64072

Working on Bug 63977, It became clear that we should have a way to beware "flattened tree" concept in XBL mean. Here is a rough idea: - At first, extract NodeFlattener from NodeRenderingContext. - Then expand its usage to where we should care about flattened tree: - Focus handling, - Text iteration, - And possibly more! NodeFlattener would look like this: ---- class NodeFlattener { public: Node* nextSibling() const; Node* previousSibling() const; Node* parent() const; Node* next() const; // dfs traversal like traverseNextNode() void moveToNextSibling(); .... // more moveTo family will come here. private: Node* m_node; ... // Pull a part of NodeRenderingContext state here. }; ---- NodeFlattener is like TreeWalker, but should be more lightweight. - It doesn't have script binding. - It's stack-based. Ideally, it would be great if the flattener can be used as a smart pointer for the node. But I think it isn't feasible because it has a many state which makes a copy operation bit expensive.