mithril.js: Recycling heuristics have false positives and false negatives

Currently, vnodes are added to a recycling pool if:

1. they are not a component (typeof vnode.tag === "string")
2. they are not a fragment or trusted (vnode.domSize == null)
3. there are no hooks that can modify vnode.dom in vnode.attrs, at the time of removal

This excludes nodes that could be recycled (fragments and components), and includes nodes that may have had problematic vnode.attrs in the past.

These heuristics are only applied to vnode that are detached (those where onbeforeremove can fire). However, once a vnode has joined the pool, its children are recycled as well even if they don’t meet those criteria, thus possibly recycling DOM nodes that have been modified in the hooks.

Proposed solution:

• Add a vnode.reuse = true field.
• Set it to false for trusted nodes and when a problematic hook fires (by modifying initLifecycle() and friends)
• Only add nodes that have vnode.reuse to the pool;
• In updateNodes(), when recycling === true, verify that vnode.reuse === true as well.

AFAICT these heuristics would allow to recycle components, fragments and don’t have the holes described above.