Fix a problem with NodeRefs and vtags, ref #2206

[WorldSEnder]

Description

Fixes #2206

This is in some sense a very poor-mans local fix. It works and has a test case - good.

There is something deeper to fix: every NodeRef should be owned by exactly one rendered vnode. If that invariant is violated, this could lead to unexpected results - like here: Complicating the situation is that during rendering, new nodes are mounted and refs are set immediately, while another part of nodes might get detached later during the same render - "owning" the same ref - leading to a time window where a NodeRef is "owned" by two nodes.

There might be other, similar bugs lying dormant until a bigger effort is spent on developing a conceptual understanding of the above and adding some more debug assertions to check for the implicit invariant. Note that for example, there are currently no checks for the user reusing the same NodeRef multiple times:

<>
  <div ref={&some_ref} />
  <div ref={&some_ref} />
</>

Which of those divs is expected to be in some_ref? Imo, some debug_assertion should trigger and inform the user not to do this in debug builds.

Checklist

• I have run cargo make pr-flow
• I have reviewed my own code
• I have added tests

[ranile]

LGTM