[ShadyDOM] assignedSlot returns null for host children, until assignedSlot is called for any element in shadow tree

[tomalec]

Description

I'm trying to inspect the host.children[~].assignedSlot. (Preferably, from within shadow root).

But in Polyfilled browsers it's null. I noticed that, if I call assignedSlot on any element from host.shadowRoot.querySelector('*').assignedSlot, shadow tree get synchronously "rendered", and host.children[~].assignedSlot starts to return correct value.

It feels like assignedSlot getter renders incorrect tree. It renders a tree that contains the given element instead of the tree that may contain a slot in question - parent element's shadow tree.

Live Demo

https://jsbin.com/tapaqoy/3/edit?html,console,output

Steps to Reproduce

1. Create shadow-host
2. Append <div slot="given"> to shadow host
3. Attach shadow to shadow-host, which contains <slot name="given">
4. Read div.assignedSlot

<template id="shadowRoot">
  <slot name="given"></slot>
</template>
<div id="shadowHost">
  <div slot="given" id="slottable">I'm distributed</div>
</div>

<script>
   shadowHost.attachShadow({mode: 'open'})
     .appendChild(document.importNode(shadowRoot.content, true));
   const slottable = shadowHost.querySelector('[slot]');

   console.log(slottable.assignedSlot); // null
   // trick Polymer to render this shadow tree to make `assignedSlot` of element accessible
   shadowHost.firstChild.assignedSlot; // does not have to be a slot

   console.log(slottable.assignedSlot); // [HTMLSlotElement]
</script>

Expected Results

A reference to the slot should be given

Actual Results

It returns null

Browsers Affected

• [ ] Chrome
• [ ] Firefox
• [x] Edge
• [x] Safari 9
• [?] Safari 8
• [?] IE 11

Versions

• webcomponents: v2.1.3

[stale[bot]]

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