[select] Consider making the popover-ness automatic

[domenic]

First, the explainer seems to be out of date and is using popup="" instead of popover="".

But that aside, this is very weird:

The element with behavior="listbox" is required to be a <div popup=popup>.

There aren't any other cases I can think of in HTML where to get the effect you want, you have to trigger some other related effect. E.g., you don't have to do <input type=text contenteditable> to make it clear that text inputs are editable. You just get that for free.

Combined with #702 this could simplify the explainer's

<selectmenu class="my-custom-select">
  <div slot="listbox">
    <div popup behavior="listbox">
      <option>Option 1</option>
      <option>Option 2</option>
      <option>Option 3</option>
      <option>Option 4</option>
      <option>Option 5</option>
    </div>
  </div>
</selectmenu>

into

<selectmenu class="my-custom-select">
  <listbox>
    <option>Option 1</option>
    <option>Option 2</option>
    <option>Option 3</option>
    <option>Option 4</option>
    <option>Option 5</option>
  </listbox>
</selectmenu>

although I guess if you did #645 then even the <listbox> wrapper would not be necessary because you could just target the listbox in CSS.

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This idea isn't free, as you need to think through some of the interactions on the API side. E.g. what happens if someone does add popover=""? What does listbox.popover return by default? Etc. But I think this things are worth thinking through, to give a better developer experience.

[josepharhar]

I agree, I am doing this in the explainer here: #798