[css-sizing] Stretch-fitting inline size next to floats. #4028

https://drafts.csswg.org/css-sizing-4/#stretch-fit-sizing
https://drafts.csswg.org/css-sizing-3/#auto-box-sizes

The spec currently doesn't provide a size keyword to achieve this effect, but there used to be one named "fill-available", then renamed to "fill", "stretch" and/or "stretch-fit".

The behavior is partially defined, though, in level 3.
"The size a box would take if its outer size filled the available space in the given axis; in other words, the stretch fit into the available space, if that is definite."

Furthermore, the spec already refers to stretch-fit sizing as part of resolving fit-content, so my question is: How is stretch-fitting in the inline dimension affected by adjacent floats?

<style>
  .outerfloat { float:left; width:100px; height:100px; background:blue; }
  .stf { overflow:hidden; width:fit-content; background:yellow; }
  .spacer { float:left; width:300px; height:10px; }
</style>
<div style="width:500px;">
  <div class="outerfloat"></div>
  <div class="stf">
    <div class="spacer"></div>
    <div class="spacer"></div>
  </div>
</div>

Should the yellow box fit to the right of the yellow box, or go below, and use the entire width of its containing block?