You need to remove the referenced tree view for the field.

Create your own View that inherits and overrides the base View:

<field name="seller_ids" position="replace">
  <field name="seller_ids" 
        context="{'default_product_tmpl_id':context.get('product_tmpl_id',active_id), 'product_template_invisible_variant': True,}" 
        nolabel="1" attrs="{'invisible': [('product_variant_count','&gt;',1)]}"/>
</field>         

The original View defines seller_ids with a context key of 'tree_view_ref':'purchase.product_supplierinfo_tree_view2'  so here we are removing this and keeping all other attributes of the field.

The popup Form View limits the selection of Variants to match those from the Product Template you are editing Vendor information for.

Thanks Ray!  This seems like a mistake by Odoo as it was working well in V13.  

UPDATE:

I tried this and it works well for products with only one variant. Once a product has multiple variants it reverts back to inline editing. I played with the attributes - changing the gt to lt - but it didn't work.

Any idea why?  Thanks for your help with this. 

UPDATE2

I have replaced the variant_seller_ids field as well, and now the form opens in a new window for products with variants.  But the variants are not passed in the context, so the variant field in the form is empty.  Here is what I have so far: