'combo' form field type validation is delayed

[paul-muckypuddle]

Steps to reproduce the issue

1. Create a form field of type combo with the required attribute set to true
2. Access a screen displaying the form field
3. Select an option

Expected result

The field contains the selected option and no validation highlighting

Actual result

The field is highlighted as invalid (i.e. red, bold text and a red border) until a second valid selection is made (or another pair of focus / blur events occur on the field)

System information (as much as possible)

Joomla 3.7.4
PHP 5.6.30
Apache 2.4
Centos 7

Additional comments

• Video of issue in action: http://support-media.muckypuddle.me/lxH0
• Field XML tested:

<field name="etd" type="combo" filter="text" required="true" label="COM_XXX_FORM_LBL_XXX_ETD" description="COM_XXX_FORM_DESC_XXX_ETD" hint="COM_XXX_FORM_DESC_XXX_ETD">
      <option value="1">1 Hours</option>
      <option value="2">2 Hours</option>
      <option value="3">3 Hours</option>
      <option value="4">4 Hours</option>
      <option value="5">5 Hours</option>
      <option value="6">6 Hours</option>
      <option value="7">7 Hours</option>
      <option value="8">8 Hours</option>
      <option value="9">9 Hours</option>
      <option value="10">10 Hours</option>
      <option value="11">11 Hours</option>
      <option value="12">12 Hours</option>
</field>

[chmst]

Very old issue and the linked Video does not exist anymore.

But valid issue. In J4 the Formfield type="combo" does not work at all.
Obviously nobody ever used this field type.