The My Community Theme has a "sticky" header that can go over the top of the IDX Registration Form pop up at some screen sizes.
If this happens on your website, you can add the following CSS so that the pop up will be on top of the sticky header.
You can go to Appearance > Customize > Additional CSS and add the following CSS:
/* Move Sticky Header below IDX Registration */ @media only screen and (min-width: 960px) { .site-header { z-index: 1000; } }