For reasons best know to itself this will be caused by your WordPress theme (although possibly but more doubtfully another WP plugjn) defining a large z-index on the display. If you do not know what that is think of it as a level of layering where zero is the actual screen.

That looks like our ‘Default’ theme so you would need to look in the stylesheet (;default.php) and find the entry for ‘ui-dialog’ (no quotes of course). It is about two thirds of the way down the file. It probably has a z-index value of 10 so you will want to increase that to bring it’s layer nearer. But you may need to experiment to get it to be larger than your Wp theme is setting itself at.

Clear the caches (admin > options > Global settings) if you are using them and clear the browser cache as well before seeing of the value you used is OK. I would start, personally, with something like 101.

As always we recommend the use of a child theme to ensure changes are not lost when upgrading. There are now Child Theme Frameworks available to download for all of our themes to make this simple and quick and full and extensive details can be found on our Codex – Creating a Child Theme page: https://simple-press.com/documentation/codex/themes/theme-basics/creating-a-child-theme/

Ah yes, that fixed it, thank you! 

I see the cache options but there’s no button to clear them. Just in case I need to in the future, how do I do that? I see both New Posts/Topics Cache and CSS/JS cache. Would I set the former to 0 and uncheck the latter’s boxes, save, then re-enable?

Sorry – I should have told you that s well.
Forum Admin > Toolbox > Housekeeping. There is a task tj=here that will clear the cache(s).