Hi Erik,
I agree there are some issues in React with styling in design time.
In general you can adjust the css in SS with effecting it runtime on the following manner by prefixing the attribute with -servicestudio:
for example, when you have a class which has a fixed position:
.myFixedContainer {
background-color: #f00;
position: fixed;
-servicestudio-position: relative;
}
So, in servicestudio it will be relative and runtime it will be fixed.
It's quite handy, because it works for most attributes like color, display, position etc.
so you can really make acid-green colors within SS to find the important containers faster which are normally hidden for example ;)
so, for the popup-widget you need to add the following in your css
[data-popup-backdrop] {
-servicestudio-position: relative;
}
This way, the backdrop and popup itself will act relatively in servicestudio.
I am not sure if this will help youi 100%, but you can see/toy with this to get what you want.
Good Luck,
J.