Add positioning context

A fallback for browsers that don't support `contain: content`, to make sure the positioning context is correct

Author: greystate

src/Umbraco.Web.UI.Client/src/less/property-editors.less

@@ -607,6 +607,7 @@
         box-sizing: border-box;
         line-height: 0;
         contain: content;
+        position: relative;
         .checkeredBackground();
 
         &:focus, &:focus-within {