Fix overflow problem in diff view — radicle-explorer

When selecting a range of lines and scrolling past the sticky header, the selection would be on top of the header. This fixes it.

     border-top-left-radius: var(--border-radius-small);
     border-top-right-radius: var(--border-radius-small);
     background-color: var(--color-background-default);
     z-index: 1;
     z-index: 2;
   }
   .sticky {