fix: prevent diff header overlapping popovers

Author: Dimillian

src/styles/diff-viewer.css

@@ -62,7 +62,7 @@
 .diff-viewer-sticky {
   position: sticky;
   top: 0;
-  z-index: 3;
+  z-index: 1;
   height: 0;
   overflow: visible;
   pointer-events: none;